FORMATIVE INFLUENCES 581 



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 organ- 

 isms to carry a very large proportion of their own weight or perish. 

 Wlien 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 up- 

 ward 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 sea- 

 weed ashore and try to stand it up; take the vine down from its sup- 

 port; 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 Eng- 

 lish gardener who trains his peach trees on the southern face of a wall 

 knows well that such trees are mechanically weaker, though more pro- 

 lific, 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 bal- 

 ance 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 runwavs between den or nest and water- 



