Chapter 1: The Characeae Plant
Chapter 1: The Characeae Plant
Chapter 1: The Characeae Plant
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1 <br />
<strong>Chapter</strong> 1: <strong>The</strong> <strong>Characeae</strong> <strong>Plant</strong><br />
1.1 General Morphology<br />
1.2 Morphology of individual species<br />
1.2.1 <strong>The</strong> Chara corallina/australis group of species<br />
1.2.2 <strong>The</strong> Lamprothamnium papulosum/macropogon group of species<br />
1.2.3 <strong>The</strong> Lamprothamnium succinctum group of species<br />
1.2.4 Lamprothamnium inflatum<br />
1.2.5 Species of Tolypella<br />
1.2.6 Species of Nitellopsis<br />
1.2.7 Species of Nitella suitable for physiological studies<br />
1.3 Cellular structures and processes<br />
1.3.1 Cells<br />
1.3.2 Gametangia<br />
1.3.3 <strong>The</strong> oospore<br />
1.3.4 <strong>The</strong> germinating oospore<br />
1.4 <strong>Characeae</strong> names<br />
1.4.1 Systematics and the concept of species<br />
1.4.2 Individual species’ stories<br />
1.4.2.1 Chara australis R. Br.<br />
1.4.2.2 Lamprothamnium inflatum (Fil. & G.O. Allen ex Fil.) A. García & K.G.<br />
Karol<br />
1.4.2.3 Chara longifolia C.B. Robinson<br />
1.4.3 Lessons to be learned<br />
1.5 Ecology<br />
1.5.1 Distribution and abundance<br />
1.5.2 Processes controlling characeae distribution<br />
1.5.3 Ecological role of characeae<br />
1.5.4 Abiotic environmental factors<br />
1.5.4.1 Light<br />
1.5.4.2 Water chemistry and physical properties<br />
1.5.4.3 Nutrients<br />
1.5.4.4 Temperature
2 <br />
1.5.4.6 Depth<br />
1.5.5 Culturing characeae<br />
1.5.6 Life History<br />
1.5.7 Population ecology<br />
1.5.8 Application of ecological knowledge<br />
Abstract<br />
<strong>The</strong> aim of this chapter is to give physiologists a thorough grounding in the morphology,<br />
taxonomy and ecology of the characeae plant. <strong>The</strong> morphology of characeae is depicted and<br />
explained, with specific examples of the morphological characteristics of different species or<br />
species groups that are used in physiological studies. <strong>The</strong> details of characeae cellular<br />
structure in growing plants and in the reproductive organs are reviewed. <strong>The</strong> history of<br />
characeae taxonomy and nomenclature is outlined, along with the most recent approaches to<br />
charophyte systematics (and what name to use for characeae e plants in physiological<br />
studies), and finally the patterns of characeae distribution and requirements for characeae<br />
growth in natural situations are explained, and related to the culture and growth of<br />
characeae for physiological studies.<br />
Michelle T. Casanova<br />
Centre for Environmental Management, University of Ballarat, Mt Helen, Vic. 3350, Australia<br />
and Royal Botanic Gardens, Melbourne. Birdwood Ave, South Yarra, Victoria 3141,<br />
Australia<br />
Current Address: 273 Casanova Rd, Westmere Vic 3351 Australia<br />
Email: amcnova@netconnect.com.au
3 <br />
1.1 General Morphology<br />
<strong>The</strong> characeae thallus (or plant body) is similar in appearance and size to the plant body of<br />
other submerged plants such as Ceratophyllum or Myriophyllum. Charophytes consist of long<br />
photosynthetic stem-like structures (axes) anchored in the soil, with whorls of leaf-like<br />
organs (branchlets) along the stem (Fig. 1.1a). Close examination reveals that the structure of<br />
characeae is very different to that of flowering plants. Instead of roots they have colourless<br />
rhizoids, instead of leaves they have whorls of branchlets of limited growth, instead of stems<br />
they have an axis of giant cells joined end-on-end, and instead of flowers and fruit they have<br />
relatively simple reproductive structures (the oogonium and antheridium, Fig. 1.1b) that<br />
produce gametes. <strong>The</strong> product of fertilisation of the gametes is an oospore (Fig. 1.1c) rather<br />
than a seed.<br />
<strong>The</strong> thallus of characeae is essentially filamentous. <strong>The</strong> axes (stems) are made up of long,<br />
multinucleate, single cells interrupted by multicellular nodes (Fig. 1.2). <strong>The</strong>re is no<br />
development of tissues such as parenchyma in characeae, although the axial nodes approach<br />
such an arrangement. Several organs of limited growth (branchlets, stipulodes, cortical<br />
filaments) arise in whorls at the nodes (Fig. 1.2). Branchlets are the ‘leaf-like’ organs that<br />
occur in spreading whorls, and below these there are often whorls of smaller cells called<br />
stipulodes. In many species of Chara the stipulodes occur in two whorls, the upper whorl<br />
pointing upwards, the lower whorl pointing downwards (Fig. 1.2a, b). In the genus<br />
Lamprothamnium and some species of Chara the axial node can also be the site of<br />
gametangial development (Fig. 1.2c).<br />
Branchlet arrangement (Fig. 1.3) and morphology (Fig. 1.4) varies among the genera, but is<br />
characterised by elongate multinucleate cells interrupted by multicelluar branchlet nodes.<br />
Other cellular structures can be produced at the branchlet nodes, namely bract cells (Fig. 1.3a,<br />
b, Fig. 1.4a, b), secondary and tertiary (et seq.) branchlet segments or rays (Fig. 1.3c, d, Fig.<br />
1.4c, d), cortical filaments (Fig. 1.4a) and gametangial initials (Fig. 1.4). Some species of<br />
Chara have elongate bract cells in whorls (verticillate) at the branchlet nodes, as well as at<br />
the apicies of the branchlets (Fig. 1.3a). Other species produce unilateral bract cells (Fig.<br />
1.4a). Lamprothamnium exhibits the same overall branchlet morphology as Chara but the<br />
bract cells are generally inserted at angles of 45° to 90° to the branchlet, forming a ‘cage-like’<br />
structure around the nodal complexes (Fig. 1.3b, Fig. 1.4b). Nitella species have branchlets<br />
that are divided or forked (furcate) into separate rays or segments (Fig. 1.4c), which can be
4 <br />
very evenly arranged (Fig. 1.3d) or messy (Fig. 1.3c). Some species of Nitella can produce<br />
more than one whorl of branchlets at the nodal complexes, a condition referred to as<br />
‘heteroclemous’ (Fig. 1.3d). Tolypella branchlets are different from those in other genera,<br />
usually with a central pluricellulate ray, secondary rays and clusters of gametangia (Fig.<br />
1.4d).<br />
<strong>The</strong> gametangial initial cell produces the gametangia (oogonia and/or antheridia), bracteoles<br />
and sometimes a bractlet (Fig. 1.5). Oogonia have a striped appearance with a small but<br />
distinctive crown of cells (coronula) at their apex. Oogonia vary in colour from green to<br />
bright orange, and in older parts of the thallus the fertilised oospore within the oogonium<br />
becomes darker as it matures. <strong>The</strong> antheridia can also be green, but are often orange to red in<br />
colour, and in dioecious species can be so large (approaching 1 mm in diameter) as to be<br />
mistaken for ‘berries’ on the branchlets. <strong>The</strong> gametangia (oogonia and antheridia<br />
collectively) are often surrounded by bracteoles (that arise from the base of the gametangia)<br />
and bract cells (that arise from the branchlet node). <strong>The</strong>se can be longer (Fig. 1.5a) or shorter<br />
than the oogonium (Fig. 1.5b). A bractlet can occur at the base of the oogonium in female<br />
dioecious plants instead of an antheridium.<br />
<strong>The</strong> distinctive characteristic of the characeae thallus is that all of the parts consist (with the<br />
exception of the gametangia and nodes) of single cells or uniseriate filaments of cells. A<br />
notable characteristic of some species of characeae is the development of a coating of<br />
calcium carbonate, ‘marl’ or ‘lime’, when growing in hard-water lakes or streams. In general,<br />
species of Lamprothamnium, Tolypella, a few species of Nitella and corticated species of<br />
Chara can develop a calcareous layer on the thallus and/or a calcified coating on the oospore<br />
(called a lime-shell or gyrogonite). However, the species that do so are not used generally in<br />
physiological studies. Calcium carbonate can be precipitated in bands on the axes and<br />
branchlets of many species of characeae as a consequence of their photosynthesis and<br />
metabolism, but this is generally distinct from the thick encrustation that occurs in calcareous<br />
habitats (see section 2.4).<br />
<strong>The</strong> growing thallus of characeae plants varies in complexity and size depending on the<br />
species. Most of the species of Chara that occur in the Northern Hemisphere (Krause 1997;<br />
Han et al 1986; Scribalo 2010) have internodes covered by a cortex of smaller, linearly<br />
aligned cells (Fig. 1.2a, b). This layer of cortical cells can prevent physical access to the
5 <br />
largest cells, and for this reason corticated species of Chara are rarely used in physiological<br />
studies (Winter et al. 1987). <strong>The</strong> rows of cortical cells are often interrupted by spine cells,<br />
which can be globular (Fig. 1.2a) or long and acute (Fig. 1.2b). <strong>The</strong>y can be sufficiently<br />
abundant in some species as to make the entire thallus appear prickly. In contrast to corticated<br />
species of Chara, all members of the genera Nitella, Tolypella, Lamprothamnium and<br />
Nitellopsis and species of Chara in subgenus Charopsis are without a cortex (ecorticate), at<br />
least in part of the thallus (Fig. 1.2c). <strong>The</strong> species most commonly used in physiological<br />
studies are Chara australis, C. corallina, C. longifolia, and species of Lamprothamnium<br />
(Table 1).<br />
<strong>The</strong>re are additional species that could be used in physiological studies: Chara braunii is a<br />
widespread ecorticate species (Proctor 1980), but has an annual life history, so is not<br />
commonly maintained in laboratory culture. <strong>The</strong>re are also several short-range endemic<br />
species that have ecorticate stems (C. simplicissima, C. lucida in Australia, C. walichii in<br />
India), or ecorticate branchlets (C. baueri in Europe, C. curtissii in North America, C.<br />
gymnopitys and C. muelleri in Australia). A large number of Nitella species produce robust<br />
thalli with long internodes, but this genus is less commonly used in physiological studies.<br />
1.2 Morphology of individual species<br />
<strong>The</strong>re are six recognised extant (as opposed to fossil) genera in family <strong>Characeae</strong>: Chara and<br />
Nitella contain the majority of species in the family, Lamprothamnium, Nitellopsis and<br />
Tolypella have several species each, and Lychnothamnus has a single species, Ly. barbatus<br />
which is rare world-wide (Sugier et al. 2009). Many species could be useful for physiological<br />
studies, but the majority of studies are done on those species of Chara or Lamprothamnium<br />
that have already been cultured, or are common and easily introduced into culture in the<br />
laboratory (Table 1).<br />
<strong>The</strong> following list of species have been used in physiological studies and are characterises by<br />
their large cell size, lack of cortication on the axis or branchlets, relatively simple<br />
morphology and ease of collection or culture. As such, they do not represent the entire range<br />
of variation that exists in characeae plants, and have some very specialised characteristics.
