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|Vol. 6(5), pp. 9-16||The McAllen International Orchid Society Journal||May 2005|
Fig. 6. Acrolophia capensis (Berg.) Fourcade. Digital photo, DSCN #1000011. G. Russell
The south side of the little coastal village of Kommetjie on the Cape Peninsula of South Africa, where I live, is bordered by a long, low ridge, rising up to 150 m above the coastal plain. Known as Slangkop, this ridge is highest on its western end, where it is crowned by a man-made cross which can be seen from quite a distance. On the higher slopes, above 100 m., here and there, in any season, one can see patches containing plants of a tough-leaved, evergreen, terrestrial, orchidaceous plant (Fig. 6). Out of flower, these plants scarcely deserve examination; in flower, hardly more, but nevertheless they represent a group of highly interesting species of the genus Acrolophia.
It is probable that the continual, slow grind of evolution's laboratory can be observed by the good student virtually anywhere where life exists, given enough time and eyes. On Slangkop, one can almost sense that evolution is going on here amongst some of the acrolophias, in front of one's eyes. For me, probably the most amazing aspect of orchids is the manifold ways which the various species have found in order to make a living, how one feature of an organism can be modified to be utilised in another way and how everything gels together to present the final product, about which nothing is final. Paradoxically, one is confronted by a sort of "perfect work in progress". Although for many, acrolophias are a bit short on the aesthetics side, for me they have a wonderful intellectual beauty, and offer many hours of observation and contemplation.
Growing in mixed patches up on Slangkop, one can find what I usually describe as "3½ species" of this genus. It is rather difficult to tell these species apart, based entirely on the vegetative characteristics. The only thing of which one can be certain is that these plants are in fact acrolophias, and even that requires closer examination, because they grow intermixed with a tough, strap-leaved daisy Corymbium africanum, which resembles an Acrolophia from a distance. It is reasonable to suspect that some form of mimicry may be taking place here; perhaps via other mammals. Potential grazers of one or other of these two species may have difficulty telling the plants apart, and because one of the two is not palatable, they simply leave them all alone. In a wider sense, these species resemble, to an extent, the grasses and restios and are perhaps part of a greater cohort of Müllerian mimics (this being defined as the sharing of a feature by a number of different species to the mutual benefit of all).
Acrolophia is a genus endemic to South Africa. In fact, it's South Africa's only Epidendroid genus. At present, seven species of Acrolophia are recognised, and they can be divided into three groups based on floral anatomy. The first group is monotypic and contains the species A. ustulata, characterised by the absence of a spur. This species is interesting for its almost black flowers and its apparent rarity. It has been dealt with by Kurze and Kurze (1990). This species has not been found on Slangkop, and I do not expect to do so, as it has only been found at altitudes of about 400 m. on the Cape Peninsula.
Fig. 7. Acrolophia cochlearis (Bol.) Schlect. & Bolus. Digital photo, DSCN #1000019. G. Russell
The spurred species can be divided further into two groups, a small-flowered group comprised of A. cochlearis (Fig. 7), A. bolusii, and A. micrantha; and the large-flowered group which includes A. capensis, A. lamellate, and A. barbata (formerly A. lunata).
This latter group, the spurred members, has a fascinating feature not seen in the two other groups: the anther-caps are horned. These horns are small dark projections arising on either side of this structure. Bearing in mind that useful dictum: Anatomy is crystallised function, the function of these horns is something to reflect upon. My thoughts on this matter will be addressed further on.
Fig. 8. Acrolophia cochlearis (Bol.) Schlect. & Bolus. Digital scan #2488. G. Russell
In the small-flowered group, two species occur on Slangkop. Based on the distribution maps published by Linder & Kurzweil (1999) as well as in other literature, one would expect to find only Acrolophia bolusii from this group on the Peninsula. This species has indeed been found here, although not very commonly, and additionally A. cochlearis has been found, even more rarely; in fact I have so far only found six plants of this species in toto on Slangkop. Although A. cochlearis (Fig. 8) has not been previously reported from the Peninsula, I have also seen material comprising a dead pod-bearing spike from Rondevlei Nature Reserve on the Cape Flats--the eastern suburbs of Cape Town--with round capsules which are almost undoubtedly those of A. cochlearis (this is a little confused by the existance of A. spherocarpa, a formerly recognised species with round seed capsules as the name suggests, which has been sunk into A. capensis). Acrolophias are so spectacularly unspectacular and poorly known that this sort of invisibility is quite understandable.
The species of the large-flowered group, those sporting horned anther-caps, represent the other "1½ species" on my Slangkop list. The distinction between A. lamellata and A. capensis is not a clear one, and revolves around overlapping flowering periods as well as flower size. Additionally, A. lamellata covers those plants found growing on the coastal plain, whereas A. capensis is found at higher altitudes. For many years the two species were considered varieties of one species, or virtually synonymous, and were resurrected as separate species by Linder & Kurzweil.
