Field of Science

Pollination biology in the greenhouse

Every semester our greenhouse has one very busy week that is the result of all the students taking non-majors biology have a laboratory exercise involving the use of floral syndromes, suites of floral characters used to determine the likely pollinators of different flowers. Sometimes the plants cooperate by flowering, and sometimes they don't. Sometimes the students cooperate by being observant, and sometimes they don't. Probably no more than one in ten of the students has any appreciation of this experience, but for a few it's revelatory, in this case because of the cooperation of a few stunning plants. Here's one of my favorites, a star flower (Stapelia), a succulent member of the milkweed family. The flowers are large, some 10-15 cm diameter in this species, and the leathery corolla is hairy, sort of dried-blood colored, and has the wonderful floral fragrance of carrion. Look closely and you'll see that even some of our temperate zone flies are quite fooled by this mimicry, and indeed, eggs were deposited and young maggots (those little oblong whitish things) were looking for what their mother thought was carrion in vane because the flower does not deliver the goods promised. No brood substrate exists, but in looking for a place for their babies, pollen gets picked up and moved from fake carrion flower to fake carrion flower. Very astute students in more advanced classes may think to ask, "Why doesn't evolution work to alter the fly's perception such that they avoid fake carrion because any fly that does is not going to waste their reproductive potential. Well, they understand the concept OK, but what does it matter when flies are so numerous and carrion flowers so rare in comparison. The few flies involved, even if under very strong selection, will not affect the much larger gene pool to which they belong. In other words the plant can afford to fool a few flies without risking the loss of their pollinators from natural selection. Star flowers are nice houseplants, so long as you don't over water them, and they don't bloom! And yes, a few students were actually impressed, and extremely pleased that they correctly diagnosed fly pollination, and actually saw it in action.

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