6 <br />
1.2.1 <strong>The</strong> Chara corallina/australis group of species<br />
<strong>The</strong> species in this group have a relatively simple morphology compared to most other<br />
species of Chara (which are illustrated in Figs 1-5). <strong>The</strong> internodes and branchlets are<br />
without a cortex (ecorticate or naked), which allows direct manipulation of the giant cells that<br />
make up the internodes (Fig. 1.6a). <strong>The</strong> gametangia occur either together (oogonium above<br />
the antheridium in Chara corallina Kl. ex Willd.) or on separate plants (in the dioecious<br />
Chara australis R. Br.), so sometimes two plants (male and female) are needed for sexual<br />
reproduction to take place (Fig. 1.6b, c). <strong>The</strong> absence of a cortex coincides with a lack of<br />
spine cells. <strong>The</strong>re is one row of stipulodes beneath the branchlets (Fig. 1.6f), and this is<br />
usually poorly developed (Fig. 1.6g). Bract cells and bracteoles are reduced in comparison<br />
with many species (Fig. 1.6e). Chara corallina and C. australis were amalgamated in the past<br />
(Wood 1962; 1965; 1972), and the two species are distinguished primarily on the basis of<br />
sexuality (i.e. C. corallina is monoecious (both male and female organs occur on the one<br />
plant) and C. australis is dioecious (separate male and female plants)). Oogonia and<br />
antheridia occur on the branchlets as well as inside the base of the branchlet whorl (Fig. 1.6a,<br />
b, c).<br />
<strong>The</strong> taxonomy of this group has been problematic and over time (see section Charophyte<br />
names) and members of this group have been variously placed in the genera Tolypellopsis,<br />
Protochara, Chara and Nitella (Table 1. Wood 1972). Recent morphological (e.g. Casanova<br />
2005) and genetic analysis indicates that Chara australis from Australia and New Zealand,<br />
and Chara corallina from south-east Asia, should be retained as separate species, and that<br />
Chara lucida A. Braun (a very thin species with short internodes from tropical northern<br />
Australia) and Chara stuartiana Kütz. (a very robust species from Tasmania) can also be<br />
distinguished. <strong>The</strong> Asian species Chara wallichii A. Braun (India) and C. fulgens Filarszky<br />
(Indonesia) have also been described and these differ from C. australis and C. corallina in<br />
the arrangement of the reproductive structures and expression of bract cells (Zaneveld 1940).<br />
1.2.2 <strong>The</strong> Lamprothamnium papulosum/macropogon group of species<br />
Lamprothamium papulosum (Wallr.) J. Groves was the first species to be described in this<br />
group. It was originally placed in the genus Chara and later transferred to the genus<br />
Lychnothamnus. Later, when it was found to be different from the other species of<br />
Lychnothamnus it was placed in a genus on its own (Lamprothamnus), which was renamed
7 <br />
Lamprothamnium in 1916 to avoid a nomenclatural conflict with a flowering plant genus<br />
(Groves 1916). Other species of Lamprothamnium were added over time, so that in 2010 the<br />
genus had approximately seven recognised species. All members of the genus<br />
Lamprothamnium have ecorticate internodes and branchlets (similar to C. australis), but<br />
differ in the presence of downward pointing stipulodes and angular whorls of bract cells at<br />
the branchlet nodes (spreading, verticillate bract cells) (Fig. 1.7a). Many specimens have<br />
distinctive, spherical white bulbils at the rhizoid nodes (McNicol 1907; Ophel 1947). <strong>The</strong>se<br />
bulbils are full of starch grains, and they allow the plant to persist when it is unable to<br />
photosynthesise, and to regenerate when the vegetative axis is uprooted or destroyed. <strong>The</strong>re is<br />
a high degree of morphological similarity among species of Lamprothamnium (García and<br />
Casanova 2004), and the most recent taxonomic treatments use the arrangement of the<br />
reproductive organs as a guide for distinguishing different species (Casanova et al. 2011).<br />
Lamprothamnium papulosum has antheridia and oogonia together on the branchlet nodes,<br />
with the antheridium above the oogonium (Fig. 1.7e). <strong>The</strong>re are a number of species of<br />
Lamprothamnium in Australia, many of which were amalgamated with European L.<br />
papulosum in the past (Wood 1972), however none of them exhibit the particular<br />
arrangement of reproductive organs found in that species. <strong>The</strong> most commonly mentioned<br />
species is L. macropogon (A. Braun) I.L. Ophel. In non-reproductive plants of L.<br />
macropogon there can be a whorl of upward-pointing stipulodes within the branchlet whorl<br />
(Ophel 1947). <strong>The</strong> oogonia are clustered inside the base of the branchlet whorl, and one or<br />
two can be found on the first branchlet node (Fig. 1.7b, c). <strong>The</strong> antheridia are rarely present<br />
singly inside the branchlet whorls (jammed in with the oogonia), they are mostly confined to<br />
the first and second branchlet nodes (Fig. 1.7d). Male and female gametangia occur together<br />
(side by side) at the same branchlet node rarely. <strong>The</strong> shoots of species of Lamprothamnium<br />
often have a dense ‘fox-tail’ appearance (‘alopecuroid’) because the upper internodes are<br />
short and the branchlets, stipulodes and bract cells overlap. <strong>The</strong>re are several additional taxa<br />
of Lamprothamnium similar to L. papulosum and L. macropogon, including L. hansenii in<br />
which the branchlet segments are rather stout and constricted at the nodes (Wood 1965), L.<br />
heraldii which is dioecious (García and Casanova 2004), L. papulosum vars toletanus,<br />
carrissoi and aragonense from Spain (Cirujano et al. 2008) and L. sonderi in Germany<br />
(Schubert and Blindow 2003), but these are less frequently used for physiological studies.<br />
Additional, undescribed species of Lamprothamnium exist in Australia.
8 <br />
1.2.3 <strong>The</strong> Lamprothamnium succinctum group of species<br />
<strong>The</strong> original collection of Lamprothamnium succinctum was made in Libya, North Africa,<br />
and was described as a species of Chara (A. Braun in Ascherson 1876). It was recognised as<br />
a species of Lamprothamnium by Wood (1962). <strong>The</strong> species was confusing because of the<br />
poor expression of the vegetative characters that characterise the genus i.e. downward<br />
pointing stipulodes and verticillate bract cells, and the presence of reproductive organs at the<br />
base of the whorl which indicated a relationship with Chara corallina and C. australis (Braun<br />
and Nordstedt 1882). Wood (1965) amalgamated specimens from Mauritius and New<br />
Caledonia with the Libyan specimens, because those specimens also had reduced stipulodes<br />
and bract cells, and gametangia at the base of the whorl. Later he placed similarly<br />
depauperate species of Lamprothamnium in Australia in L. papuluosum, describing a new<br />
form, f. succintoideum (Wood 1972) for them (Fig. 1.8). However, despite records of the<br />
species in Australia (García et al. 2002) there is no evidence that L. succinctum occurs over<br />
such a wide area, from Northern Africa to islands in the Indian and the Pacific Oceans, and in<br />
southern Australia. It is more likely that there are additional species with similar<br />
characteristics. <strong>The</strong> illustrations provided of the Mauritian species in Imahori and Wood<br />
(1965) display a different arrangement of reproductive organs from the type material, so that<br />
specimen, at least, is unlikely to be conspecific with L. succinctum. In the absence of an<br />
updated taxonomy specimens of L. succinctum that have been collected from places other<br />
than the Libyan desert (Braun and Nordstedt 1882) and used in physiological studies, can be<br />
called L. sp aff. succinctum. Australian specimens in this group are generally long and<br />
slender, with few or short stipulodes below the branchlet whorls (Fig. 1.8a), and few or short<br />
bract cells (Fig. 1.8b). <strong>The</strong> fertile whorls can be contracted into the typical ‘fox-tails’ of the<br />
genus, but the antheridia occur on short stalks outside the base of the whorls, and on the<br />
branchlets (Fig. 1.8c, d), and the oogonia are clustered within the whorl of branchlets as well<br />
as on the branchlets (Fig. 1.8c, e). This taxon is particularly useful in physiological studies<br />
because of its elongated internodes.<br />
1.2.4 Lamprothamnium inflatum<br />
<strong>The</strong> ‘inflated’ charophyte Lamprothamnium inflatum (Fil. & G. O. Allen ex Fil) A. García &<br />
K.G. Karol has had a long and confused taxonomic history, largely because of its unusual<br />
morphology (Fig. 1.9). When growing in the field the young thallus appears as a (dense or<br />
sparse) mat of green bubbles erupting from the surface of the mud, the bubbles being the<br />
swollen tips of the branchlets. As plants mature the narrow axis elongates and dense whorls
9 <br />
of inflated branchlets develop, clustered towards the top of the axis. <strong>The</strong> inflated parts of the<br />
plant are the branchlet cells, and often only the second (or penultimate) cell of the branchlet<br />
(Fig. 1.9b). In comparison the axis cells are narrow and thick-walled. <strong>The</strong> plants are<br />
monoecious and the oogonia and antheridia are clustered at the bases of whorls and at the<br />
branchlet nodes (Fig. 1.9c, d). Many specimens of Lamprothamnium have inflated sterile<br />
branchlets, but L. inflatum has also inflated fertile branchlets. In common with other species<br />
of Lamprothamnium it tolerates fluctuations in the salinity of its medium, and probably<br />
doesn’t thrive in fresh water. Its natural distribution is in brackish to saline temporary<br />
wetlands in the south of Western Australia, South Australia and Kangaroo Island.<br />
1.2.5 Species of Tolypella<br />
<strong>The</strong> genus Tolypella (Fig. 1.10) appears to be restricted to latitudes greater than c. 40° in both<br />
the Northern and Southern Hemispheres (Wood 1965). <strong>The</strong> genus is characterised by its<br />
relatively disorganised thallus, with large numbers of sterile and fertile branchlets, branches<br />
and reproductive organs arising from the nodal complexes. <strong>Plant</strong>s usually consist of a long<br />
protonema or primary axis cell from which a series of complex nodes arise, each with many<br />
simple sterile branchlets, branches and reproductive organs arranged in a disorganised<br />
fashion (Fig. 1.10a). <strong>The</strong> structure of the branchlets is different from that of members of<br />
Chara, Lamprothamnium or Nitella, consisting of a primary branchlet cell, a nodal complex<br />
that produces the continuation of the primary ray, a number of secondary rays and<br />
reproductive organs (Fig. 1.10b, c). <strong>The</strong> antheridia and oogonia are often stalked (stipitate)<br />
and the oogonia have 10 coronula cells (Fig. 1.10d). In some of the species of Tolypella the<br />
coronula is deciduous (i.e. falls off at maturity) and the spiral cells become swollen (Fig.<br />
1.10e). Some species produce gyrogonites (lime-shells) around the oospores. <strong>The</strong> oospores<br />
are terete in cross-section (c.f. Nitella in which the oospores are flattened in cross-section),<br />
and often flanged on the striae.<br />
<strong>The</strong> common name for some species of Tolypella is the ‘birds nest’ or ‘tassel’ stonewort,<br />
eflecting the highly divided and disorganised nature of the thallus. Many species can tolerate<br />
salinity to varying degrees (Winter et al. 1996), but they do not grow in strictly marine<br />
systems. Although species of Tolypella occur in permanent lakes in North America, there<br />
appears to be a higher diversity of species in temporary systems, requiring disturbance or<br />
drying of their habitat for the maintenance of populations (Stewart and Church 1992).
