Page:Popular Science Monthly Volume 18.djvu/92

82 integration. The organisms which subsist upon it are functionally low, the forces which might have been otherwise employed being used up in the formation of protoplasm and other high organic compounds.

The conditions of alimentation limit the vegetable world to one general form. Plants do not need to seek food; food seeks them. Thus no motive powers are requisite, and they remain fixed in one position. But food seeks them in two different localities, the earth and the air. The earth constituent comes to them dissolved in water, the atmospheric constituent dissolved in air. They must, therefore, have powers of extension sufficient to occupy both these fields. This power is obtained by growth, root-extension seeking the liquid food, leaf-extension the gaseous food.

These general conditions confine plants to one generic form, a connecting link of stem between earth and air, and an extension of root-mouths into the soil and of leaf-mouths into the atmosphere. A tree is a society, or family; the main stem being the patriarch of the flock, the earth and air branches its descendants, and the leaves and rootlets its latest unisexual offspring.

As the type of animal form is the stomach, so the type of plant form is the mouth. It has not yet developed into the formation of a central stomach, nor has it attained powers of digestion. It builds up protoplasm by successive steps of chemical integration. It is a laboratory for the production of chemical synthesis—not of chemical analysis, as in animals.

Thus the food-taking requisite is provided for in the production of numerous leaf and root mouths, extending themselves into the two great reservoirs of food. But defense must be provided for as well. Plants are attacked by various foes. Fierce storms assail them. These can only be resisted by an inmate strength or elasticity. Wintry cold congeals their food-supply. They must therefore be capable of hibernating. Animals seek to devour them. They can only escape by inclosing themselves in a rigid armor, or by becoming unfitted for animal food.

This leads us to the most significant adaptation in plants. Their life duration is limited, and they must have powers of reproduction, the best adapted in this respect crowding out the less adapted. Obviously the seed-bearers are best fitted for survival; and of these, those bearing the most seeds, and having the best facilities for dispersing them.

But the fixed plant can not, of itself, spread its seeds beyond its own locality. It must be aided by other agencies. Many plants avail themselves of air-currents for this purpose, the seeds being provided with curious appendages to aid them in flying or rolling before the winds. In other cases the seed is surrounded by a store of palatable food, offering an inducement to the higher animals to devour it, and thus to disseminate the seeds.