Based on the current definitions of these two separate species, it appears that the material on Slangkop belongs in the species A. capensis, but what a variety of material there is! The various plants on Slangkop, and there are many, exhibit a large range of variation in size of plant, width of leaf, thickness of leaf, pigmentation of spike, colour of flower, flowering time, length of floral bracts and many other characteristics. With this sort of variation, one gets the feeling that here we have some sort of big evolutionary cauldron in which species are moving apart, or together, or perhaps in several directions. At times I have had the sense that there are perhaps two basic forms, a robust December-flowering form and a more gracile January-flowering form, but many plants cannot be reliably separated into either group, and some plants seem to be almost protean. For the moment, "1½ Species" is the most reasonable number I can come up with here.
The best method of learning to understand the nature and limits of any species is to get to grips with the reproductive biology of that species. Insects and their relatives are probably the worlds greatest taxonomists and by observing them, one is likely to get more information on the taxonomy of the plants associated with them than any dried herbarium specimen could offer. It is usually the nature of the interaction between a plant and its pollinator that determines the flow of genes within a group of closely related plants, and sets up those barriers which define a species and promote the maintenance of species integrity. The latter speaks to the very essence of what is a species, biologically and evolutionarily.
An estimation of the ploidy-levels of the various species by means of the measurement of stomatal guard-cells. For technical details, see Russell (2004) or the website with stomata in the address line. Clinical work suggests the two species of the small-flowered group are diploids, whereas Acrolophia capensis is a tetraploid, with no notable differences observed between the measurements of the robust and gracile forms. One or two particularly heavy-leaved specimens have a guard-cell measurement suggesting hexaploidy. Of course, all this will need to be corroborated by means of chromosome counts in the future. This would allow us to conclude that all the forms of A. capensis on Slangkop are not precluded from interbreeding and producing fertile offspring on grounds of a difference in ploidy-levels, something which I had initially suspected, based on the the fact that various plants in this species-complex appear to display differing leaf thicknesses.
Getting back to Acrolophia cochlearis of the small-flowered group, this species is characterised, on Slangkop and apparently elsewhere, by the production of numerous, small, rounded seed capsules. About one in every five of the many flowers produces a seed capsule. Such a high production figure hints at some form of self-pollination (autogamy), or possibly agamospermy. The lips of these non-resupinate flowers are furnished with long papillae and rest forward on the column, meaning that the flowers do not offer free access to any but the smallest of insects. This helps support this idea of autogamy. I initially thought the wind (these plants flower in October/November--quite windy months) might aid self-pollination. The wind's action could cause the lips to move up and down and somehow the papillae could become engaged with the viscidium of the pollinarium, dislocating the pollinarium and ultimately causing it to topple into the stigma, thereby effecting pollination.
Oeceoclades maculata has been found to set fruit following disturbance of the anther by rain. Although the mechanism involved could not be directly translated to Acrolophia cochlearis, as O. maculata has resupinate flowers; the idea that self-pollination in this acrolophia could be mediated by rain became worth exploring. A mechanism similar to the one suggested above for wind-mediated pollination could be in operation, with raindrop-impact being the energetic force involved. Acrolophia cochlearis lips are characterised by an interesting basal constriction and fold and this fold might act as a spring-loaded hinge allowing the lip to bend and return to its original position after being depressed.
Although this all sounds quite good in theory, I have not found any reliable way to manually pollinate these flowers using the lip as a medium of transfer, although I have managed to do this occasionally. However, A. cochlearis is the species of the genus with the largest distribution range, a characteristic suggesting that there is no great reliance on any specific pollinator. Rain in the flowering period, September to December, can be expected in both winter and summer rainfall areas, and it would be interesting to see if plants from winter rainfall areas flowered, on average, earlier than those in the summer rainfall areas; something that would make sense if pollination was rain-mediated.
Fig. 9. Acrolophia bolusii Rolfe. Digital scan #2929. G. Russell
Because Acrolophia bolusii (Fig. 9) is the most susceptible to the depredations of a species of Criocerine leaf beetle, plants of this species have presented little opportunity for pollination work to date, as the flowers disappear as fast as they are produced. In many cases, the leaf beetle damage to the plant is so severe that flowers are not produced at all. Thus, as an ancillary pursuit, I am also studying this Criocerine leaf beetle. In the second season (2001/2) following the big burn of early 2000, I first found plants of this species, unfortunately after their flowering had ceased, which had produced occasional seed pods (the fire having temporarily reduced the population of beetles, thus allowing for more normal reproduction). This has led me to suspect that this non-resupinate species, with upright lip, probably has a more normal, (probably insect-mediated) pollination biology. Hopefully, within the next two or three years, the beetle problem will be ameliorated by the balancing action of beetle parasites, allowing more access to suitable plant and floral material.