10 <br />
1.2.6 Species of Nitellopsis<br />
Nitellopsis (Fig. 1.11) is a Northern Hemisphere genus, so far absent from Africa, Australia<br />
and South America. <strong>The</strong> genus occurs in relatively deep water in European lakes (Shubert<br />
and Blindow 2003), as well as in North America. Nitellopsis obtusa (Desv. in Lois.) J.<br />
Groves can be dominant in lakes and frequently co-occurs with the rare charophyte<br />
Lychnothamnus barbatus (Pełechaty et al. 2010). It reproduces sexually as usual, but also<br />
asexually via distinctive star-shaped bulbils (Fig. 1.11d) sometimes called starch-stars<br />
(Fritsch 1948), attached to the lower stems and rhizoids. <strong>The</strong> capacity for asexual<br />
reproduction could improve the success of long-term culturing in the laboratory. <strong>The</strong> long,<br />
ecorticate internodes and branchlets can be large enough for physiological studies (Winter<br />
and Kirst 1990; Winter et al. 1999), and it is tolerant of low levels of salinity.<br />
<strong>The</strong> plant body is relatively simple (Fig. 1.11a) with whorls of elongate branchlets and simple<br />
bract cells. Stipulodes are absent or rare at the base of the whorl, there is no cortication and<br />
few accessory structures (Fig. 1.11c). <strong>The</strong> oogonia and antheridia are on separate plants<br />
(dioecious), and the oogonia can be calcified, with the oospore surrounded by a gyrogonite<br />
(Fig. 1.11b). Oospores and gyrogonites of Nitellopsis can be among the largest of extant<br />
charophyte reproductive organs.<br />
1.2.7 Species of Nitella suitable for physiological studies<br />
Nitella species are identified on the basis of a coronula of 10 cells on the oogonium (Fig.<br />
1.12b) and furcate or forked branchlets, at least in the fertile parts (Fig. 1.12a, c). <strong>The</strong><br />
terminal branchlet segments or rays are called ‘dactyls’ and the arrangement of cells in the<br />
dactyls is used to determine the subspecies and species of Nitella (Fig. 1.4c). <strong>The</strong> species of<br />
Nitella that have been used for physiological studies in the past are generally characterised by<br />
long internodes, interrupted by few, sparsely forked whorls of branchlets. Species with short<br />
internodes and dense whorls (e.g. Fig. 1.12a) are unlikely to be useful, simply because access<br />
to their large cells would be difficult. In general species of Nitella have branchlets in whorls<br />
of 6 to 9 (Fig. 1.3c, d, Fig. 1.12c), except where there is more than one whorl of branchlets at<br />
the node (a condition termed ‘heteroclemous’ Fig. 1.3d). Branchlets can be once, twice or<br />
more times furcate (Fig. 1.4c). Fertile branchlets can be similar to the sterile ones, or<br />
contracted into ‘heads’ or ‘spikes’. <strong>The</strong>y can sometimes be covered with a jelly-like<br />
substance called ‘mucus’. <strong>The</strong> reproductive structures occur at the branchlet nodes (forks),
11 <br />
where the antheridia are terminal or central in the fork (Fig. 1.12d), and the oogonia are<br />
lateral. <strong>The</strong> taxonomy of Nitella species is gradually being sorted out by various authors, and<br />
the overall morphology as well as the characteristics of the oospore are useful clues to<br />
identity (Sakayama et al. 2002; Casanova 2009). Species with single-celled dactyls (terminal<br />
branchlet segments) (e.g. Fig. 1.12d) are in two groups; Palia and Nitella, those with<br />
pluricellulate (multicelluar) dactyls are in subgenus Hyella, and those with two-celled<br />
dactyls, of which the terminal one is small, conical and acute (bicellulate e.g. Fig. 1.12c), are<br />
in subgenus Tieffallenia. <strong>The</strong> validity of these higher taxonomic groupings is under<br />
investigation, as is the membership of those groups.<br />
1.3 Cellular structures and processes<br />
1.3.1 Cells<br />
<strong>Characeae</strong> organs (internodes, branchlet cells, stipulodes, bract cells etc.) are single cells or<br />
groups of single cells. In general they have a typical plant-cell structure with a cellulose cell<br />
wall, plasma membrane enclosing a thin layer of cytoplasm containing chloroplasts,<br />
mitochondria, nuclei, protein bodies and statoliths, surrounding a large tonoplast-bound<br />
vacuole (Fig. 1.13). At the ultrastructural level charophyte cells have characteristics in<br />
common with the cells of other algae (Zygnematophytes), mosses, liverworts and vascular<br />
plants (Casanova 2007). <strong>The</strong> plant axis is produced via sequential divisions of a dome-shaped<br />
apical cell, the products of which differentiate into discoid nodes and elongate internodes<br />
(Fritsch 1948). Axis nodal initials divide into central cells and a series of peripheral cells that<br />
differentiate into branchlet, stipulode, cortical and gametangial initials (Bharathan and<br />
Sundralingham 1984). Branchlet initials divide further into branchlet internode cells and<br />
nodal complexes (Fig. 1.13). <strong>The</strong> cortical filaments, bract cells and gametangial initials<br />
develop from the branchlet nodes.<br />
<strong>The</strong> cell wall construction in charophytes is basically fibrillar, crystaline cellulose (Casanova<br />
2007), sometimes with a superficial layer of mucilage (check Mary). <strong>The</strong> chloroplasts are<br />
lodged in the peripheral layer of the cytoplasm, close to the cell wall, and are arranged in<br />
longitudinal lines. Internode cells contain multiple copies of the nucleus formed via amitosis<br />
(Fritsch 1948). <strong>The</strong> axial node complex consists of different numbers and arrangements of<br />
cells and can be used to distinguish among genera: in Chara there are two central cells in the<br />
nodal complex, and in Nitella, Tolypella and Lamprothamnium one of the central cells<br />
undergoes further division into daughter cells (Frame and Sawa 1975).
