Introduction

The genus Isoglossa Oerst. (Acanthaceae: Acanthoideae: Justicieae: Isoglossinae; Kiel et al. 2006; Manzitto-Tripp et al. 2021) currently comprises c. 70 species, distributed across the palaeotropics and subtropics, with centres of species richness in eastern and southern Africa (POWO 2022). It is currently delimited by having the combination of a bilabiate corolla with ascending-cochlear aestivation, two bithecous stamens that lack appendages on the thecae, 4-seeded stipitate capsules, and (usually) bipororate pollen (i.e., with two compound apertures, each with the ecto- and endo-apertures both porate), circular in apertural view and with a pronounced interapertural “girdle” of tectate exine, termed “gürtelpollen” (Darbyshire et al. 2023). The limited molecular phylogenetic evidence currently available indicates that Isoglossa s.l. is paraphyletic, with monothecous genera of Isoglossinae arising out of it (McDade et al. 2021). However, clear synapomorphies to support the splitting of Isoglossa have not yet been identified, and further molecular data, including the use of Next Generation Sequencing techniques, is required before further decisions on generic delimitation in Isoglossinae can be made (Darbyshire et al. 2023).

In the Flora of Tropical East Africa account for Isoglossa, 26 species were documented in Tanzania, of which three were undescribed and poorly known due to incomplete material (Darbyshire et al. 2010). Isoglossa s.l. is noteworthy for including a significant number of narrowly range-restricted taxa, both in Tanzania and in wider tropical and southern Africa and Madagascar (Brummitt 1985; Darbyshire 2009; Darbyshire et al. 2010, 2011, 2021, 2023; Balkwill et al. 2017; Champluvier & Fischer 2020). Although the genus occurs in a range of habitats, many of these range-restricted species are endemic to specific mountain ranges. For example, the Eastern Arc Mountains of Kenya and Tanzania — well known for their exceptionally high biodiversity and endemism (Burgess et al. 2007; Gereau et al. 2016) — support seven endemic species of Isoglossa. These are I. asystasioides I.Darbysh. & Ensermu, I. bondwaensis I.Darbysh., I. ixodes Lindau and I. oreacanthoides Mildbr. from the Uluguru Mountains, I. candelabrum Lindau from the Taita Hills and East Usambara Mountains, I. variegata I.Darbysh. from the East Usambara and Nguru Mountains, and I. ventricosa I.Darbysh. from the southwestern-most extremity of the Eastern Arc in Lulanda Forest and the Mufindi Escarpment (Darbyshire 2009; Darbyshire et al. 2010).

Here, we report on a further new species of Isoglossa endemic to the Eastern Arc Mountains in Tanzania, found by the second author during ongoing ecological studies of forest biodiversity on East African mountains. The type locality for this new species, at Mwala in the South Pare Mountains of Kilimanjaro Region, Tanzania (Fig. 1), is of high botanical interest. A new orchid species, Rhipidoglossum pareense Cribb & Hemp, was documented recently from this site (Cribb & Hemp 2022), and the recently described Asplenium arcumontanum Hemp & N.R.Crouch (Aspleniaceae) occurs here and on the neighbouring North Pare and West Usambara Mountains (Hemp & Crouch 2018). Furthermore, the rare Asystasia masaiensis Lindau was encountered in the dry foothill savannas at Mwala at 700 m elevation in December 2019 (A. Hemp #7348; Fig. 1D). This latter species was previously known only from two collections from Mkomazi town to the south of the South Pare Mountains, and is assessed as Critically Endangered on the IUCN Red List (Vollesen in Darbyshire et al. 2010; Luke et al. 2015).

Fig. 1.
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Mwala, South Pare Mountains, Tanzania. A aerial view of submontane forest and associated habitats; B within the forest understorey with shaded rock outcrops; C ground fog within the forest, a major source of moisture at this site; D Asystasia masaiensis recorded from the dry foothill savannas at Mwala, collected as A. Hemp 7348. photos: a. hemp.

