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From its hairy feeder roots below, up through its trunk to topmost twigs and leaflets, its cells are so arranged that they imbibe moisture and, by osmosis (diffusion through membranes), elevate it from the ground to the crown, where air and light can act on it in the system's upper terminals. Evaporation from the leaf pores supplies an added pull to overcome gravity. The action is microscopic in its parts, but it proceeds so fast and constantly, with trillions of cells incessantly functioning as a bucket brigade, that the water pumped up by a good-sized tree may exceed 200 gallons a day. The water is not put forth at the tree-fountain's top in fluid form or even, except momentarily on the leaves' surfaces, as a detectable vapor. Upon arrival in the leaves' external cells the droplets combine with carbon dioxide in the air to form carbohydrates—sugars and starches, which the leaves absorb as nourishment—and oxygen, much of which is released into the air. Oxygen is as vital to fauna as carbon dioxide is to flora. Thus, like all other plants, trees are potent aids to the health of the animal kingdom. They actually filter and enrich our very breath. This service, apart from the fiber and food and fuel that trees supply, may well have been suspected by primitive man and led him to such worshipful imaginings as Yggdrasil, the earth-sheltering, dewdropping Tree of All Existence. Modern man's understanding of plants as air conditioners is more practical. When he ventures away from earth in spaceships he plans to take along some tiny vegetable organisms called algae, to help purify his air supply and solve his food problem. These little "trees of existence" will ride and grow in tanks of water, every drop of which will have to be recaptured and recycled within the sealed vehicle. |
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