12 <br />
<strong>The</strong> contents of the cell are in continual movement in healthy internodal cells, with active<br />
streaming clearly visible under relatively low magnification (10-20 x magnification). <strong>The</strong><br />
lines of chloroplasts in the peripheral cytoplasm are interrupted by a less-dense part of the<br />
cytoplasm, around which the cytoplasm rotates (Fritsch 1948). <strong>The</strong> vacuole that takes up the<br />
majority of the cell volume in the centre of elongate characeae cells is responsible for<br />
generating and maintaining cell turgor (the internal pressure that maintains the rigidity and<br />
shape of the cell) (Pickett-Heaps 1975).<br />
1.3.2 Gametangia<br />
<strong>The</strong> charophyte plant body is haploid (i.e. n chromosomes) and gametes (also n<br />
chromosomes) are produced via mitoitic division in the reproductive organs. <strong>Characeae</strong> male<br />
gametes consist of motile sperm cells (n chromosomes) (Fig. 1.14a) that swim through the<br />
water to fertilise the female egg cell (n chromosomes). Specialised, diploid (2n), seed-like<br />
oospores (Fig. 1.1c) are the product of fertilisation of the egg cell by a sperm cell. <strong>Characeae</strong><br />
gametangia, or reproductive organs, have a distinctive morphology (Fig. 1.5). Unlike other<br />
algae, the gamete-producing cells (spermatogenous threads (Fig. 1.14b) and egg cell) are<br />
surrounded by sterile cells that protect them. <strong>The</strong>se organs are variously called (for male<br />
organs) the globule or antheridium; and (for female organs) the nuclule, oogonium or<br />
oosporangium. Development of the oogonium progresses from a single peripheral cell on the<br />
branchlet node, producing the five spiral cells and an oosphere mother cell. <strong>The</strong> spiral cells<br />
elongate around the mother cell which divides to produce the oosphere or egg cell (Leitch et<br />
al. 1990). <strong>The</strong> oogonium has one or more basal cells. <strong>The</strong> spiral cells divide to produce a<br />
crown-like structure (coronula) of either five cells (in Chara, Lamprothamnium, Nitellopsis<br />
and Lychnothamnus) or ten cells (in Nitella and Tolypella). Antheridial development<br />
similarly proceeds from a peripheral cell (Pickett-Heaps 1968), and the mature antheridium<br />
consists of a mass of spermatogenous filaments or threads attached to manubria cells via<br />
capitula cells, surrounded by (usually) eight shield cells (Fig. 1.14b). When the male gametes<br />
(spermatids (Pickett-Heaps 1968b): Fig. 1.14a) are mature they exit the spermatogenous<br />
threads via pores (Fritsch 1948), and the shield cells separate slightly to release them,<br />
eventually falling apart altogether (Fig. 1.14c). When the egg cell is ready to be fertilised the<br />
cells below the coronula become swollen and gaps are formed between the spiral cells<br />
allowing entry of the sperm cells. After fertilisation the egg cell becomes the diploid zygote<br />
(i.e. 2n chromosomes) and a complex, multilayered wall is formed by both the developing
13 <br />
zygote and the maternal spiral cells (Leitch et al. 1990). This specialised zygote is called the<br />
oospore and its multilayered wall allows it to resist desiccation and destruction during<br />
dispersal and dormancy.<br />
1.3.3 <strong>The</strong> oospore<br />
<strong>Characeae</strong> oospores are distinctive and easy to recognise (Fig. 1.1c). In all the genera except<br />
Nitella there is sometimes the development of a gyrogonite, or calcified ‘shell’ around the<br />
oospore, deposited during the last stages of development on the parent plant (Fig. 1.15a).<br />
Gyrogonites can gradually decompose over time, or break open when the oospore germinates.<br />
<strong>The</strong> oospore wall, within the gyrogonite, is formed by both the egg cell (internal layers) and<br />
the oogonium cells (external layers), and is characterised by spiral sutures or lines on its<br />
surface (Leitch 1989). <strong>The</strong> flat part of the oospore wall is called the fossa, and the sutures are<br />
called the striae. Sometimes the striae have flanges (where the oospore wall is formed on the<br />
side walls of the spiral cells; Fig. 1.15b), and sometimes a thinner structure called a ‘ribbon’<br />
(John et al. 1989) or thicker structures (pachygyra; Fig. 1.15c) are formed. <strong>The</strong> fossa can be<br />
variously ornamented with grains (typically so for Lamprothamnium) or fibres. <strong>The</strong><br />
ornamentation becomes amazingly diverse in the genus Nitella, with verrucae, tuberculae,<br />
reticulae and every combination of wall construction imaginable (Fig. 1.15d, e, f) (John and<br />
Moore 1987). <strong>The</strong> base of the oospore is impressed with the shape and arrangement of the<br />
basal cell complex. In Chara there is a single basal cell (Fig. 1.15g), in other genera there can<br />
be more (Fig. 1.15h). <strong>The</strong> apex of the oospore exhibits a ‘w-shaped’ suture where the five<br />
spiral cells join up (Fig. 1.15i). Oospores can be different colours, ranging from pale yellow<br />
to black. In some Nitella species oospore colour is an indication of the maturity of the<br />
oospore, and degree of development of the oospore wall (Casanova 1991). Sometimes<br />
oospore colour is useful for distinguishing species of Chara (Zaneveld 1940). Oospores are<br />
densely packed with starch grains (amyoplasts), the primary reserve for the germinating<br />
plant. Healthy oospores are turgid and resistant to external forces. <strong>The</strong>y can be handled easily<br />
with fine forceps. <strong>The</strong> distinctive differences among charophyte oospores allows them to be<br />
used in identification of different charophyte species (Haas 1994, Sakayama et al. 2002,<br />
Casanova 2005, Casanova 2009). Cultures of charophytes for physiological studies can be<br />
started by collecting mature oospores, sterilising their surface and germinating them under<br />
controlled or known conditions (see section 1.5.5).
14 <br />
1.3.4 <strong>The</strong> germinating oospore<br />
At some time between fertilisation of the egg cell on the parent plant, and germination of the<br />
oospore, reduction division (meiosis) occurs, resulting in the regeneration of haploid (n)<br />
thallus cells (Fritsch 1948). Despite some thorough studies (e.g. Leach 1989) the exact timing<br />
of meiosis has not yet been discovered. <strong>The</strong> start of the germination process sees the oospore<br />
wall split along the ‘w-shaped’ suture at the apex. <strong>The</strong> first two cells of the germinating<br />
charophyte develop into the protonema (first thread) and the rhizoid initial (Ross 1959). Axes<br />
and whorls of branchlets develop from the first node of the protonema (Fritsch 1948; Ross<br />
1959). <strong>The</strong> rhizoids are positively geotropic (Kiss and Staehelin 1993) and grow into the<br />
substrate. <strong>The</strong> protonema is negatively geotropic and positively phototropic (Fritsch 1948)<br />
and grows up through the soil or substrate, often from depths of 25 mm or more,<br />
exceptionally from depths of at least 100 mm (Dugdale et al. 2001). Germination rates can be<br />
enhanced by light of certain wavelengths (Sokol and Stross 1992), although germination is<br />
possible in the absence of exposure to light (de Winton et al. 2004). However, successful<br />
establishment of the young plant appears to be light dependent when the oospore starch<br />
reserves are exhausted (de Winton et al. 2004).<br />
1.4 <strong>Characeae</strong> names<br />
<strong>Characeae</strong> are commonly called ‘stoneworts’ because of the deposition of calcium carbonate<br />
on the axes and branchlets, especially in lime-rich habitats. However, many species,<br />
particularly in the genus Nitella, never accumulate calcium carbonate, and the name is quite<br />
unsuitable for these often delicate, mucus-covered, flexible species. Occasionally characeae<br />
are referred to as ‘musk-grass’, due to the strong, pungent, sometimes rank smell associated<br />
with certain corticated species (Anthoni et al. 1980). <strong>The</strong> smell can be so strong and<br />
distinctive that a single shoot can be detected in a handful of other vegetation. However, not<br />
all species produce the odour, so the term ‘musk-grass’ is inappropriate in many cases. For<br />
this reason, in this text, we use the term ‘characeae’ as a name for the group. This, of course,<br />
does not replace the need for recognised scientific binomial names. <strong>Characeae</strong> taxonomy and<br />
the application of binomials has been difficult for non-specialists, and only slightly easier for<br />
specialists. <strong>The</strong> application of genetic analytical techniques has helped to solve many<br />
problems.
15 <br />
1.4.1 Systematics and the concept of species<br />
<strong>The</strong> superficial resemblance of characeae to flowering plants has led, in the past, to confusion<br />
about their place in botanical systematics (Fritsch 1948). Early botanists discussed whether<br />
they were the simplest of flowering plants or the most complicated of algae. Recent<br />
phylogenetic studies (Karol et al. 2000) have shown that they are the closest living relatives<br />
of the ancestors of land plants. <strong>The</strong>y have evolved an analogous morphology to aquatic<br />
angiosperms, in the same environment. <strong>The</strong> common and ancient ancestry of characeae and<br />
land plants means that many of their cellular processes and mechanisms are the same as those<br />
of land plants, a fact that makes the study of characeae physiology directly relevant to the<br />
physiology of land plants, including plants and processes of economic interest.<br />
<strong>Characeae</strong> have been recognised as fossils in sediments dating as far back as the Silurian<br />
(Grambast 1974) and have been through various changes in diversity and abundance over the<br />
millennia. Living characeae were among the species first described in Linnaeus’ Species<br />
<strong>Plant</strong>arum in which he sought to develop a consistent taxonomy. <strong>Characeae</strong> were not<br />
recognised as being substantially different from other plants, and most workers at that time<br />
saw them as a watery Equisetum or horsetail. It was later that their unique differences were<br />
recognised, and they were set apart in Division Charophyta.<br />
Although a number of characeae species are regularly used in physiological studies,<br />
physiologists seeking to have their plant identified to species can obtain erroneous or<br />
conflicting answers. <strong>The</strong>re are a number of reasons for this. Firstly characeae morphology is<br />
affected by the environment for growth, and there is variation in plant size and appearance<br />
depending on the light availability and water chemistry of the habitat. Secondly, although<br />
characeae have been well known for a long time, the taxonomy of characeae is undergoing a<br />
revolution, based on recent advances in the understanding of variation and speciation in the<br />
group and genetic data about relationships among species and species groups.<br />
<strong>The</strong> taxon level called ‘species’ is not the same in all groups of organisms. In birds and<br />
mammals relatively small genetic differences are present between ‘species’. In plants there<br />
are often larger genetic differences. In algae the genetic distances among ‘species’ can be<br />
larger still. <strong>The</strong> detection of characteristics that allow us to reliably discriminate among<br />
different taxa are often based on historical precedent or subjective assessment (i.e. what the<br />
first taxonomist used) rather than objectivity or experimentation (Proctor 1980), and linking
16 <br />
what has been described as a species with a genetically discrete ‘clade’ or group of<br />
individuals is one of the frontiers of taxonomy today. Traditionally characeae species have<br />
been organised into groups within a genus (subgenera, sections, subsections) and ‘complexes’<br />
of closely related species have been recognised. A charophyte ‘species complex’ is a group of<br />
morphologically similar, closely related taxa that have, historically, had the morphological<br />
extremes described as different species by some authors (e.g. Braun and Nordstedt 1882), and<br />
described as different varieties or forms by other authors (Wood 1965; Boegle et al. 2007;<br />
Blume et al. 2009). Examples of these are the Nitella hookeri complex of Australia and New<br />
Zealand (Casanova et al. 2007) and the Chara baltica complex of central Europe and the<br />
Baltic Sea (Boegle et al. 2007; Blume et al. 2009).<br />
Historically the first comprehensive taxonomic treatment on characeae was by Braun and<br />
Nordstedt (1882), and was based on the specimens from around the world available to those<br />
experts at the time. <strong>The</strong>y distinguished between the genera Chara and Nitella, but the other<br />
genera were not delineated and named until later in the 19 th (Tolypella, Lychnothamnus,<br />
Nitellopsis) or 20 th centuries (Lamprothamnium). Researchers in the past often saw few<br />
specimens of each species, so the conclusion that many characeae ‘species’ had a broad range<br />
of characteristics was a natural conclusion. Subsequent taxonomists used Braun and<br />
Nordstedt (1882) as a basis for their taxonomy (Zaneveld 1940, Wood 1962), but it was not<br />
until 1965 that a world-wide treatment of family <strong>Characeae</strong> was attempted. Wood (1965)<br />
tried to rationalise the species concept in <strong>Characeae</strong>, amalgamating monoecious and<br />
dioecious species and grouping morphologically similar specimens from around the world.<br />
His work resulted in a reduction of the 350 recognised species to just 18 species of Chara,<br />
three of Lamprothamnium, 90 of Nitella, three of Nitellopsis and two of Tolypella.<br />
Subsequent research has shown Wood’s species concept to be erroneous, and rather than<br />
there being c. 180 species in family <strong>Characeae</strong>, the older treatments (c. 380 species) give a<br />
better representation of the true diversity. This has consequences for physiologists seeking to<br />
identify charophytes. Firstly Wood’s work resulted in confusion rather than consensus among<br />
taxonomists. Long-held species concepts had been demolished and in many places in the<br />
English-speaking world Wood’s treatment was accepted without challenge. Few people using<br />
Wood’s taxonomy were able to reliably identify characeae to species. This was especially<br />
true for some of the species most frequently used by physiologists: Chara<br />
