Oxalidaceae

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Oxalidaceae

Description

Herbs, sometimes with scaly rhizomes, bulbs, bulbils or stolons, or woody perennials, shrubs, lianas or trees. Leaves penninerved, digitately or pinnately trifoliolate, imparipinnate or paripinnate, basal, alternate, subopposite or apically tufted. Stipules sometimes present. Inflorescences basal, axillary or pseudoterminal, cymose to pseudumbellate, rarely racemose, 1-many-flowered, bracteate and bracteolate. Flowers bisexual, very rarely also ♂ specimens (Dapania), actinomorphic, 5-merous, hetero-tri-, -di-, or homostylous, sometimes cleistogamous. Sepals imbricate, free or connate at base, sometimes with apical calli (Oxalis), persistent. Petals contort, quincuncial or cochlear, free but usually cohesive above the base ('pseudosympetal'), clawed (sometimes minutely so), glabrous or inside sometimes with minute papillae or pilose. Ovary 5-celled, superior; Fruit capsular, loculicid, 5-celled, dry, rarely fleshy and indehis-cent. Seeds usually with an aril;

Distribution

Asia-Tropical:, New Guineapresent Luzon: present Madagascar: present Old World: present S. Africa: present S. America: present West Malesia: present
6(7?) genera with c. 850 spp. Of the Malesian representatives Oxalis, the largest genus, is most numerous in S. America and S. Africa and Biophytum in S. America and Madagascar; Dapania has 2 spp. in Malesia and 1 in Madagascar; Sarcotheca (11 spp.) is endemic in Malesia, while Averrhoa (2 spp.) assumedly also originated here; it is now cultivated pantropically.
In Malesia there are 5 genera with 29 species, of which 14 endemic.
The generic distribution of the family offers in the Old World two remarkable patterns. First, that of Dapania which shows the characteristic disjunction between West Malesia and Madagascar. Second, that of Oxalis sect. Acetosellae which almost resembles that of Euphrasia, that is, a temperate genus with two stepping stones across the tropics (Luzon, New Guinea), otherwise bipolar. .

Pollination

Heterostyly is a common feature in the family, heterotristyly is observed in Oxalis; Biophytum, and Averrhoa bilimbi. This must be assumed to be the primitive condition, as is found in the allied Connaraceae. From it is derived a heterodistylous condition in Sarcotheca and Dapania, in certain species of Oxalis and Biophytum, and in Averrhoa carambola. A further derived, homostylous condition is found in certain species of Oxalis and Biophytum.
DARWIN concluded that the heterotristyly in a Ceylonese Biophytum which he examined, was also functional in analogy with Oxalis while in cleistogamous flowers he observed that in some way the incompatibility factor seemed to be removed (). SALTER () confirmed by experiments that functional heterotristyly occurs in Oxalis. This was also found in S. American species of Oxalis. Miss P. MAYURA DEVI () found in Indian Biophytum in her experiments a significant incompatibility in illegitimate crossings, except for MF selfed with pollen of the long stamens. This means a loss of compulsory he-terotristyly. In a later article () she described another mid-homostylous form which proved excellently self-compatible. It is regrettable that she did not conserve voucher specimens bound to her experiments, as the identification of Indian Biophytum is in distinct confusion and more than one species is cited as B. sensitivum. The plants used by DEVI certainly do not belong to B. sensitivum, as in her pictures the corolla is much longer than the calyx.
Functional heterotristyly is apparently present in the introduced species of Oxalis, O. tetraphylla and O. latifolia, which in Malesia occur as SF only and have never been observed with fruit. O. corymbosa, although present with SF and MF, does not fruit either, but the flowers are often monstruous in this species. All three reproduce very successfully by bulbils and can become obnoxious weeds.
Biophytum fruticosum, B. adiantoides and B. microphyllum are also heterotristylous. No experiments have been done.
Biophytum sensitivum and B. reinwardtii sens. str. are both in India and Malesia mid-homostylous.
Reduction to a single stylar form (LF) is found in Oxalis magellanica and O. acetosella subsp. griffithii; reduction to homostyly (MF) has very far progressed in both O. corniculata where LF occurs rarely (6 out of 105 plants examined; cf. also ) and Averrhoa bilimbi, where LF and SF were only observed once. The introduced Biophytum dendroides is apparently self-compatible, as only one form (LF) has been found at Bogor and in the glasshouses of the Botanical Gardens of Leyden and Groningen, where in the apparent absence of pollinators it is fully fertile and weedy.
Heterodistyly (LF and SF) occurs in Biophytum petersianum, Sarcotheca, and Dapania pentandra from Madagascar. It is plausible that the ♂ flowers of the Malesian Dapania originated by an extreme reduction of the pistil in SF-flowers. Averrhoa carambola is heterodistylous for LF and MF. The latter type might well correspond with SF, as the shorter stamens are much reduced and without anthers.
More information about heterostyly and literature concerning this matter is given by ORNDUFF and by MULCAHY ().

