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|Vol. 7(9), pp. 4-7||The McAllen International Orchid Society Journal||September 2006|
Associating the term parasite with orchids usually brings forth howls of protest, followed by a lecture on epiphytism. However, it is reported that in Latin America, the word "parasitas" is commonly used to refer to epiphytic orchids. On the other hand, the chlorophyll-deficient, so-called "saprophytic" orchids really do not satisfy the strict definition of saprophytism, which is the quality of using dead or non-living organic material as a food source. These latter orchids are in fact parasitic on saprophytic fungi and I suspect that very many other orchids are to some degree parasitic on their mycorrhizal fungi.
There are quite a number of plants which are parasitic on other plants; mistletoe and witchweed being examples. As far as I am aware however, orchids are never directly parasitic on any other higher plants, and so far there are no reports that I can find of any instance of a higher plant parasitising an orchid.
Now for a change of tack, the family Lauraceae contains 2000 plus species in 50-60 genera. Members of this family are generally aromatic shrubs and trees with insignificant flowers and fleshy fruit. Some of the better known members of the family are the avocado, the camphor tree, cinnamon, sassafras (for root beer!) and Lindera, the spicebush. However, the "prime" member of this family is the Bay Laurel (Laurus nobilis), famous as the provider of bay leaves for cooking, also a useful hedging plant, and the source of branches from which laurel wreaths were created, the latter widely used in Greek and Roman times as crowns for victors. The word "laureate," as in Nobel Laureate or Poet Laureate, was derived from this latter practice, and "resting on one's laurels" is another concept for which we can thank the bay tree (while resting on our laurels?).
In South Africa, Lauraceae is a poorly represented family, our most famous example being Ocotea bullata, a now rare tree celebrated for yielding a fragrant wood, much in demand for the production of furniture, especially linen chests, and having the common, if unfortunate name of "stinkwood."
Fig. 1. Cassytha ciliolaris painted by Elsie Garrett Rice (1950). Used with permission of the Botanical Society of South Africa, with thanks.
More interesting, however, is the totally anomalous genus Cassytha (Fig. 1), which comprises 20 species, 15 of which are Australian, with a few in Africa and Asia and one pantropical species (C. filiformis). Three of these are to be found in South Africa. This genus is the only one which is parasitic in this family, and surprisingly looks for all the world like dodder (Cuscuta species). As Cassytha is usually capable of some photosynthesis and hence food-production, it is classified as a hemiparasite. In contrast, Cuscuta, when attached to a host, is virtually chlorophyll-free and is thus almost a holoparasite.
Fig. 2. Acrolophia capensis growing in a tuft of grass and parasitised by Cassytha ciliolata. Here, two leaves have been affected, and the bulges indicating haustoria can be clearly seen.
Cassytha ciliolata is a Western and Southern Cape endemic, and is commonly found on Slangkop--the hilly area behind my home--usually above an altitude of about 80 m. Here it is to be found scrambling through the fynbos, wrapping its tendril-like stems around anything in its path (Fig. 2). The list of host plants is a long one, including ericaceous, proteaceous, and composite shrubs as well as restios, the rush-like plants so characteristic of the Cape Flora. It is a leafless plant (occasional scales along the stem represent the remnants of the leaves) with thin, stringy, greenish-yellow stems, varying in thickness from 1.2-2.5 mm., which readily form haustoria on coming into contact with other plants. These haustoria penetrate into the host's tissue to seek out its vascular tissues.
Fig. 3. Photomicrograph of a print of the surface of a stem of Cassytha ciliolata, made according to the leaf print technique of Ferry (Ferry 2000).
The stems of Cassytha are furnished with a very large number of stomata, so that this plant is able to transpire rapidly, thereby creating the necessary negative pressure to "suck" the nutrient-containing fluids out of the host plant. The arrangement of these stomata is an interesting feature. The stomatal openings are elongated transversely and the stomata are found in rows running along the length of the stem, adjacent stomata virtually touching one another (Fig. 3). Although there may be some irregularity in this pattern, there are essentially 10-20 rows of stomata around the circumference of each stem. (Determining this was a difficult task, and was accomplished by rubbing stems with finely-divided activated charcoal and then wiping off the excess; the charcoal acting as a disclosing agent. The stem was then examined by reflected light under the microscope, and the count was made.) This unusual arrangement of stomata exists for some reason; one that may be suggested is that they play an important part in the host-recognition system of this parasite, which probably uses the stomata to "sniff out victims."
In common with other members of the Lauraceae, Cassytha has small insignificant flowers, and from these flowers, small, red, fleshy berries develop following insect-mediated pollination. The ripe berries are harvested by fruit-eating birds, which tend to eat the fruit pulp from around the single seed and drop the latter to the ground. It is possible that residual fruit pulp or some other specially developed material adhering to the seed is attractive to ants, causing them to collect the seed with resultant transferal underground. The seeds are long-lived and require the prolonged effect of bacterial action, or possibly heat, on the tough seed coat before germination can be initiated.
Following germination, once the seedling has established its primary haustorium, the connection with the soil is lost. As this plant has no subterranean organs, it is completely destroyed by the occasional fires which sweep through the fynbos*, and following one of these, the banked seeds commence germinating to create the next generation of plants.
Having now created the necessary context, I can describe my findings one late spring day in 2003. Following the major fire on Slangkop in early 2000, I identified quite a large number of plants of the orchid genus Acrolophia, which I have been studying, and on which I have written recently (Russell 2005). For some time after this fire, Cassytha was only seen in small patches, but the reasonably good winter rains of 2002 and 2003 promoted the rapid growth of plants of Cassytha now feeding upon well-watered and hence larger shrubs. By spring 2003, extensive mats of the parasite were present and in one particular area, the stems were coming into contact with plants of Acrolophia capensis. On closer examination it was found that one Acrolophia plant had Cassytha stems wrapped around its leaves and what appeared to be haustoria had developed. I harvested a leaf, took it home and proceeded to make some rather crude, hand-cut sections for examination under my microscope.
Fig. 4. Photomicrograph of a hand-cut section of one of the parasitised leaves of the Acrolophia capensis, with the plane of the cut made through one of the haustoria, the latter seen here as the dark mass of tissue which has grown toward the light-coloured vascular bundle of the leaf from which it is surely drawing nutriment.
These sections, although not beautiful, demonstrated well the haustorial tissue growing within the tissue of the Acrolophia leaf and impinging on its vascular apparatus (Fig. 4). Rather more complicated work would need to be undertaken to demonstrate conclusively that there was actual transfer of nutrients taking place between the orchid and the cassytha, but I am satisfied to a high degree that this would in fact be the case.
So perhaps here we have the first reported case of an orchid being parasitised by another higher plant? Let me know if I am wrong.
Visser, J. 1981. South African Parasitic Flowering Plants. Cape Town & Johannesburg: Juta. xiv & 178 pp.
Russell, G. 2005. Acrolophias on Slankop--Part 1. MIOS Journal 6(5): 9-16 (May 2005). http://www.miosjournal.org/journal/2005/05/AcrolophiasOnSlangkopPartI.html
Ferry, R.J. Sr. 2000. Leafprints and Statistical Analyses: Findings and Implications. Orchid Digest 64(1): 21-6 (Jan.-Feb.-Mar. 2000).
Rice, E. G. & Compton, R.H. 1950 Wild Flowers of the Cape of Good Hope. Kirstenbosch: Botanical Society of South Africa. 24, CCL & vi.