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The new Isoglossa species was first collected in November 2017 when it was found primarily in fruit; the specimen was shown to the first author during a research visit to the Kew herbarium, and was found not to match any of the species documented in the Flora of Tropical East Africa (Darbyshire et al. 2010). Flowering material was later collected and the species was also grown from seed in the glasshouses at the University of Bayreuth where an additional herbarium specimen was made, allowing for confirmation that this is an undescribed species. Based on similarities in corolla and seed morphology, it is hypothesised to be allied to Isoglossa gregorii (S.Moore) Lindau and I. punctata (Vahl) Brummitt & J.R.I.Wood, which are both widespread in the mountains of eastern Africa, but the new species displays clear and consistent morphological differences such that they are unlikely to be confused.

Materials & Methods

This work is based on the study of both herbarium collections and live plants of the new species of Isoglossa from Mwala in the South Pare Mountains of Tanzania and from cultivated plants at the University of Bayreuth grown from seed collected from the type locality. The herbarium material was analysed and measured at K. Prior to dissection, a flower was soaked in Aerosol OT 5% solution; all other characters were measured on dry material. Herbarium abbreviations follow Thiers (continuously updated).

The species conservation (extinction risk) assessment follows the Categories and Criteria of the IUCN Red List (IUCN 2012) and the guidelines for their use (IUCN Standards and Petitions Subcommittee 2022).

Taxonomic Account

Isoglossa pareensis I.Darbysh. & Hemp sp. nov. Type: Tanzania, South Pare Mountains, Mwala, 28 Nov. 2017, A. Hemp 6953 (holotype K; isotypes B, NHT, UBT).

http://www.ipni.org/urn:lsid:ipni.org:names:77323326-1

Slender herbs 10 – 25 cm tall, trailing and rooting towards base, with decumbent leafy stems; stems green, with conspicuous cystoliths, angular (sulcate in dried material), with inconspicuous short pale retrorse or mixed antrorse and retrorse hairs mainly on two opposite sides, hairs more dense along the nodal line. Leaves ovate or proximal leaves ovate-elliptic, pairs somewhat anisophyllous, largest leaves 4.2 – 6 × 2.5 – 3.9 cm, each leaf with base broadly cuneate to obtuse or rounded or distal-most leaves shallowly cordate, margin entire, apex acute to shortly attenuate, surfaces largely glabrous except for antrorse multicellular hairs along midrib above and margin, or adaxial surface with scattered multicellular hairs appearing deflated in dry state; cystoliths linear, conspicuous in dry state; petiole 20 – 33 mm long, antrorse-pubescent along adaxial groove. Inflorescence a slender panicle-like thyrse 74 – 105 × 28 – 37 mm including primary peduncle 26 – 32 mm long, few-branched, the branches dichasial or partially monochasial; inflorescence axes shortly glandular-pubescent and eglandular-puberulous to -pubescent, glandular hairs 0.1 – 0.25 mm long, sometimes more dense on the secondary branches; bracts and bracteoles purplish, linear-lanceolate, 1.3 – 3.6 mm long, up to 0.5 mm wide. Calyx purple, lobes linear-lanceolate, 3.4 – 3.8 mm long in flower, 4.6 – 5.5 mm long in fruit, shortly glandular-pubescent and eglandular-puberulous. Corolla white suffused pink and with deeper pink markings on palate of lower lip, 10.5 – 11.5 mm long, pubescent externally mainly on dorsal side of tube; tube cylindrical, 6.7 – 7 mm long, with basal cylindrical portion ± 1.5 mm in diam., only slightly widened towards mouth where 2 – 2.3 mm in diam.; upper lip 2.7 – 2.8 mm long, apex 2-lobed for ± 1.6 mm, lobes somewhat reflexed; lower lip 3.6 – 4.7 mm long, with 3 lobes 2 – 2.8 mm long, palate somewhat raised with central furrow, lacking prominent herring-bone patterning, glabrous. Stamens inserted ± 4 mm from base of corolla tube; filaments free for 2.8 – 3 mm, glabrous; anthers barely exserted or held in corolla mouth, thecae slightly oblique to parallel, overlapping for half their length, upper theca 0.65 – 0.75 mm long, lower theca 0.6 – 0.7 mm long, muticous. Pistil glabrous; style ± 5 mm long; stigma shortly bilobed. Capsule stipitate with expanded fertile portion, 9.5 – 10.5 mm long, puberulous with eglandular and occasional glandular hairs; seeds subflattened, ± 3 × 2.7 mm, with elongate, minutely glochidiate tubercles particularly towards the rim Figs. 2, 3 and 4.