Dispersal

The fleshy fruits of Averrhoa, Dapania, and Sarcotheca are no doubt eaten by various animals as bats, birds, and monkeys, and dispersed by them. The seeds of Oxalis and Biophytum have a peculiar ejaculative aril originally enveloping the entire seed which at maturity shoots them away for some distance (). The cultivated species of Oxalis which set no fruit in Malesia, are locally tenacious weeds through their bulbils. The native Oxalis species propagate also vegetatively, by stolons and root-stocks.
All Oxalidaceae have an arillate seed except the species of Sarcotheca and Averrhoa bilimbi.
On germination some remarks are made under the genus Biophytum.

Morphology

Episeptal rimae are found in some species of Oxalis, in most of Dapania and all Sarcothecas; in Averrhoa they are only inconspicuous, apical furrows. The most primitive state is probably represented in Oxalis corniculata. The septs fail to enlarge towards fructification, whereby the cells of the fruit are only united by their attachment to the central axis. The walls of the cells are pressed together and slits are formed, especially conspicuous by transparent ridges. These ridges are wide apart in the Malesian Dapanias and are slightly bent inwards. In Sarcotheca the septs are developed, at least in the lower half; in the upper half the rimae may be open and papillose inside, or closed and glabrous. Episeptal rimae are absent in all Biophytums and in Dapania pentandra from Madagascar.
Dehiscence of the fruits in Biophytum and Dapania is into a 5-rayed star. In Oxalis the valves remain united and only longitudinal loculicid slits are formed. Sarcotheca and Averrhoa have indehiscent fleshy fruits.

Anatomy

.

Taxonomy

In Engler's Syllabus () SCHOLZ divided the genera in two groups, A and B, on the aestivation and the number of ovules per carpel. Through this Averrhoa was joined to the affinity of Oxalis and Biophytum. However, the aestivation is inconstant. Moreover, the affinity of Averrhoa is doubtless with Sarcotheca and Dapania in all other characters. These three genera form a very clear reticulate affinity.
The family is considered to be the most primitive of the Geraniales by HALLIER f. () and ENGLER (). This primitive position is possibly the reason for the affinity with the Connaraceae, at which some authors have pointed (). The latter family is usually placed near the Leguminosae. As a matter of fact specimens of Rourea and Sarcotheca were often confused and are very similar, the pistil and fruit excepted.
The closest allied family is Geraniaceae in which they were merged by BENTHAM & HOOKER f. In fact the American Hypseocharis seems to link both families. The only constant character was said by HALLIER f. () to be the tenuinucellate ovules in Oxalidaceae and crassinucel-late ovules in Geraniaceae, but Hypseocharis and many species of other genera have not been checked on the general validity of this character, as HALLIER f. himself admitted. And one must be very careful in this respect; for example HUTCHINSON () stated that Oxalidaceae have albuminous seeds, but some species of Oxalis are exalbuminous (cf. ); he stated also that Oxalidaceae are exstipulate, but stipules are found in all Biophyturns and also in some species of Oxalis (sect. Acetosella, O. corniculata, etc.).
HUTCHINSON'S division of Oxalidaceae into three different families belonging to three different orders seems not to have a reliable basis as Averrhoa cannot be divorced from Sarcotheca and Dapania and there is no reason for admitting a preponderant importance to their ligneous habit; besides many species of Biophyturn and Oxalis are dwarf shrubs.

Uses

Several species of Oxalis are cultivated as ornamentals, Averrhoa for its edible fruit. The wood of the ligneous Oxalidaceae is useless as timber. In Malaya the fruits of Sarcotheca are sometimes eaten. See also .

Phytochemo

Few phytochemical investigations were performed with members of the family. In fact, distinct constituents have been isolated only from a few species of Oxalis. The tendency shown by many species of Oxalis to accumulate large amounts of oxalic acid in water-soluble form is known since a long time. Leucocyanidins and leucodelphinidins have been demonstrated to be present in the leaves of some species of Oxalis and of Averrhoa carambola L. This agrees with the idea that Oxalidaceae represent the most primitive family of Geraniales. Probably the species described by plant anatomists as possessing 'tannin' cells are the ones which contain leucoanthocyanins in leaves. The bright yellow flower pigments of Oxalis cernua THUNB. are the aurone glycosides aureusin and cernuoside. A yellow quinonoid pigment was isolated from the bulbs of one species of Oxalis (O. purpurata JACQ. ?) and later identified with rapanone, a benzoquinone occurring frequently in myrsinaceous plants. Too little phytochemical information is available at present for a chemotaxonomical appreciation of systematic relationships of Oxalidaceae. It might be significant, however, that rapanone does also occur in Connaraceae. General reference: . — R. HEGNAUER.