Marty Epstein wrote: >I speculate that this makes cross pollination more common in orchids >than in most other flowering plants leading to a broadening of the >gene pool, rapid evolution and the fantastic diversity we see in >orchids today. > >Comments, anyone????
You make two points, one of which is about the separation of the orchids from the liliales, and the other about orchid diversity. Here's my take on this. Orchids are indeed sexual oddities. Laying aside the technicalities, what makes them most odd is (1) the enormous number of seeds which most produce from a given capsule and (2) the strange solution at which they have arrived for providing the male gametes for these seeds. A Cymbidium, for example, may drop three million seeds from a capsule: each of these needs an ovum and a pollen gamete to fertilize it. That means that three million gametes need to get to base, more or less all at once and with good repeatability. The solution is to pack these into one to eight pollinia, stick these onto an animal and give this such a fantastic experience - anthropomorphising heavily, but in technical speak they need to deliver deep and fast imprinting - that it will go to great lengths to repeat this. The bearer of the pollinia cannot go to just any old orchid, but to exactly the one species from which the pollen came. Many pollinators are species-specific, both others - bees, humming birds - are not. The odds are shortened when the species flowers all at once and are closely co-located; and much lengthened when they are dispersed and flower at random. Many tropical orchids adopt the high risk end of this spectrum, whilst most temperate plants seem to go for the cluster-and-synchronise solution. I suggest that this strategy is responsible for much of the variation which is observed. The plant needs a flower, habitat and niche which is both attractive to its pollinator and sharply differentiated from other species with which it might be diluted. There are swarm-forming species - such as Arundina - but most do not and are, in fact, tightly segregated. Many closely related sympatric species seem to keep themselves aloof from each other - one sees a dozen cane stem Dendrobiums intertwining their roots in the Himalaya, but almost no hybrids. Where else drives speciation in orchids? They are habitat and niche plants, not prone to forming vast tracts as are, for example, the grasses. They are opportunists, grabbing niches when they open and wasting huge numbers of seeds that fall on barren ground. (Remember Darwin's calculation on Orchis maculata: if all the seeds from one plant succeeded and were evenly distributed, then they would cover an acre in the first generation, carpet the isle of Wight on the second and the planet on the third.) Additionally, there is a secular rise in the number of species found in any tract of land as one moves from the pole to the tropics. Why this is so is unknown, but more diversity implies more niches and potential pollinators, which drives speciation. Note that this is not necessarily an adaptive response, but rather a consequence of the overall engine that drives genetic drift. The more niches, the more specialised an baroque a set of reproductive organs the plant will need if the precious pollinia are to be delivered on time and on target. The feedback is self-evident, and the result baroque. _____________________________________ Oliver Sparrow Tel: UK (0)20 7736 9716 www.chforum.org www.treknepal.org www.datafreeze.com
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