Martin Epstein wrote:
I believe that the defining characteristic, that is, what separates orchids from other flowering plants, is the union of the male and female sexual organs into a single entity, the column... It is very difficult for an insect to remove the pollinia on the way in. It is relatively easy for an insect to remove it on the way out. Cleistogamy aside, this makes self pollination difficult (except with a toothpick).
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.
This is an interesting idea and perhaps someone on the list will have data about the frequency of self vs cross pollination in orchids as compared to other flowering plants. There are several things to consider, however, that will probably skew the results, perhaps preventing a simple correlation such as you propose. First, many orchids are multifloral so pollinators have the opportunity to transfer pollen from one flower to another on the same plant, thereby accomplishing self pollination. Second, since self pollination is disadvantageous, many flowering plants, including many orchids, have developed biochemical, post-pollination barriers that prevent self-pollen from successfully fertilizing the ovules. Third, it has been argued by Dotson and others that the mechanical, prepollination barriers used by orchids (such as the ease of extracting pollen only on the way out) are error prone. Occasional failures of these barriers can allow self pollination and the possibility that a single plant can colonize a new habitat, a possibility that might increase evolutionary diversification.
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Steve Beckendorf Berkeley, California
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