Limonia Sec. Citropsis Engl. (sec. Swingle and Reece 1967)
Citropsis (Engl.) Swingle & M. Kellerm. (sec. Swingle and Reece 1967, Mabberley 2004, Bayer et al. 2009; sensu Tanaka sec. Cottin 2002)
Crown compact or dense, not weeping. First-year twig surface glabrous; second- or third-year twig surface mottled; thorns straight; prickles absent or not persistent. Petiole glabrous, length very long; wings medium, adjoining the blade. Leaflets three or five to seven, margins entire or bluntly toothed, rachis wings medium or wide, shade leaflet blades flat or weakly conduplicate, sun leaflet blades weakly conduplicate. Scent of crushed leaflets faintly sweet candy-like. Fruit as broad as long or longer than broad; rind green-yellow (6), yellow (7-10), or yellow-orange (11); rind texture slightly rough (4-5); firmness membranous; navel absent; flesh yellow.
Swingle and Reece (1967) provided the following additional notes on the genus:
"A genus of Near-Citrus Fruit Trees found in tropical Africa. Shrubs or small trees with 1 or 2 well-developed spines (C. angolensis is said to be spineless) in the axils of the leaves; leaves pinnate, sometimes 3-foliate [sic], rarely 1-foliolate (simple in C. tanakae); rachis and petiole usually broadly winged (wingless in C. le-testui), flowers 4- or 5-merous, usually in dense axillary clusters or short racemes; stamens twice as many as the petals, filaments free, flattened laterally and in some species cohering laterally at the base in groups; ovary with 4 or 5 locules, each with a single ovule; fruit spherical, small (2-3 cm diam.), with pulp-vesicles broad at the base where they are attached and tapering to an acute tip; seeds large (10 X 6 X 4 mm); cotyledons hypogeous in germination.
This genus is certainly closely related to Citrus, in spite of the fact that all but one species of Citropsis have pinnate or trifoliolate leaves. It belongs, along with Atalantia, to group B, the Near-Citrus Fruit Trees, of the subtribe Citrinae. Both of these genera have small, subglobose fruits, 1.5 to 3 cm in diameter, with 3 to 5 locules, in which occur broad-based, sessile, conical, radially arranged pulp-vesicles. All the species of Citropsis have leaves, or leaflets, very similar to those of Citrus in shape, venation, and texture, whereas the leaves of the species of Atalantia are thick, more leathery, and much more veiny than those of Citrus.
The seeds of Citropsis are plump and broadly ovoid with a very hard, smooth testa. The hilum is an oval or ovate aperture bordered by the incurved testa, which is here very hard and thick. No other genus of the subtribe Citrinae has seeds as plump and as hard-shelled. The flowers of Citropsis differ greatly in the various species with respect to size and proportion of parts, and especially with respect to the morphology of the pistil. The leaves differ even more than the flowers in the different species, but fortunately for the taxonomist the leaf and flower characters are not closely correlated; that is, a pair of species having very similar leaves, like C. schweinfurthii and C. angolensis, may show easily recognized differences in the flowers. Citropsis schweinfurthii and C. gilletiana have similar flowers, with only slight differences in the pistil, but very different leaves.
The species of Citropsis look like missing links, i.e., surviving forms of the remote ancestors of Citrus and may, in fact, be such forms that migrated tens of millions of years ago from the southeastern Asiatic homeland of the tribe Citreae and found safe and congenial refuge in the tropical forests of Africa, where they have persisted with little change ever since. Once safely in Africa, the ancestors of Citropsis were freed from competition with the numerous species of related genera of the subtribe Triphasiinae as well as from the danger of hybridization with vigorous new genera that evidently arose in rapid succession in southeastern Asia and the East Indian Archipelago. Thanks to this age-long isolation, even today the leaflets of Citropsis (and especially the unifoliolate leaves of certain forms of C. gabunensis) resemble very closely the leaves of Citrus (and doubtless those of the common ancestor of Citrus and Citropsis) in shape, texture, venation, and color. That this resemblance of the leaves is no mere analogy is proved by the fact that the species of Citropsis, so far as they have been tested, graft readily on Citrus, and vice versa. Citropsis and Pleiospermium are the two genera that most nearly represent today the remote ancestors of Citrus."
"A striking proof of the inadequacy of our botanical exploration of tropical Africa is shown by the fact that, of the eleven species of Citropsis here recognized, eight were discovered after 1900 and five of these after 1920. The last three species to be found—C. gilletiana, C. tanakae,and C. daweana —were described as recently as 1940! The two last-named are strikingly different from all the other known species and from each other. They are not mere small species made by splitting large species, but are new and aberrant members of the genus, which when better known may need to have new subgenera made to include them. One subgenus, Afrocitrus, has already been named to include three allied species, all growing in western tropical Africa.
In the Congo, it was found that Gillet’s cherry-orange, C. gilletiana, the most vigorous species in the genus, growing to be a tree 10 meters high, could be used to advantage as a rootstock for cultivated varieties of Citrus because of its resistance to foot rot (very injurious to cultivated varieties of Citrus there). This species proved to be an even better rootstock than the sour orange for the orange, the mandarin, the pummelo, and the lemon. It is one of the more important rootstocks for Citrus found in this century.
In view of the beauty of their foliage, the refreshing fragrance of their flowers, and the attractive aroma of their tiny fruits, coupled with their rapid growth and free flowering while still small trees, an effort should be made to test all the species of Citropsis as greenhouse ornamentals in the cooler climates and as out-of-door ornamentals in subtropical regions."
Swingle and Reece (1967) provided the following additional information on the species: "The genus Citropsis, native to tropical and subtropical Africa, has eleven species, all but one with pinnate or trifoliolate leaves resembling in several important characters those of the genus Hesperethusa. "
Bayer, R.J., D.J. Mabberley, C. Morton, C.H. Miller, I.K. Sharma, B.E. Pfeil, S. Rich, R. Hitchcock, and S. Sykes. 2009. A molecular phylogeny of the orange subfamily (Rutaceae: Aurantioideae) using nine cpDNA sequences. American Journal of Botany 96: 668–685.
Cottin, R. 2002. Citrus of the World: A citrus directory. Version 2.0. France: SRA INRA-CIRAD.
Mabberley, D.J. 2004. Citrus (Rutaceae): A review of recent advances in etymology, systematics and medical applications. Blumea 49: 481–498.
Swingle, W.T. and P.C. Reece. 1967. The botany of Citrus and its wild relatives. In: Reuther, W., H.J. Webber, and L.D. Batchelor (eds.). The Citrus industry. Ed. 2. Vol. I. University of California, Riverside. http://lib.ucr.edu/agnic/webber/Vol1/Chapter3.html.
Search for this genus in NCBI Entrez or NCBI Nucleotide
Citrus ID Edition 2
October, 2011
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