Fig. 2.
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Isoglossa pareensis. A habit, flowering branch; B flower in situ; C face view of corolla with stamens and stigma visible at mouth; D dissected corolla with stamens; E partial infructescence with mature capsules; F indumentum of calyx lobe, side view, external face to left; G indumentum of peduncle; H capsule with seeds, and dorsal view of capsule valve; J mature seed, with detail of sculpturing. A & D from Hemp 8165; B & C from photographs of plants in the field; E – J from Hemp 6953. Drawn by andrew brown.

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Fig. 3.
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Isoglossa pareensis. A – C in situ at Mwala, Tanzania, collected as A. Hemp 7355; D & E in cultivation at UBT. photos: a – c a. hemp; d, e u. meve.

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Fig. 4.
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Fruits of Isoglossa pareensis. A fruit valve within calyx, showing dense glandular indumentum; B dehiscing capsule with seeds; C capsule valve showing immature seeds with glochidiate-tuberculate sculpture. Scale bars = 2 mm. photos: u. meve.

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RECOGNITION. Isoglossa pareensis is similar to I. gregorii in foliage and to both I. gregorii and I. punctata in corolla form and in having seeds with elongate, minutely glochidiate tubercles, but differs from both in the inflorescence being a panicle-like thyrse with pedunculate dichasial or monochasial cyme units (vs inflorescence a simple spike or, if branched, the branches being spiciform with (sub)sessile cyme units); in the anther thecae overlapping for c. half their length (vs thecae fully superposed to widely separated); in the capsule being puberulous with eglandular and occasional glandular hairs (vs capsule glabrous or occasionally with few eglandular hairs towards apex and/or with scattered glandular and/or long eglandular hairs); and in the inflorescence axes and calyces having a more dense indumentum including more numerous glandular hairs. It additionally differs from I. gregorii in having linear-lanceolate bracts, 1.3 – 3.6 × 0.3 – 0.5 mm (vs bracts elliptic, somewhat obovate or basal pairs ovate, (3.5 –) 4.5 – 14 × 0.7 – 6 mm) (Fig. 5; Table 1).

Fig. 5.
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Isoglossa gregorii. A habit of flowering plants; B inflorescence for comparison to I. pareensis; Ngozi, Tanzania. photos: i. darbyshire.

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Table 1. Main diagnostic characters for separation of Isoglossa pareensis from I. gregorii, I. paucinervis and I. punctata.
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DISTRIBUTION. This species is known only from the South Pare Mountains of Kilimanjaro Region, northeast Tanzania.

ADDITIONAL COLLECTIONS STUDIED. TANZANIA. Kilimanjaro Region, South Pare Mountains, Mwala, 1700 m, fl. & fr., 17 Dec. 2019, A. Hemp 7355 (NHT, UBT); cult. at Univ. Bayreuth, fl., 13 June 2021, A. Hemp 8165 (K, NHT, UBT).

HABITAT & ECOLOGY. This species occurs in the herb layer of lower montane Newtonia forests between 1600 and 1700 m, with an underlying geology of gneisses (Geological survey of Tanzania, Quarter degree sheet 108, Hedaru) (Fig. 1). Dominant associated trees include Newtonia buchananii (Baker f.) G.C.C.Gilbert & Boutique (Fabaceae), Pleiocarpa pycnantha (K.Schum.) Stapf (Apocynaceae; newly recorded for South Pare), Strombosia scheffleri Engl. (Olacaceae), Drypetes gerrardii Hutch. (Putranjivaceae), Syzygium micklethwaitii Verdc. sensu stricto (Myrtaceae), Dasylepis integra Warb. (Achariaceae), Xymalos monospora (Harv.) Baill. ex Warb. (Monimiaceae), and Myrsine melanophloeos (L.) R.Br. ex Sweet (Primulaceae). The dense shrub layer in these forests is mainly composed of Dracaena fragrans (L.) Ker Gawl. (Asparagaceae), Psychotria goetzei (K.Schum.) E.M.A.Petit, Chassalia parvifolia K.Schum., C. discolor K.Schum. (all Rubiaceae), and Maytenus acuminata (L.f.) Loes. (Celastraceae).