australis/corallina/inflata, Lamprothamnium macropogon/paulosum/succinctum. With the<br />
use of molecular taxonomy (e.g. McCourt et al. 1996) and an experimental approach to the
17 <br />
definition of species (Proctor 1975), taxonomists in the 21 st century can provide a better<br />
taxonomy.<br />
<strong>The</strong> use and citation of designated clones in physiological studies is to be encouraged, given<br />
the past taxonomic difficulties with the group. Specimens used for physiological studies<br />
should also be vouchered (preserved by pressing or in 70% alcohol, and lodged in an<br />
internationally recognised herbarium) as a standard practice. Misidentification of taxa in the<br />
past (at both species and genus level) could go some way towards explaining difficulties in<br />
generalisation about characeae physiological processes.<br />
1.4.2 Individual species’ stories<br />
1.4.2.1 Chara australis R. Br.<br />
One of the most popular species used in physiological studies is the dioecious species Chara<br />
australis. <strong>The</strong> species was collected first from the vicinity of Sydney in New South Wales,<br />
Australia, by Robert Brown, a botanist on Matthew Flinders’ exploratory expedition to<br />
circumnavigate Australia in 1802-03. It was one of only two species in family <strong>Characeae</strong><br />
described for Australia in the early 19 th century (Brown 1810). At least two collections in the<br />
1800s were named Chara australis (which means ‘southern Chara’), but the one named by<br />
Brown has priority, so it is the one that retains the name. Later, Alexander Braun (1843)<br />
described a similar specimen as Chara plebeja, which he later amalgamated with C. australis<br />
as a subspecies (Braun and Nordstedt 1882). In the same publication (Braun and Nordstedt<br />
1882) Braun was inclined to distinguish more varieties and forms of Chara australis, so he<br />
presented a descriptive list including C. australis var. nobilis form stuatiana, var. lucida form<br />
tenerior, var. vieillardii and forms vitiensis and simplicissima as well as subspecies plebeja.<br />
<strong>The</strong> group was reviewed by Zaneveld (1940) who had the advantage of seeing more material<br />
from a diversity of places, but he followed Braun’s taxonomy closely. In the 1960s R.D.<br />
Wood amalgamated the entire complex with monoecious Chara corallina Kl. ex Willd. from<br />
south-east Asia (Wood 1962), on the basis of its morphological similarities. Wood (1965)<br />
retained many of the subdivisions recognised by Braun and Zaneveld, but this time within the<br />
species Chara corallina. Thus since the 1960s we have had Chara corallina with three<br />
varieties and eight forms, at least half of which were dioecious, the other half monoecious.<br />
Proctors work (summarised in Proctor 1975) established that monoecious Chara corallina<br />
and dioecious Chara australis were different species, but the status of many of the other
18 <br />
entities within the complex has not been fully examined. We can be sure, at least, that<br />
dioecious specimens that originated in eastern Australia and have the morphology illustrated<br />
in Fig. 1.5, can be called Chara australis, and that monoecious specimens that have a similar<br />
morphology are likely to be Chara corallina.<br />
1.4.2.2 Lamprothamnium inflatum (Fil. & G.O. Allen ex Fil.) A. García & K.G. Karol<br />
This curious species (Fig. 1.8) has been well known, and misnamed for a long time. <strong>The</strong> first<br />
collection of this species was made by Nancy Burbidge, the eminent Australian taxonomist,<br />
in 1933, while swimming in Lake Parkeyerring near Wagin in Western Australia. It is an<br />
attractive plant when actively growing, looking like small bunches of glistening green grapes.<br />
<strong>The</strong> preserved specimens were sent to G.O. Allen (in England) who was the world expert on<br />
charophytes in the English-speaking world at the time. He sent a subsample to F. Filarszky in<br />
Hungary, who described the species as Nitellopsis inflata. In recognition of the role that Allen<br />
had in the discovery, Filarszky included Allen in the authorship of the species, although Allen<br />
was not consulted about the determination (Wood 1964). Next, this odd species was included<br />
in the genus Protochara by South Australian phycologists Womersley and Ophel (1940)<br />
although they had not examined any specimens (Womersley and Ophel 1940). When<br />
Macdonald and Hotchkiss (1956) investigated the genus Protochara (specifically Protochara<br />
australis) their results indicated that the genus should be amalgamated with Chara, so<br />
Protochara inflata became Chara inflata (Fil. & G. O. Allen ex Fil) Macdon. & Hotch.,<br />
again, without a thorough examination of any specimens of the species. <strong>The</strong>n, in a revision of<br />
the Australian charophyte flora Wood (1962, 1965, 1972) reduced the species to a form of<br />
Chara corallina (C. corallina var. nobilis f. inflata) despite the fact that it was monoecious<br />
and all other Australian specimens in that group were dioecious. Thus the species was<br />
allocated to three different genera. However, recent evidence places the species firmly within<br />
the genus Lamprothamnium (García and Karol 2004). This placement is supported by<br />
morphological characteristics such as the presence of gyrogonites (calcium carbonate ‘shells’<br />
around the oospore), contracted apical whorls and occasional downward-pointing stipulodes<br />
(Fig. 1.8).<br />
1.4.2.3 Chara longifolia C.B. Robinson<br />
This enigmatic species has been described as three different species, in two genera in the<br />
past: Chara longifolia (which is the currently accepted name), C. buckellii and Nitellopsis
19 <br />
bulbilifera. <strong>The</strong> natural range of the species is in North, South and Central America in<br />
brackish habitats (Mann et al. 1999). Chara longifolia was originally collected from Kansas,<br />
USA by M.A. Carleton in 1891 (Robinson 1906). <strong>The</strong> species was dioecious, had an<br />
irregularly corticated axis, poor development of stipulodes (absent in older nodes) with c. 8<br />
ecorticate branchlets in each whorl with distinctive decumbent bract cells. Robinson<br />
commented on the similarity of this species with Nitellopsis, but described it as a new species<br />
of Chara (Robinson 1906). <strong>The</strong> species was known only from its type collection (Wood<br />
1965). In 1928 Dr Edward Buckell, a friend of the British charophyte specialist James<br />
Groves, collected another specimen from British Colombia, Canada which was described by<br />
Allen (1951) as Chara buckellii. Again, the specimens had irregularly or incompletely<br />
corticated axes, with ecorticate branchlets and posterior bract cells distinctively deflected<br />
downwards. Again, this species was known only from the type locality (Allen 1951; Wood<br />
1965). In 1959 a specimen was cultured from soil collected from Laguna La Brava near<br />
Buenos Aires, Argentina, by Carlotta Carl de Donterberg, who described it as Nitellopsis<br />
bulbilifera (Carl de Donterberg 1960). Tindall et al. (1965) reported N. bulbilifera (cited as N.<br />
bulbillifera) from New Mexico, and illustrated the morphology of the thallus. Although<br />
Tindall et al. (1965) received Chara buckellii from V.W. Proctor in Texas for examination at<br />
the time of their study of N. bulbilifera, they concluded that despite the common<br />
characteristics with C. buckellii (dioecious nature, and similarity of morphology of the<br />
branchlets and bract cells), the presence of a corticated stem and stipulodes in C. buckellii<br />
was sufficient to distinguish the two taxa. Daily (1967) also examined the taxon, and<br />
transferred all the previously described species to the genus Lamprothamnium, but this<br />
taxonomic change was very short-lived, as in the next article in the same issue of J. Phycol.<br />
Proctor et al. (1967) presented evidence that all of the species (Chara longifolia, C. buckellii<br />
and N. bulbilifera) were conspecific. <strong>The</strong> evidence consisted of 1) similar morphology, 2) the<br />
same number of chromosomes, and 3) the production of viable oospores when crossed. This<br />
finding, and the designation to the genus Chara, has been further supported by genetic studies<br />
(Meiers et al. 1999). Since all these taxa are the same species of Chara, the oldest name has<br />
priority, thus the species is Chara longifolia C. B. Robinson.<br />
1.4.3 Lessons to be learned<br />
<strong>The</strong> morphology of ecorticate charophytes is relatively simple, and there are few outstanding<br />
characters with which to distinguish species. Those that are available (arrangement and<br />
angles of stipulodes and bract cells, arrangement of gametangia) are not always obvious or
20 <br />
expressed on plants in culture. Added to this is the fact that there is more species-level<br />
diversity among ecorticate characeae than has been recognised in the past. <strong>The</strong> only way to<br />
be sure that the entity used in experimentation is the same as that used by someone else is to<br />
use the same culture. However, approaches to taxonomy are improving all the time, and<br />
chances of correct identification are increasing with every new taxonomic work undertaken,<br />
especially with the establishment of gene sequencing approaches, DNA fingerprinting and<br />
DNA bar-coding. Isolation of new culture material from wild populations is likely to have<br />
significant benefits for physiological studies, and could produce novel insights in relation to<br />
old problems. In all cases preservation of reference voucher material in a recognised<br />
herbarium is vital so that the species used can be reliably identified in the future, even if the<br />
taxonomy changes. It should be standard protocol for the herbarium material to be referred to<br />
in the description of methods for physiological studies. This can contribute to making the<br />
research timeless and applicable into the future.<br />
1.5 Ecology<br />
1.5.1 Distribution and abundance<br />
<strong>Characeae</strong> occur in non-marine water bodies of all kinds, on every continent except<br />
Antarctica. <strong>The</strong> only occurrence of characeae in marine waters is in estuaries and saltmarshes<br />
associated with marine influence, and in the Baltic Sea, where they are restricted to<br />
areas of relatively low salinity, or sheltered bays and inlets (Blindow 2000; Appelgren and<br />
Matilla 2005). <strong>The</strong>y can grow in water as shallow as a few centimetres deep, or down to c. 40<br />
m at the limits of plant growth under water (Casanova et al. 2007). <strong>The</strong>y grow in both<br />
temporary and permanent water bodies (Casanova 1993), lakes, rivers (Bornette and Arens<br />
2002), swamps, seeps, bogs, fens, marshes, creeks, lagoons, clay-pans, wadis, water-holes<br />
and billabongs (Casanova and Brock 1999b). Most species prefer water with little current or<br />
wave action, but some can grow in relatively fast-flowing water, water that experiences<br />
waves, as well as in the backwaters and shallows of rivers and streams (Bornette and Arens<br />
2002). <strong>Characeae</strong> populations can occur as dense monospecific ‘beds’ across the bottom of<br />
lakes and wetlands, or as isolated single plants, restricted to a particular depth or soil type.<br />
Early studies into characeae distribution distinguished Chara-lakes (shallow, calciumcarbonate-rich<br />
lakes) from other kinds of lakes (e.g. John et al. 1982) and there was<br />
recognition of the value of temporary wetlands as characeae habitat (e.g. Corillion 1957).<br />
Other historical and current studies of characeae ecology have focused on their distribution in<br />
the landscape (García and Chivas 2007). <strong>The</strong> two questions ‘what species occur?’ and ‘where
21 <br />
do they occur?’ are central to such studies and have been vital in the formulation of<br />
hypotheses concerning characeae distribution and abundance. In other studies ecological<br />
experiments have been undertaken, in order to understand the mechanisms (i.e. ‘how?’) and<br />
reasons (i.e. ‘why?’) for the diversity and distribution of characeae that we see (Blindow et al.<br />
2002).<br />
1.5.2 Processes controlling characeae distribution<br />
Many characeae species develop a long-lived bank of oospores in the soil of permanent and<br />
temporary wetlands and riparian zones (Casanova and Brock 1990). <strong>The</strong> density of oospores<br />
in this ‘seed’ bank is dependent on 1) the longevity of oospores; 2) the rate of oospore<br />
addition to the bank; 3) the loss from the seed bank via germination of new plants, death or<br />
dispersal (via invertebrate ingestion, or dispersal of the substrate (mud) attached to animals)<br />
(Fig. 1.16). Oospore density can vary from just a few oospores/m 2 (Casanova and Brock<br />
1990) to hundreds of thousands of oospores/m 2 or per litre of surface soil (van den Berg<br />
1999; Porter 2007). <strong>The</strong> presence and density of oospores in the seed bank depends on<br />
oospore production by adult plants, and in most systems there is likely to be a mass<br />
deposition of oospores of a single age in the late summer and autumn, or as temporary water<br />
bodies dry up, as this is the time when the oospores of most species mature (Casanova and<br />
Brock 1990; Casanova 1994; Casanova and Brock 1999a). In permanent water bodies there<br />
can be immediate germination of fresh oospores, although many oospores are dormant upon<br />
release (Casanova and Brock 1997). Oospores can remain viable in seed banks for years<br />
(Casanova and Brock 1997) and even decades (Simons et al. 1994). <strong>The</strong>re is one record of<br />
oospores germinating from sediment estimated to be at least 45 years old (Rodrigo et al.<br />
2010). Oospore viability and subsequent germination generally decreases over time<br />
(Casanova and Brock 1994), as even very low respiration rates can exhaust the oospore starch<br />
reserves over time, and there is likely to be loss to bacterial infection of oospores as well as<br />
direct herbivory. Germination followed by failure to establish is also a mechanism by which<br />
oospore banks can be depleted (de Winton et al. 2004). As a result of deposition of oospores<br />
over many seasons and the longevity of oospores, oospore banks can contain oospores of<br />
various ages, accumulated over time. Germination from the seed bank, and the differential<br />
germination requirements of characeae species can partly explain characeae distribution in<br />
the landscape (Casanova and Brock 1996).
