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Tree Flowers
| Article
# : |
11797 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
4 / 1987 |
854 Words |
| Author
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Frederick D. Atwood
Frederick Atwood teaches biology at St. Stephen's School
in Alexandria, Virginia, and has been a nature enthusiast
since childhood. |
Trees must overcome a number of problems if they are to reproduce successfully. They cannot search for mates, set up territories, or perform courtship displays as animals can. Unlike their evolutionary ancestors (and today's algae, ferns, and mosses), their male sex cells can't swim from one plant to another in search of eggs. The majority of temperate-zone trees have neither a scent nor flashy petals to attract pollinators, and they rarely have nectar rewards. So pollinating insects generally avoid them. In addition, since the tree's flowers are reproductively mature for only a few days, the blooming needs to be synchronized with that of others of its species so cross-pollination can occur.
How does a tree solve these problems? Timing of flowering is easily solved. Because there are definite seasons in the temperate zones, the blooming of trees can be synchronized by specific cues in the changing spring environment: the lengthening day, the warm weather, and the abundant rain. The long, cold winter has destroyed the inhibitors that prevented the premature sprouting of the buds during a winter warm spell, enabling them now to burst out of their imprisoning scales.
The other problems are solved by wind pollination - the curse of those humans who suffer from hay fever. Wind-pollinated (anemophilous) trees like oak, ash, maple, elm, willow, birch, aspen, poplar, sycamore, hickory, and all the conifers, which include pine, spruce, fir, cedar, and yew, are wonderfully adapted for this process.
The first major adaptation is the lack of petals. Petals would not only be a waste of energy and matter but would also block the removal of pollen from the male flowers and the reception of pollen by the female. Such tree flowers, though abundant and ubiquitous, are inconspicuous to the untrained eye. Some types blossom unnoticed along the bare branches of a tree before the leaves come forth. Others hang from the newly emerged leaves in the form of long, dangling spikes of unisexual flowers called catkins or aments. With this particular formation the wind can whip through the trees and carry the pollen much further, improving the tree's chances of contributing its genes to the next generation.
If a tree drops its pollen when there is no wind, the pollen will not travel far enough and will be wasted. Rain is also a problem. It washes the pollen out of the air within a short distance of a parent tree. To decrease the occurrence of these problems, the openings in the sides of birch and alder aments are obstructed by scalelike structures.
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