Page:Popular Science Monthly Volume 74.djvu/585

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The force of gravity, which enables a man to stand erect, defeats the unbalanced efforts of the baby learning to walk; it directs the growth of root and shoot from the sprouting seed; it forces organs and organisms to carry a very large proportion of their own weight or perish. When plants and animals live in water, over seven hundred and fifty times as much of their weight is carried for them as if they were growing in air. When plants are supported on trellises, or grow on trees, their mechanical strength, the development of their supporting tissues, corresponds to the lessened load. The buoyancy of the water and the mechanical support of trellise or tree, opposing the pull of gravity, modify and reduce its formative influence.

If we compare the great brown kelps growing along the rocky parts of the Pacific Coast, attached by hold-fasts to the bottom, floating upward and along the surface of the ocean till they become the longest plants known, with land plants, we find only some trees and such vines as the rattan at all approaching them in length. But in structure and in mechanical strength, what a difference there is! Bring the seaweed ashore and try to stand it up; take the vine down from its support; neither will be able to sustain its own weight. The force of gravity opposed, in the one case by water, in the other by the forest trees on which the rattan grows, has not exerted its full influence on either. The tree stands, and its trunk is composed of mechanically strong tissues, in part at least because of the pull of gravity only feebly opposed by the air.

Experiment proves the formative influence of gravity. The English gardener who trains his peach trees on the southern face of a wall knows well that such trees are mechanically weaker, though more prolific, than other peach trees growing unsupported in the same enclosure. The delicate stalks of the blossoms of apple, peach or prune, thicken and strengthen as the fruit sets, grows and ripens, the increased pull of gravity stimulating the living stalk to meet the greater strain by greater strength.

When we take into account the fact that the force of gravity acts constantly, that though we ordinarily ignore it or take it unthinkingly for granted (as we do the quality of our milk), it is an unchanging force, the same night and day, from season to season, from cycle to cycle, we begin to realize that it must exercise a formative influence of the utmost importance on all living things, stimulating the growing plant and animal to develop an adequate skeleton and to attain a balance of parts which will tend to stability.

Water opposes the force of gravity by buoying up, and carrying so large a fraction of the weight of, the creatures living in ponds, streams and the sea. In addition, it has a positive influence of its own. We are used to the directive influence which causes the wild creatures of field and forest to make the runways between den or nest and water-