22 <br />
Dispersal of characeae into new habitats occurs most commonly via water-fowl (Proctor<br />
1962; Bonis and Grillas 2002; Casanova and Nicol 2009), and in riparian systems oospores<br />
are probably transported downstream with sediment by flow (Bornette and Arens 2002).<br />
Although characeae occur in a diversity of systems, they are not always a dominant or<br />
permanent component of the submerged vegetation. Classical succession theory suggests that<br />
when characeae are early or ‘pioneer’ components of aquatic vegetation (Stewart and Church<br />
1992), they are replaced by angiosperms and phytoplankton as nutrient concentrations and<br />
turbidity increase (Crawford 1981; Blindow 1992). Thus characeae can be rare in eutrophic<br />
or highly modified systems (Casanova and Brock 1999b), despite the fact that they are<br />
capable of tolerating very low light conditions (de Winton et al. 2004). <strong>The</strong> alternative stable<br />
states hypothesis (Sheffer et al. 1993 van den Burg et al. 2001) postulates that characeae<br />
(along with other submerged vegetation) produce a stable, macrophyte-dominated state at a<br />
variety of nutrient levels, a condition that can be restored via turbidity reduction. An<br />
appropriate disturbance regime is likely to be vital in maintaining characeae populations and<br />
diversity in ecosystems. Such disturbance can be imposed by water-bird herbivory<br />
(Noordhuis et al. 2001), or by drawdown and drying (Casanova and Brock 1991). Too much<br />
disturbance can reduce or prevent characeae establishment (de Winton et al. 2002). In general<br />
we find that characeae can be permanent components of the vegetation in a number of<br />
situations, and can dominate the submerged flora of calcareous lakes (Forsberg 1960), saline<br />
lakes (Porter 2007) and temporary wetlands (Casanova 2003). <strong>The</strong>y are found more rarely in<br />
farm ponds, although oospores can be present in a majority of farm pond seed banks<br />
(Casanova and Brock 1999b).<br />
1.5.3 Ecological role of characeae<br />
<strong>Characeae</strong> undertake many ecosystem services including provision of resources to other<br />
organisms and maintenance of processes that enhance biodiversity. Charophytes provide<br />
habitat and shelter for algal epiphytes, invertebrates and fish. <strong>The</strong> density and diversity of<br />
invertebrates associated with characeae populations can be greater than those associated with<br />
other macrophytes (Kingsford and Porter 1994, Kuczyńska-Kippen 2007). In Great Lake,<br />
Tasmania, characeae are thought to provide resources for a number of rare and endangered<br />
invertebrate species (Davies 2001) and most wild-collected characeae thalli have a diversity
23 <br />
of animal and algal epiphytes (Kairesalo et al. 1987). In general, characeae are thought to be<br />
indicators of healthy, clear-water ecosystems (Krause 1981).<br />
<strong>Characeae</strong> provide food, both directly for invertebrates (Proctor 1999) and vertebrates<br />
(Noordhuis et al. 2002), and as particulate organic matter (Pereya-Ramos 1981). <strong>The</strong>y<br />
remove nutrients from the water column and stabilise sediment (Kufel and Kufel 2002),<br />
reducing turbidity and nutrient release from the sediment. <strong>The</strong> presence of characeae is<br />
correlated with high species diversity in phytoplankton communities (Casanova and Brock<br />
1999b) and when characeae are removed experimentally phytoplankton (particularly bluegreen<br />
algae) concentrations have been shown to increase (Villena and Romo 2007),<br />
coincident with increased nutrient concentrations and sediment resuspension. Where dense<br />
characeae beds exist they can be responsible for patches of clear water through their capacity<br />
to dampen water movement and allow sediment to settle (Scheffer et al. 1994). <strong>The</strong>re is<br />
thought to be a positive feed-back mechanism operating between the development of<br />
characeae populations in shallow lakes and the maintenance of the clear water conditions<br />
they need to thrive (Fig. 1.17). <strong>The</strong> thalli also provide habitat and refuge for cladocerans<br />
(water-fleas) that consume phytoplankton, thus reducing the biological turbidity. Both these<br />
processes maintain conditions conducive to charophyte growth (clear water and low watercolumn<br />
nutrient concentrations). When charophyte establishment is inhibited the opposite<br />
occurs, leading to high turbidity, high water-column nutrient concentrations and a stable,<br />
phytoplankton-dominated state. Studies have shown a statistically significant relationship<br />
between charophyte abundance and a reduction in wind-driven resuspension (Hamilton and<br />
Mitchell 1996), and a high correlation between characeae abundance and water clarity<br />
(Blindow et al. 2002), but there is still discussion about the stability and mechanisms by<br />
which the different states are achieved (Blindow et al. 2002). <strong>The</strong> capacity of charophytes to<br />
modify community dynamics through allelopathy (production of substances that adversely<br />
affect the growth of other organisms) has also been under investigation (Wium-Andersen et<br />
al. 1982). <strong>The</strong>re is general consensus, however, that characeae populations are important in a<br />
variety of ecosystem processes in shallow lakes.<br />
1.5.4 Abiotic environmental factors<br />
1.5.4.1 Light<br />
Light intensity affects the morphology of the characeae thallus and the distribution of<br />
characeae species within lakes (Schwarz et al. 1996). Different species exhibit different
24 <br />
degrees of acclimation to low or high light intensities (Wang et al. 2008). In general, high<br />
light intensities (that can be experienced in very shallow water, or very clear water) lead to a<br />
contracted thallus with short internodes (Wang et al. 2008), whereas low light intensities (in<br />
deep water or shaded situations) produce a more diffuse plant (Wang et al. 2008). <strong>Characeae</strong><br />
also respond to light intensity by modifying their pigment ratios (Wang et al. 2008) and in the<br />
timing of reproduction (Wang et al. 2008). Exposure to light, and certain wavelengths of light<br />
can also be important for stimulating germination of oospores (Sokol and Stross 1992),<br />
although there is experimental evidence that although germination can take place in darkness,<br />
light is required for establishment of the young thallus (de Winton et al. 2004).<br />
1.5.4.2 Water chemistry and physical properties<br />
<strong>The</strong> physical properties pH and conductivity (as an analogue for salinity or total dissolved<br />
solids) are relatively easy to measure and have frequently been reported for charophyte<br />
habitats, and it is common for tables of species to be listed in relation to these factors<br />
(Corillion 1957, García and Chivas 2007). In general the most common species of<br />
charophytes tolerate a range of pH values, with species of Chara preferring slightly alkaline<br />
pH and species of Nitella preferring slightly acid to circum-neutral pH. <strong>The</strong>re are a few<br />
species that are found only in water with specific pH, but this is the exception rather than the<br />
rule. To date there are no reports of a mechanism by which pH might select for, or influence<br />
the distribution of characeae species. Salinity, on the other hand, imposes specific constraints<br />
on the physiology and osmotic processes in charophyte cells. Several species of Chara and<br />
Nitella can be found in, and appear to prefer slightly saline conditions. However, species in<br />
the genera Lamprothamnium and Tolypella are frequently found in water of varying salinity,<br />
and Lamprothamnium species can be the only plants occurring in waters with high salinity<br />
(Brock and Lane 1980).<br />
1.5.4.3 Nutrients<br />
<strong>Characeae</strong> can take up nutrients through the above-ground thallus as well as through the<br />
rhizoids (Littlefield and Forsberg 1965; Andrews 1987; Vermeer et al. 2003). Nitrogen is<br />
preferentially accumulated as ammonium, and it can be circulated throughout the plant body<br />
via cytoplasmic streaming (Vermeer et al 2003). In field studies characeae can have a major<br />
impact on the nitrogen budget of lakes (Rodrigo and Alonso-Guillén 2008), responsible for<br />
12 to 95 % of the total nitrogen content of the system. <strong>Characeae</strong> can also accumulate<br />
phosphorus (Kufel and Ozimek 1991). Mean daily rates of phosphate uptake can exceed 27<br />
nmol g -1 fresh weight h -1 (Andrews 1987).
