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|Vol. 10(4), pp. 9-13||The McAllen International Orchid Society Journal||April 2009|
The vast majority of orchids of the Cape Flora are terrestrial, with only a few epiphytes having drifted into some forested areas from further north. The Cape Floristic Region occupies less than 10% of the area of South Africa, but accounts for about 40% of the species of flowering plants on the national list and about 50% of the orchids. Winter rainfall and dry, warm summers characterise this Cape Floristic Region, so that the majority of our orchids are deciduous geophytes, hiding underground from summer's drought and heat. The only evergreen orchids found here are the tough-as-nails genus Acrolophia, about which I have written previously; and the evergreen disas, such as Disa uniflora, which reside at high altitudes where the moist air carried by the south-easter winds condenses on a regular basis to keep the environment damp.
Fig. 1. June06 034-1.jpg Liparis capensis in flower on the 14th June 2006 in the above habitat. This is not a large plant; the scape shown here is all of 60 mm.
The "little Cape greasy" I refer to in my title is my invented name for Liparis capensis (Fig. 1). Liparis is a large genus of about 420 species of world-wide distribution. The genus includes terrestrial, lithophytic or even epiphytic plants. The majority of the species are shade-loving denizens of tropical to equatorial forests and the plants usually comprise aerial pseudobulbs carrying a few soft, deciduous, plicate, petiolate leaves, and spikes of small, shiny [hence the "greasy" implied by the Greek λιπαρος (liparos)], relatively large-lipped, green to brown flowers of no great aesthetic value. The other species of Liparis found on the South African list more-or-less conform to this description, coming as they do from the moister summer-rainfall areas of the east half of the country.
As species of Liparis are found further and further from their "ideal habitat," they show those modifications in life-style and anatomy that are needed to cope; and thus the American and European species of this genus found in more temperate areas usually have subterranean pseudobulbs as a protection from frost and they exhibit a strict seasonality.
Liparis capensis grows in open, sunny, habitats, the dominant vegetation of which is narrow-leaved shrubs (fynbos), in sandy soil where nothing of the plant can be seen during the dry summer months; its growth cycle beginning in early autumn (fall) as the soil commences cooling. So what characteristics does Liparis capensis have that allow it to survive in its seasonally hostile environment?
Fig. 2. June06 039.jpg. Two plants of Liparis capensis, two plants. The right one is the same plant shown in Fig. 1. Note the rocky, sandy habitat, and basal leaves. (The plant on the right is about 60 mm tall; about 2.36 to 2.5 inches).
Fig. 3. June06 032.jpg. The habitat of Liparis capensis in the shallow valley behind the hill above my house in Kommetjie, as seen in mid-June 2006, following the fire of November 2005. A regenerating plant of Acrolophia capensis can be seen in the bottom right-hand corner of the photo.
Well firstly, it is a strict geophyte, and its pseudobulbs are usually fully subterranean, except for perhaps the extreme tip (Fig. 2) (Fig. 3). When growth recommences in early autumn, the new pseudobulb comes forth from the base of the old, and because of the generally nutrient-poor environment in which it lives, a single replacement growth is produced annually (except perhaps when fertilized by ash following a fire). This species is thus not colony-forming and recruitment is only by seed. The subterranean habit also allows the plant to survive the periodic fires that would usually occur during the dry, dormant period of its life cycle.
It is the leaves of this plant, however, that are the most interesting feature. There are usually two and occasionally three of these, and they grow out flat on the ground forming the slightly convex, softly ribbed, rounded, green, ground-hugging organs of photosynthesis. This form of foliage is a special feature of a small guild of geophytes of the Cape Floristic Region, with some similar species found in adjacent areas of southern Africa; but it is not something encountered elsewhere in the world to any degree. These so-called "geophyllous" plants, or more acceptably "prostrate-leaved geophytes," have initiated a new little field of theoretical and practical research here in the Western Cape.
What is particularly interesting is the occurrence of this feature across a range of monocotyledonous geophytes. There are such species in Amaryllidaceae, Colchicaceae, Eriospermaceae, Hyacinthaceae, Iridaceae and Orchidaceae. In Orchidaceae, prostrate-leaved geophytes are found in the following genera: Satyrium, Holothrix, Bartholina, Habenaria, and our little Liparis; a rather genetically diverse group. There is, however, sometimes an obvious genetic basis to this characteristic as, for example, nine of the species of Satyrium that are more-or-less prostrate-leaved, including the beautiful S. carneum, are more closely related to one another than to any of the other 24 or so species of the genus that are indigenous to South Africa. But then there is also one species, S. humile, which although prostrate-leaved, is rather distantly related to those nine.