The Acrolophia capensis complex shows all the signs of being insect-pollinated. The flowers are generally but not reliably resupinate ("mixtosupinate"?), and when resupinate present a landing platform and alignment pigmentation to guide the pollinator. There is a fragrance present during the heat of day, which to my nose could only be described as honey-urine, suggesting fly pollination (also called myophily). There is some sort of reward offered in the spur, apparently within thin-walled cells which line the spur, which is more thick and oily than sweet, once again hinting at fly-pollination. The purpled-green and white coloration of the flowers also supports the idea of myophily; there are no bright, bee or butterfly colours here. Plants of this species complex also exhibit sexual self-incompatibility. For additional self-incompatibility information, the reader is referred to an article by the author in CSA Journ. 4(4): 186-191, July-August, 2004, or the "incompat" listing in the website references.
Fig. 10. Acrolophia capensis (Berg.) Fourcade (w/capsules). Digital scan #2584. G. Russell
In this species, utilising pollen from a different flower gives rise to a capsule filled with seed. Self-pollination results in a thin, malformed pod, and examination of the contents of this pod on ripening reveals the presence of seed coats, but no embryos. Acrolophia seed capsules ripen rapidly, doing so in about 8 weeks following pollination (Fig. 10). The strange behavior of the production of sterile capsules following self-pollination, must be advantageous to the plant, otherwise such they would be shed at an earlier stage, without the apparently unnecessary production of seed coats. Judging by insect damage to capsules in the field, it appears that self-pollinated capsules act as tasty decoys, offering food for voracious caterpillars, which then appear to be more likely to leave good pods alone. Sexual self-incompatibility obviously does not occur in self-pollinating plants.
In attempts to discover what insect(s) may be involved in the pollination of plants of the A. capensis-complex, I have spent many hours sitting around in the midday, mid-summer sun, often with an umbrella in one hand and an insect net in the other, in the middle of a national park. This has unfortunately been almost as unproductive as it has been surreal. I have twice seen flowers visited briefly by those high-speed syrphid flies of the impossible-aerobatic-display type, but have yet to find the knack of catching these six-legged "Scarlet Pimpernels."
Now we get to the part about the horned anther caps. Looking at the anatomy of the flower of Acrolophia capensis, the viscidium of the pollinarium is likely to be stuck to the top of the visiting pollinator (this is known as nototriby), perhaps on its head as it is backing out after a visit. If one removes a pollinarium of A. capensis, the anther cap comes with, fairly intimately attached, and a period of drying of some four to six minutes is required before the anther cap can be shaken free. As the anther cap is likely to interfere with the aerodynamics or vision of the fly, I suggest that the fly is sufficiently irritated to settle and groom, away from the flower from which the pollinarium has been withdrawn. Grooming in house-flies involves dragging the forelegs together across the head and eyes and it is probably reasonable to assume that the same applies in syrphid flies. It would be interesting to determine whether the horns of the anther cap actually lock physically into some part of the foreleg, allowing the fly to remove it "manually". This little distraction probably also causes the fly to lose its train of thought (that is, of course, if flies indeed have any train of thought whatsoever), and when it is in a position to continue with its perambulations, another vector has been set, meaning that the next visit is to a totally different plant from the one recently left, increasing the chances for cross pollination to take place.
Much of the above is supposition and has yet to be validated, indeed, such a method has yet to be worked out in many cases. This project should keep me busy for years, and I hope to keep readers updated regularly.
One of the purposes of this article is to point out that anybody who can take the time to get out in the field, can start studying one or other orchid species or group of species, and after sufficient effort, start seeing and finding and considering things that no-one else has ever seen, found or considered. Common sense will tell you that "orchid twitching," that is, rushing around the fields and trying to see as many species as possible in the shortest period of time, is frequently just a great, big waste of time. Concentrating on a single population of a single species, or even a group of species in a single locality, with frequent revisits over a number of years, is ultimately going to yield valuable data, and additionally is likely to be far more relaxing than "twitching". Those who are interested in their floral heritage should adopt a local species, get to know it in its native habitat, and begin to try to understand it. By this means, one may find something of value to add to the inherited knowledge of the ages.
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Dressler, R. L. 1993. Phylogeny and Classification of The Orchid Family. Portland OR: Dioscorides Press. 314pp.
Kurze, O., and Hilde Kurze. 1990. Acrolophia ustulata--The "Black Orchid." SAOS Journ. 21(2): 39-40 (June)
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Richards, A. J. 1986. Plant Breeding Systems. London: George Allen & Unwin. 530pp.
Russell, G. 2004. Stomatal Guard Cell Measurements Using Leaf Prints. Cymb. Soc. Amer. Journ. 4(3): 137-139 (May-June)
________ . website: http://www.geocities.com/pennypoint9/stomata.html.
________ . website: http://www.geocities.com/pennypoint9/incompat.html.
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