25 <br />
1.5.4.4 Temperature<br />
Storage temperature can affect germination of characeae (Casanova and Brock 1996), so that<br />
winter germinating species will only germinate after they have been exposed to cues to the<br />
onset of winter. <strong>The</strong> rate at which characeae germinate can be related to the ‘day-degrees’<br />
they have experienced (Casanova 1994). Some species occur only in tropical regions, others<br />
have a temperate distribution (Casanova 1993) and many populations experience seasonal<br />
changes in biomass (Fernández-Aláez et al.2002).<br />
1.5.4.6 Depth<br />
<strong>Characeae</strong> are often the deepest inhabitants of lakes (Dale 1986), and there can be a high<br />
diversity of species in individual lakes, zoned in relation to depth (de Winton et al. 1991).<br />
<strong>The</strong>y have been recorded to 30 m in USA (Stross et al. 1995), 15m depth in New Zealand (de<br />
Winton et al. 1991; Casanova et al. 2007) and 12m depth in Tasmanian lakes (T. Dugdale,<br />
personal communication). Classical studies relating characeae distribution to depth have<br />
generally held light attenuation to be responsible for the determining the lower depth limits of<br />
characeae communities (Wood 1950; Spence 1982; Andrews et al. 1984; Vant et al. 1986).<br />
Some species exhibit definite biomass maxima at certain depths (de Winton et al. 1991;<br />
Asaeda et al. 2007), and allocate more biomass to sexual reproduction at shallower depths<br />
than in deeper water (Casanova 1994; Asaeda et al 2007). Depth can influence allocation of<br />
resources to, and therefore the size and morphology of oospores (Boszke and Bociag 2008).<br />
Depth, along with light availability, can affect the development of internodes and shoot<br />
lengths, with thalli collected from deeper water exhibiting elongated internodes and<br />
branchlets compared to thalli collected from shallower parts of the same water bodies<br />
(Asaeda et al. 2007).<br />
1.5.5 Culturing characeae<br />
In many cases characeae can be harvested from the field and introduced into informal culture<br />
with very little effort. Shoots, along with water from their habitat, can be placed in a vessel,<br />
and given adequate light (e.g. on a window-sill; Karling 1924; Casanova and Brock 1996),<br />
will persist, and sometimes grow, for weeks, if not months. Simple aeration of the water<br />
through a bubbler can prevent the proliferation of bacteria and production of anoxic<br />
conditions, a particular problem for species from brackish and saline water-bodies. This<br />
method can provide simple, if undefined, cultures of characeae for experimental purposes.
26 <br />
Some species produce vegetative reproductive organs that can be used to initiate cultures<br />
(e.g. starch-filled branchlet whorls: Starling et al. 1974; bulbils: Fritsch 1948).<br />
Another very common method of developing long-lived laboratory cultures of characeae is to<br />
collect the sediment or soil from under characeae stands, place it in a tank or culture vessel<br />
and flood it with rainwater or distilled water, allowing characeae to germinate under<br />
controlled light and temperature regimes. Drying the sediment prior to flooding can enhance<br />
germination and establishment of characeae (Casanova and Brock 1990, Casanova and Brock<br />
1999a), even when drying is not a common event in the natural habitat (de Winton et al.<br />
2004).<br />
Unialgal, axenic or sterile cultures can be attempted, but success is by no means certain.<br />
Surface sterilised oospores have been used to initiate cultures (Imahori and Iwasa 1965).<br />
Artificial culture conditions that have been used in the past include simple distilled water<br />
with no physical substrate (Ross 1959), soil-water cultures (Proctor 1960), soil-nutrient<br />
solution cultures (Clabeaux and Bisson 2009), and sand (particle size < 710 µm in size) in<br />
distilled water with the addition of organic salts (Andrews et al. 1984). Nutrient agar (0.5 %)<br />
in distilled water can provide good conditions for oospores to germinate (Forsberg 1965; Ray<br />
and Chatterjee 1989; Casanova et al. 1998), but probably doesn’t support growth once the<br />
oospore reserves are exhausted (Casanova and Brock 1994). Amino acids, gibberellic acid<br />
and other plant hormones have been found to promote or inhibit the growth of characeae in<br />
culture (Imahori and Iwasa 1965) and further examination of the factors that affect characeae<br />
growth in vitro are currently under examination (Clabeaux and Bisson 2009).<br />
Wild populations of characeae are frequently home to by a variety of herbivores, and can<br />
be relatively free of epiphytes. Epiphytic growth can be a nuisance in cultures, and can be<br />
reduced by decreasing nutrient concentrations, lowering light intensities or introducing<br />
herbivores (e.g. snails or tadpoles) into the culture.<br />
1.5.6 Life History<br />
<strong>The</strong> plants that are grown and used in physiological studies are the haploid (gametophyte)<br />
generation in these algae. <strong>The</strong> mature antheridia release the spermatocytes which swim to the<br />
oogonium, enter it and fuse with the egg cell to form the oospore, which is the only diploid<br />
(sporophyte) cell in the life history. <strong>The</strong> timing of germination, growth, reproduction and<br />
senescence varies among species (Casanova and Brock 1999a). Some species are monocarpic
27 <br />
annuals (i.e. reproducing once in their lives), germinating in either Spring or Autumn,<br />
growing, producing sexual organs once, then dying. <strong>The</strong>se species are highly dependent on<br />
oospore production and storage in a seed bank for survival (Casanova and Brock 1994; Bonis<br />
and Grillas 2002). Other species are polycarpic perennials (i.e. reproduce several times in<br />
their lives) which can survive inclement periods either through dormancy or production of<br />
vegetative reproductive structures (bulbils). Such species are less dependent on oospore<br />
germination for the maintenance of populations. It is likely that many species maintained for<br />
physiological studies are perennial species, because these can persist in culture. Whether a<br />
species is monoecious (with both sexes on a single plant) or dioecious (with separate male<br />
and female plants) can modify the timing of life history events such as germination and<br />
timing of reproduction (Casanova and Brock 1999a). Monoecious annual species can<br />
germinate and grow rapidly, completing their life cycle in a relatively short period of time.<br />
Dioecious annuals can be slower to germinate and grow, and dioecious perennials can have<br />
very specific germination requirements. All these life history types maximise reproductive<br />
success in a variety of habitats (Casanova and Brock 1999a).<br />
1.5.7 Population ecology<br />
Studies into the ecology of characeae populations relies on examination of the transition rates<br />
between the states of propagule dormancy, germination, growth, reproduction, perenniation,<br />
senescence and death. <strong>The</strong> processes of inter- and intra-specific competition and loss to<br />
predation or herbivory can modify the development of populations. In established populations<br />
charophytes deposit their oospores into the soil or sediment, forming a bank of viable<br />
propagules that can become both diverse and dense. Oospores can be dispersed to new<br />
habitats by water birds, through their digestive systems (Proctor 1962) or attached to the feet<br />
and coverings of animals and people. Oospores in the seed bank can be readily germinable<br />
(Casanova and Brock 1990), or more frequently, dormant (Skurzynski 2009). <strong>The</strong> dormancy<br />
can be innate or enforced, depending on the adaptations of the species, and the type of habitat<br />
in which they occur. Enforced dormancy is defined as a maintenance of viability when<br />
conditions are not adequate for germination. Innate dormancy is defined as the failure of a<br />
viable oospore to germinate when exposed to conditions that are adequate for germination<br />
(Harper 1980).
28 <br />
<strong>The</strong>re is often a high degree of innate dormancy in charophyte oospores. Germination of<br />
fresh or field-collected oospores is typically less than 10%, and often less than 1% of the total<br />
oospore pool (Bonis and Lepart 1994; Skurzynski 2009). Rarely germination rates in excess<br />
of 40 % have been recorded (Casanova and Brock 1996; Dugdale et al. 2001). <strong>The</strong> role of<br />
such a uniformly high degree of dormancy is to ensure that populations remain viable, even if<br />
the habitat becomes unsuitable for characeae growth for long periods of time. A possible<br />
additional role is to allow transport of oospores long distances within soil, or the intestinal<br />
tracts of herbivores (Proctor 1962). <strong>The</strong> conditions that break dormancy vary for different<br />
species, so that species that germinate in Spring and Summer can require chilling, or<br />
stratification before germination will occur, and other require conditions that simulate winter<br />
inundation (Casanova and Brock 1996). <strong>The</strong> timing of germination in relation to dormancy<br />
breakage can vary as well. <strong>The</strong> high degree of dormancy and variability in the conditions<br />
required to break that dormancy make germination ‘on demand’ for laboratory experiments<br />
difficult to achieve.<br />
<strong>Characeae</strong> growth rates vary in relation to species (Blindow 1988; Casanova 1994; Casanova<br />
and Brock 1999a) and season (Blindow et al 2002) and an extension rate of 5 mm day -1 in the<br />
field is not exceptional. When water levels are constant some populations have been shown to<br />
maintain their position in the water through apical growth while the basal portions of the<br />
plant decay, initiating new rhizoids from sequentially higher nodes as the plant sinks through<br />
the water (Andrews et al 1994). Where water levels increase apical growth can maintain the<br />
upper photosynthetic parts of the thallus in the photic zone, and where water levels fall the<br />
thallus can lean over in the direction of water movement and initiate rhizoids at the nodes that<br />
are in contact with the substrate (Casanova 1994). In contrast, characeae can be found almost<br />
emergent at the edges of shallow and temporary water bodies, often dying back as the<br />
exposed shoots dry. In some exceptional circumstances populations of characeae can exist in<br />
a film of water in moist terrestrial habitats.<br />
Initiation of sexual reproduction in characeae populations can be a consequence of time since<br />
germination (e.g. for monocarpic annuals) or as a response to environmental conditions or<br />
resources (Casanova 1994). Polycarpic perennial species, such as Chara australis can<br />
produce oospores every year through the warmer months (Casanova 1994). Annual species<br />
can produce reproductive organs on the first whorl of branchlets before significant vegetative
29 <br />
growth has taken place (e.g. Chara muelleri). Other species establish a vegetative thallus<br />
before fertile shoots are produced (e.g. Nitella leonhardii).<br />
Although there are few studies on the interactions among characeae and between characeae<br />
and animals, the dense structure of characeae stands indicates that competition for light could<br />
affect growth, and the occurrence of characeae meadows that exhibit an abrupt, unexplained<br />
growth boundary (in plant height or depth distribution) suggest the presence of biotic<br />
interactions or controls.<br />
<strong>Characeae</strong> species vary with respect to their palatability to animal herbivores (Proctor 1999),<br />
so that the existence of perennial beds of characeae could be explained by their resistance to<br />
herbivory, and the absence of particular species to their susceptibility to herbivory (Proctor<br />
1999). In general, interactions with herbivores are difficult to detect except through direct<br />
observation and experimentation. However characeae would be exceptional (i.e. unlike all<br />
other plants) if herbivory or creation of structures or behaviours to resist herbivory did not<br />
play a role in their ecology.<br />
1.5.8 Application of ecological knowledge<br />
Knowing something of the ecology of characeae can have at least two positive consequences<br />
for physiological research: 1) the development of enhanced techniques and protocols for<br />
culturing and maintaining characeae for laboratory work, and 2) as a stimulus for<br />
physiological research. Observations of characeae in the natural environment can lead us to<br />
asking ‘why is it so?’, and physiologists and physicists will look for answers in relation to the<br />
physiology and biophysics of characeae plants and cells. Observations of the distribution of<br />
characeae in relation to environmental constraints can allow development of better<br />
germination and growth protocols, and a better understanding of the relationships among<br />
species, as photosynthesisers, as food for herbivores and as part of an integrated community<br />
of plants and animals. <strong>The</strong> rest of this book contains information about some of the studies<br />
that have been undertaken on the physiology of characeae plants, informed by a knowledge<br />
of charophyte morphology, cytology and ecology.