Isoglossa pareensis grows on the shady forest floor together with many ferns, particularly Asplenium gemmiferum Schrad., A. arcumontanum, A. erectum Bory ex Willd., A. elliottii C.H.Wright (all Aspleniaceae), Lomariopsis warneckei (Hieron.) Alston (Lomariopsidaceae), and Blotiella stipitata (Alston) Faden (Dennstaedtiaceae). Other herbs belonging to the Acanthaceae include Crossandra tridentata Lindau and Isoglossa lactea Lindau subsp. saccata I.Darbysh. Isoglossa pareensis is not common within these forests; using data from 30 forest plots (A. Hemp, unpubl. data) we estimate that it occurs in 13% of the area of Mwala with perhaps 10 – 20 individuals per 0.1 ha.

The climate at Mwala is humid and foggy. Mean annual rainfall is only about 700 mm; however, fog water interception provides more than two times this amount in addition (data of 5 years; A. Hemp, unpubl. data). Consequently, mean annual relative humidity is 94%. Mean annual temperature is 15.7°C with a minimum temperature of 8.7°C and a maximum temperature of 29.0°C. Because of the high humidity, tree trunks and branches are densely covered by mosses (30 – 50% coverage) and vascular epiphytes (5 – 10% coverage) (A. Hemp, unpubl. data).

CONSERVATION STATUS. This species is known only from the type locality, and using the recommended 2 × 2 km grid cell size for calculation, its Area of Occupancy is 4 km2. However, the forest in which this species is found covers an area of c. 3.5 km2, within which suitable habitat for the Isoglossa is estimated to be 1.2 km2, and as noted above, it is not common here (A. Hemp, pers. obs.). The area where the species occurs is currently unprotected although it has been proposed as a Forest Reserve, and during the visit of the second author (A.H.) no illegal logging activities or other ongoing threats were observed. However, there is a plausible future threat from forest fires at this site; such events have been recorded from similar forest patches in the region and larger burning events can be highly damaging to the forest understorey (A. Hemp, pers. obs.). Given the extremely small range size and this plausible threat, Isoglossa pareensis is assessed as Vulnerable under criterion D2 – VU D2.

ETYMOLOGY. The species epithet “pareensis” denotes that this species is, so far as is known, endemic to the Pare Mountains of Tanzania.

NOTES. Although this species appears to be most closely allied to Isoglossa gregorii (Fig. 5) and I. punctata based on corolla and seed morphology, it is easily separated by the characters listed in the Recognition section and in Table 1, and the inflorescence form, in particular, is so different from those species that they are unlikely to be confused.

One further species from adjacent areas — Isoglossa paucinervis I.Darbysh. from the Eastern Arc Mountains (West Usambara, Ukaguru and Udzungwa) and Mt Kilimanjaro — could potentially be confused with I. pareensis because they share a similarly small stature, small leaves with few lateral veins, and an open, branched inflorescence with small bracts. However, I. pareensis is easily separated from I. paucinervis by a range of characters, including the capsule being puberulous (vs glabrous), the seeds having glochidiate tubercles (vs seeds with elongate but non-glochidiate tubercles), a markedly more slender corolla tube and a more dense inflorescence indumentum (see Darbyshire 2009; Darbyshire et al. 2010). We hypothesise that these two species are not closely related and that the resemblance between them is likely to be superficial. Isoglossa paucinervis is included in Table 1 for completeness.

To date, none of the Isoglossa species discussed in this paper have been sampled in molecular phylogenetic studies pertaining to Isoglossinae (Kiel et al. 2006; McDade et al. 2021), and so relationships based on morphology are speculative. In their study of Justicieae using five molecular loci (nrITS, trnT-L and trnS-G, trnL-F and rps16 intron), McDade et al. (2021) documented two clades of Isoglossa s.l. within Isoglossinae. Included within Isoglossa Clade 1 were two species from Madagascar, one of which — I. aff. justicioides Baker — is from a group of Malagasy species that appear morphologically similar to I. gregorii and I. punctata (I. Darbyshire, pers. obs.). We could hypothesise, therefore, that I. pareensis may be resolved within Isoglossa Clade 1, but this needs to be tested with further molecular phylogenetic data, ideally using Next Generation Sequencing phylogenetic techniques to improve data resolution. With this in mind, a RADseq analysis of the tribe Isoglossinae is now being prepared to investigate the delimitation and evolutionary history of Isoglossa and its allies (C. A. Kiel & I. Darbyshire, in prep.).