GEOTROPISM 275 



to abate, and in autumn it gradually declines to a 

 minimum prior to entire arrest in winter. 



We know that light has a most important chemical 

 value for plant-life, that in its absence green plants 

 cannot assimilate carbon; but it has also an influence 

 which produces mechanical results, among which re- 

 tardation of growth must be noted. All other condi- 

 tions of growth being equal, it is certain that it is greater 

 by night than by day. Plants which grow in feeble 

 light are as a rule taller and have larger leaves than 

 those of the same species that grow under conditions of 

 intense illumination. Window plants exhibit a well- 

 known mechanical result of one-sided illumination. 

 They bend towards the light, and the curvature of the 

 stems is due to retarded growth on the side facing the 

 light, and the more rapid growth of the side which is 

 not strongly illuminated. The bending of the stem has 

 the happy result of inclining the leaves towards the 

 light, so that it may exert its chemical influence to the 

 fullest possible extent. Thus, assimilation proceeds at 

 full speed by day, when growth is 'slow, and growth 

 advances apace by night, when assimilation is arrested. 

 Heliotropism (Gr. helios, the sun; tropos, a turn) is 

 the term applied to the movements of plants in relation 

 to light. The stalk which bends towards the light dis- 

 plays positive heliotropism, and the root, which seeks 

 darkness rather than light, is negatively helio tropic. 



Geotropism (Gr. ge, the earth; and tropos), or the 

 natural tendency of the root to grow downwards and 

 penetrate the earth, is at any rate in part due to gravity. 

 The upward growth of the stem and the downward 

 growth of the main axis of the root is in line with the 