30 <br />
1.6 Figure and Table captions<br />
Figure 1.1 Thallus of a characeae plant (Chara sp. r862). a) stem showing whorls of<br />
branchlets, scale bar = 10 mm. b) reproductive organs, scale bar = 1 mm. c) Scanning<br />
electron micrograph of an oospore of Chara sp. (r862), scale bar = 200 µm.<br />
Figure 1.10 Tolypella glomerata (p798), a) whole plant showing basal axis, first nodal<br />
complex with elongate simple branchlets, and numerous fertile branches. b) a single fertile<br />
node with 8 branchlets in a whorl, each of which has a primary node with multicelluar rays<br />
and gametangia. Oogonia and antheridia are clustered at the branchlet nodes, c) oogonium<br />
prior to fertilisation with coronula intact, d) oogonium after fertilisation when the coronula<br />
has fallen and the spiral cells have swollen.<br />
Figure 1.11 Nitellopsis obtusa (t810). a) Axis of a female plant, with six branchlets in whorls<br />
at the nodes, and elongate bract cells at the branchlet nodes. scale bar = 5 cm. b) Oogonium at<br />
the branchlet node, subtended by bract cells, with a 5-celled coronula. Oogonia are usually<br />
heavily calcified with the oospore developing within a calcareous lime-shell or gyrogonite. c)<br />
Apex of a shoot showing absence of stipulodes and simple branchlets with elongate bract<br />
cells. d) vegetative reproductive organ characteristic of Nitellopsis (after Fritsch 1948).<br />
Figure 1.12 Morphology of Nitella. a) whole plant and b) corounula at the top of the<br />
oogonium, c) terminal branchlet segments with central antheridium and bicellulate dactyls, d)<br />
terminal branchlet segments with lateral oogonia and a central antheridium.<br />
Figure 1.13 Diagrammatic representation of a transverse section through the lowest three<br />
cells of a corticated branchlet of Chara. Cells are bounded by a cell wall, the cytoplasm is<br />
bounded by the plasma membrane. <strong>The</strong> peripheral cytoplasm contains numerous discoid<br />
chloroplasts (along with mitochondria, nuclei, golgi apparatus etc.) surrounding a central<br />
tonoplast-bound vacuole. <strong>The</strong> branchlet structure consists of intermodal cells and nodal<br />
complexes with cortical cells, bract cells, and gametangia arising at the nodes (after Fritsch<br />
1948, Pickett-Heaps 1975 and Moestrup 1970).<br />
Figure 1.14 Structures associated with spermatogenesis in characeae. a) spermatid of Nitella<br />
with two flagellae, mitrochondria, nucleus and plastids (after Pickett-Heaps 1975 and<br />
Moestrup 1970), b) part of the contents of an antheridium after removal of the shield cells c)<br />
four of the eight shield cells.
31 <br />
Figure 1.15 <strong>The</strong> characeae oospore: a) gyrogonite of Lamprothamnium macropogon, b)<br />
oospore of Chara sp. with flanged striae, c) oospore of Chara sp. with thickened (pachygyra)<br />
striae, d) oospore of Nitella lhotzkyi with a verrucate ornamentation, e) oospore of Nitella<br />
tasmanica with a reticulate ornamentation, f) oospore of Nitella hyalina with fibrous oospore<br />
wall construction, g) basal view of the oospore of Chara sp showing the single, pentagonal<br />
basal cell impression. h) basal view of the oospore of Nitella sonderi showing the two basal<br />
cell impressions, i) apical view of the oospore of Chara sp. showing the ‘w-shaped’ suture<br />
where the striae meet.<br />
Figure 1.16 <strong>The</strong> relationships among processes that contribute to the size of the seed bank of<br />
characeae in lakes and wetlands.<br />
Figure 1.17 Diagram of a simplified model of the relationships and processes leading to<br />
characeae abundance or phytoplankton abundance in shallow lakes, according to the<br />
Alternative Stable States model (Scheffer et al. 1993). Arrow type indicates whether the<br />
effect is positive (solid) or negative (dashed). <strong>The</strong> size of the circle indicates the relative<br />
abundance of each element. a) High water clarity leads to establishment of high characeae<br />
biomass (which enhances water clarity by stabilising sediment, removing nutrients available<br />
for phytoplankton growth and providing habitat for cladoceran herbivores). b) High nutrient<br />
availability leads to high phytoplankton abundance which reduces charophyte abundance by<br />
decreasing water clarity. Absence of charophytes increases sediment resuspension, further<br />
decreasing water clarity and decreases the habitat available for cladoceran herbivores, which<br />
further enhances phytoplankton biomass.<br />
Figure 1.2 <strong>Characeae</strong> axial nodes. Scale bar = 1 mm. a) Chara sp. (r822) node with two rows<br />
of stipulodes, short cortical cells and small spine cells. b) Chara canescens (r020) node with<br />
longer stipulodes and spines cells. c) Lamprothamnium sp. (r028) with no cortication,<br />
oogonium and stipulodes below the base of the whorl, and spreading bract cells at the<br />
branchlet nodes.<br />
Figure 1.3 Whorls of characeae branchlets viewed in cross-section, with the branchlets<br />
spread around the sectioned node. a) Chara gymnopitys whorl with 12 branchlets in a whorl,<br />
12 stipulodes at the base of the whorl, long bract cells at each branchlet node and branchlets<br />
terminated by a group of bract cells. b) Lamprothamnium sp aff. macropogon with 7<br />
branchlets in a whorl, no apparent stipulodes, long bract cells at the branchlet nodes and an<br />
oogonium at the base of the whorl. c) Nitella sp. with 7 branchlets in a whorl, the branchets
32 <br />
divided into 3-5 secondary rays and 3-5 tertiary rays. d) Nitella hyalina with two whorls of<br />
branchlets at the node (a longer, primary whorl of 8 branchlets, and a smaller, inner,<br />
secondary whorl of 7 branchlets), with the branchlets divided into 3-6 secondary rays, 3-5<br />
tertiary rays and 3-4 quaternary rays. Scale bars = 5 mm.<br />
Figure 1.4 Branchlet morphology in the different genera of charophyte plants. a) Chara, b)<br />
Lamprothamnium, c) Nitella d) Tolypella.<br />
Figure 1.5 Branchlet nodes and gametangia of Chara spp. In Chara the oogonium (female) is<br />
above the antheridium, and bract cells, bracteoles and cortical filaments arise from the nodal<br />
complex.<br />
Figure 1.6 General morphology of Chara australis (r186, r612, r692). a) whole plant<br />
showing main axis and whorls of branchlets. b) whorl of branchlets from a male plant with 6<br />
branchlets in a whorl and antheridia at the branchlet nodes. c) whorl of branchlets from a<br />
female plant with 6 branchlets in a whorl with oogonia at the base of the whorl and on the<br />
branchlet nodes. d) oogonium with coronula of appressed cells, and spiral cells surrounding<br />
the internal egg cell. e) branchlet node with bract cells and two oogonia, f) base of branchlet<br />
whorl with relatively elongate stipulodes, g) base of whorl with obscure stipulodes and two<br />
basal oogonia.<br />
Figure 1.7 Lamprothamnium macropogon (r809, r822) a) whole thallus showing whorls of<br />
branchlets, downward pointing stipulodes and verticillate bract cells on the branchlets. b)<br />
fertile whorl with antheridia on the branchlets and oogonia inside the base of the whorl, or<br />
beside the antheridia. c) internal view of whorl with a single antheridium squeezed among the<br />
oogonia. d) branchlet and bract cell morphology. e) Lamprothamnium papulosum (r729) with<br />
conjoined gametangia, the antheridium above the oogonium.<br />
Figure 1.8 Lamprothamnium sp aff. succinctum from Australia (r 272). a) whole plant<br />
showing elongated internodes and few, small stipulodes and bract cells. b) whorl of<br />
branchlets with antheridia and oogonia on the branchlets. c) detail of basal gametangia,<br />
oogonia within the whorl, a single antheridium below the whorl in the place of a stipulode. d)<br />
antheridium at a branchlet node with two bract cells. e) two young oogonia at a branchlet<br />
node with three bract cells.<br />
Figure 1.9 Lamprothamnium inflatum (r901, r902, r905). a) Shoot of highly inflated<br />
specimen with almost spherical second segments on the branchlets, obscure stipulodes and<br />
contracted apicies. b) elongated shoot. c) whorl of fertile branchlets with oogonia and
33 <br />
antheridia clustered at the base of the whorl and on the first branchlet nodes. d) dissected base<br />
of a whorl showing stipitate antheridia below the whorl and stipitate and clustered oogonia<br />
within the whorl.<br />
Table 1 Names and synonyms of characeae species used in physiological studies.
34 <br />
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