CHAPTER VI 

 ADJUSTMENT TO GRAVITY, CONTACT, AND SHOCK 



156. The relation of the plant to gravity. Gravity differs from 

 all the factors previously considered in being constant and in 

 affecting all plants essentially alike. Although it occurs in every 

 habitat, exerting a profound control upon the relation of root, 

 stem, leaves, and flowers, no essential differences between stemmed 

 plants arise from its action. This is an immediate result of its 

 constancy, and consequently under normal conditions gravity 

 has no power to produce modification. In fact, the control exerted 

 by it is stabilizing rather than modifying. The first terrestrial 

 plants in all probability possessed flat thalloid bodies. Through 

 the action of two opposite media, air and soil water, the thallus 

 became differentiated, a change further emphasized by the different 

 light intensity at the leaf and the root surface. Any tendency 

 upon the part of the leaf surface to grow upward, or to become 

 upright, tended to increase the light energy available, and the 

 downward growth of the hair-like roots increased the water supply. 

 Plants that thus became polarized were placed at a great advantage 

 over the thalloid forms. They were doubtless the ancestors of 

 the vascular plants. It does not seem probable that gravity 

 played a considerable part in producing the polarity shown by 

 stemmed plants. As this habit became more and more fixed, 

 however, it necessarily acquired a constant relation to gravity. 

 The roots grew downward in line with the pull exerted by it, while 

 stems grew constantly in opposition to it. After countless genera- 

 tions, the relation has become so firmly established that the control 

 exercised by gravity is much greater than that of light. 



157. Geotropism. The relations of plants to gravity are com- 

 l^rised in the term </eotropism. The actual bending or turning o( 

 an organ in response to gravity is evident only when the normal 



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