Oxalis L. is a huge genus of about 270 species within 6 subgenera of the family Oxalidaceae native to Europe, Asia-Tropical, Africa, Falkland Island, Pacific, Southern America, Northern America, Australasia and Asia-Temperate. A large number of species are found in South Africa and South America. Oxalis was the subject of the PBS list topic of the week in November 2003. To read the introduction by Robin Attrill for this topic view this link. Another post with a lot of good background information on this subject was in this list post from David Victor. For the most recent comprehensive information about the names of the species see Clifton, 2004 in References. Some of the species do not fit the criteria for inclusion on the wiki, and there is great variety in the ones that have storage organs (bulbs, tubers, rhizomes, tap roots) that can be included. For ease of comparison, we've gathered a collection of photos of Oxalis storage organs.
Photos 1-2 below taken by Bill Dijk are a collection of Oxalis species painted by botanical artist Zoë Carter from Nelson, New Zealand. Bill has kindly added the names in the second photo. Photos 3-4 show flowers and leaves scanned by Ron Vanderhoff. To see more clearly the leaves identified with the species names click on the higher resolution file. These photos show the great number of flowers that can be in bloom at the same time in his collection and the great diversity of leaf types of this genus. Photo 5 by Nhu Nguyen shows a bouquet of various species.
The genus Oxalis has a reputation as a weed, and a few species really are weeds in parts of the world. But most are not. The late Michael Vassar, Curator of Floristic Gardens at the Huntington Botanical Garden, played a big role in popularizing Oxalis bulb culture by donating several South African collections to charity fund-raisers in the late 1990s. Many of his selections turned out to be relatively easy to grow, but not invasive in the garden. Download a list of his complete collection here. The non-invasiveness of some Oxalis species in cultivation seems to be caused in part by their unusual pollination system.
PBS member Christiaan van Schalkwyk, who gardens in South Africa, shared this on the PBS discussion forum:
South African Oxalis species employ a system of tristyly, that is there are three whorls of styles, two of them bearing anthers and the third the stigma (5 to a whorl). If the uppermost whorl contains the stigma, it is referred to as a longistylar form, mediostylar for the middle and brevistylar if at the bottom. Not all Oxalis species seem to employ tristyly, but it is common in the bulbous forms. (Most Oxalis from the Americas are either weedy, i.e. without a bulb, or have more or less a rhizomatous underground structure. Most (all) South African Oxalis have true bulbs, and tristyly is the rule for sexual reproduction.) I presume that there might be exceptions, as always.
Only flowers with two different stylar forms can be pollinated to form seeds. Thus if you have two different clones of the same species, but both are of the same stylar form, they are sterile towards each other.
Often only one stylar form is available (for example because only one plant/bulb is collected or because one stylar form exhibits better characteristics from a cultivator's point of view (i.e. bigger, brighter, taller flowers), or the one stylar form is more vigorous than the other and the weaker does not get distributed). Personally I think this is an excellent state of affairs, because it precludes seed forming which could otherwise turn quite a few species real weedy real quick. (I am still battling to get rid of O. haedulipes. This one is "a local" and managed to get into nearly every pot of plants I have. Luckily the bulbs are quite distinctive and can be culled during transplanting). Also think of the pest O. pes-caprae have become in some parts of the world, and that only by vegetative reproduction. Think of the havoc if it could set seeds as well).
The first picture below is of the different stylar forms, longistylar, mediostylar and brevistylar, in that sequence (O. annae) The second show a seedpod of O. flava var. fabifolia, with some fresh seeds and a three day old seedling. The white stuff are "pseudoseeds" (actually don't recall the correct term right now). They are found in-between the seeds and absorbs moisture when the seeds are ripe, and this then forces the capsule to explode and helps to shoot the seeds a great distance. The third shows some seedlings, a few days old. The top fabifolia, the bottom a more conventional flava.
Some research has suggested that the pollen size of the different stylar morphs differ, whether it has been proven conclusively, I do not know. Whatever the case, heterostyly might be the only barrier to self fertilization, but alas, a camel hair brush is not going to break through it that easily.
To complicate matters, the basic number of chromosomes vary widely between species, with 5, 6, 7, 8, 9 10, 11, and 12 being possibilities. A multitude of ploidy levels are available, sometimes quite restrictive in a particular species, but, as in the case of O. obtusa, about anything is possible(from 2n to 8n plus?). Even plants from the same locality can display different ploidy levels and are thus incompatible. I once saw three very divergent forms of obtusa growing intermingled with each other without any obvious hybrids inbetween. Salter mentions more than 10 species growing and flowering together in nature, without any hybrids seen.
Hybrids between species are unknown. Maybe because of lack of effort and interest, or maybe because there are too many factors inhibiting pollination. Even when my collection exceeded 80 different accessions of O. obtusa (and about 400 for Oxalis in total), natural seed set were rare, and even hand pollination yielded poor results (bad ploidy selections, I think). The only other Oxalis seedlings I've seen (other than obtusa, haedulipes and that pesky corniculata), are from O. flava var. fabifolia and O. helicoides. I've heard of O. hirta forming seeds, but it does not grow well for me (actually I do not think I've got any left).
Nhu mentioned that Oxalis seeds are very short lived. More than 60% of the South African species does not have endospermous seed. They need to either fall in a suitable place or be planted within 24 hours to grow. The tiny seedling with two lobes and a small root gets ejected from the seedpod, it starts to grow immediately and it forms true leaves within two days.
Oxalis obtusa does have endospermous seed, and it flowers close to the end of the growing season. I suspect they might last longer, and may even germinate the next season, but I have not investigated it or read about it. Maybe a little experiment is called for....
See these wiki pages Miscellaneous Oxalis - Oxalis articulata - Oxalis flava - Oxalis hirta - Oxalis obtusa - Oxalis pes-caprae - Oxalis purpurea - Oxalis tuberosa - South African oxalis A-B - South African oxalis C -South African oxalis D-E - South African oxalis F-G - South African oxalis H-K - South African oxalis L-M - South African oxalis N-O - South African oxalis P - South African oxalis R-S - South African oxalis T-Z - South American oxalis or click on the name in the table below to learn about some of the species.
Miscellaneous Oxalis - Oxalis articulata - Oxalis flava - Oxalis hirta - Oxalis obtusa - Oxalis pes-caprae - Oxalis purpurea - Oxalis tuberosa - South African oxalis a-b - South African oxalis c - South African oxalis d-e - South African oxalis f-g - South African oxalis h-k - South African oxalis l-m - South African oxalis n-o - South African oxalis p - South African oxalis r-s - South African oxalis t-z - South American oxalis