What all these prostrate-leaved geophytes do have in common is that they all live in the Cape Floristic Region, or are species that appear to have radiated out of this region. So their existence owes something to their geography, although precisely what that is, is hard to finger. The theories so far put forward for the function of the prostrate leaves are many and inventive. They include:
suppressing competing vegetation;
reducing the chance of being eaten;
reducing water loss from the immediate area of the roots;
buffering the temperature of the leaf due to its proximity to the soil;
reducing water loss through the reduction of transpiration;
supplementing water supply through the precipitation of dew on leaves;
benefiting CO2 uptake.
Working with this last idea in mind, Cramer et al. (2007) have studied the C13 levels in prostrate leaves, comparing it to that in leaves of the same species which have been prevented from prostrating normally, and they have deduced through this that about 7% of the fixed carbon in the prostrate leaves is derived from the soil CO2 bank. The science here is based on the fact that this subterranean CO2 bank is large and arises from the microbiological breakdown of plant products. Because much of this carbon has not been part of the atmosphere for a protracted period, it has a relatively low C13 level. It can also be readily displaced from the soil by the inflow of water and will move out through the soil surface following rain; and thus valid theories and procedures can be formulated to work all this out.
Fig. 4. 301009m.jpg The plate of Liparis capensis from Harry Bolus's Icones Orchidearum Austro-Africanarum Extra-Tropicarum (better known as Orchids of South Africa), Vol. 1, part 1 of 1893, and it is the first plate in this marvellous work. The details of the flower are more apparent here than in my photo.
I hardly see this small CO2 contribution as being of any great importance to the plant. And thus I believe that there is still a little mystery here in need of solution. My biggest problem in thinking around this problem is dealing with differences in scale. The leaves of Liparis capensis (Fig. 4) are very similar in appearance to those of Haemanthus sanguineus of the Amaryllidaceae, however the leaves of the former are 20-45 mm wide, whereas the latter can be 250 mm wide; so I am not certain that these two plants can be compared fairly, and may perhaps be prostrate-leaved geophytes for two completely different reasons.
The scape of Liparis capensis arises between the two leaves, which represents a terminal position, in common with other plants of the genus Liparis. It emerges quite early in the season, usually in May, and by the winter solstice (which is 21st June in this part of the world), the plant is in full bloom. Flowering plants are notably more frequently encountered in the first two years following a fire.
Green flowers usually imply pollination by flies (myophily), and the flowers probably emit compounds detectable by their pollinator, although I can find no scent myself. Bill Liltved (pers. comm.) has seen sciarid flies (fungus gnats) visiting the flowers; and he has seen tiny pollinaria on said flies, although he was not able to tie these two events together with complete certainty. My only experience of sciarid flies is as pests of the seedlings of clivias.
Bill has also reported that baboons have been seen digging and eating the pseudobulbs of this orchid at Hermanus, near Cape Agulhas (the southernmost point of the African continent). The distribution range of Liparis capensis extends from the Cape Peninsula eastward to Knysna on the southern Cape coast and then more sporadically as far east as Port Elizabeth. It is found mostly at low altitudes but also up as high as 700 metres.
Somewhy (ain't that a lovely word? - pity nobody uses it), a diverse range of plants have found benefit from prostrating their leaves, and surely the reason for this must be fairly obvious. Unfortunately, so far, nobody seems to appreciate why this is the case. So if anybody has a brilliant idea why this very individual little orchid has such interesting and wonderful leaves; please put us all out of our misery. We would be forever indebted.
Cramer M. D., Kleizen C. and Morrow C. 2007. Does the Prostrate-leaved Geophyte Brunsvigia orientalis Utilize Soil-derived CO2 for Photosynthesis? Annals of Botany 99(5): 835 - 844.
Esler, K. J., Rundel P. W. and Vorster P. 1999. Biogeography of prostrate-leaved geophytes in semi-arid South Africa: hypotheses on functionality. Plant Ecology 142(1-2):105-120
Stewart, J. 1981. The South African Species of Liparis. American Orchid Society Bulletin 50(5): 547-552.