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How Did the Orchids Get That Way? March Meeting Recap, Part Two
by SEAN EATON
Dr. Warren H. Wagner was our guest speaker at the March meeting. He began his talk with an overview of how the structure of flowers may evolve over time, from simple and undifferentiated blooms to the highly specialized and complex forms we see in orchids. Although many have the perception that orchids are delicate and rare, they are in fact one of the largest and most successful group of plants in the world. Why should this be so? Probably the answer lies in their tremendous diversity of form and adaptation. What follows is a summary of Dr. Wagner's comments regarding adaptations specific to orchids.
Roots: The roots of orchids lack the root hairs that most plants use for obtaining nutrition from the soil. They have delegated this task to fungi in a special, (symbiotic) relationship called mycorrhiza. The fungi live on or in the roots of the orchid and allow it to process the nutrients it needs. In fact, so close is this relationship that orchid seeds require the presence of the fungus in order to germinate. (Dr. Wagner later showed slides illustrating how seeds are penetrated by the hyphae of the fungus, which dissolves the seed capsule and frees up nutrients for use by the few plant cells that form the embryo of the orchid.) One theory about plant evolution holds that the first land plants relied on micorrhiza to serve as root hairs.
Leaves: New shoots develop under leaves to form pseudobulbs, topped by leaves with parallel veins. This is a characteristic growth pattern of orchids and other monocots like palms, grasses, and lilies.
Flower structure: Dr. Wagner went into considerable detail about the organization of the orchid flower, discussing and showing slides to illustrate such parts as the tepals, pollinaria, caudicles, stigma, and spur. Tepals, which refers to petals and sepals that appear nearly identical, is a characteristic also shared with other monocots, like lilies. The lip or labellum of the orchid is especially interesting because of its elaboration and specialization. The lip of the orchid bloom is originally on the top, but when ready for pollination, the flower resupinates, (that is, twists)--no other flower does this. Presumably, this is to provide a "landing pad" specially evolved to attract certain pollinating insects. When the orchid flower resupinates, the ovary becomes inferior, with the flower parts above to form a "glove".
Seeds: Orchid seed is very tiny, with as much as 2 million seeds per ovary. The embryo within the seed is only a couple of cells big in size, perhaps at most 5-10 cells. When this "pro-embryo" germinates with the help of symbiotic fungi it "turns into something awful--globules of goo" and is initially Continued next page nonphotosynthetic, that is, unable to generate its own food in the presence of light, as most advanced plants are able to do. This was an area of special interest for our speaker--the special adaptations of size, number, and reliance on fungi that allow for widest dispersal.
Pollen: Orchids put "all their eggs in one basket" so to speak, by keeping their pollen organized as two to four globular masses of 1-2 million pollen grains, held in such a way that they adhere to visiting insects who carry it to the next flower. Effectively, this means that the flower can supply pollen on only one visit.
Hybridization: Orchids can hybridize very easily, with relatively little limitation of compatibility between species and between genera. There are over 16,000 hybrids, and at least 160 different genera have been hybridized. Most hybrids do not of course occur in nature, because the specialization of the flowers limits the pollinators that can visit them in the wild--but this is easily circumvented by horticulturists in the lab.
Dr. Wagner showed slides of a series of plants showing the development of flower structure from the "original monocot", a flowering rush, through tulips, Easter lilies, lily of the valley, amaryllis, iris, and finally representative orchids. He also showed an example of what may have been one of the earliest orchids, a nonresupinate flowered pickerel weed. Some other interesting orchid examples included an albino fringed orchid that relies on mycorrhiza instead of photosynthesis, a frog orchid, (Polyrrhiza lindenii), and best of all, a type of praying mantis from Borneo that had evidently co-evolved with the particular species of orchid which it closely resembles and on which it lies in wait for its prey.
Dr. Wagner left us with many questions to ponder and new areas to explore in coming to understand these wonderful plants.
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