400 Evolution and Adaptation 



to the species ; but even if this is so, there is no need to 

 assume that the response has been gradually acquired for 

 this particular purpose. If it were characteristic of a new 

 form to open at a particular time, and there were insects in 

 search of food at this time that would be likely to fertilize 

 the plant, then the plant would be capable of existing ; but 

 this is quite different from supposing that the plant devel- 

 oped this particular response, because this was the most 

 advantageous time of day for the fertilization of its flowers. 



We can apply this same point of view, I believe, to many 

 of the remarkable series of tropisms shown by plants, whose 

 whole existence in some cases is closely connected with 

 definite reactions to their environment. Let us examine 

 some of these cases. 



When a seed germinates, the young stem is negatively 

 geotropic, and, in consequence, as it elongates it turns up- 

 ward towards the light that is necessary for its later growth. 

 The root, on the contrary, is positively geotropic, and, in 

 consequence, it is carried downward in the ground. Both 

 responses are in this case of the highest importance to the 

 seedling, for in this way its principal organs are carried into 

 that environment to which they are especially adapted. It 

 matters very little how the seed lies in the ground, since the 

 stem when it emerges will grow upward and the root down- 

 ward. The young stem, when it emerges from the soil, will 

 turn toward the light if the illumination comes from one 

 side, and this also may often be of advantage to the plant, 

 since it turns toward the source from which it gets its 

 energy. The leaves also turn their broad surfaces toward 

 the light, and as a result they are able to make use of a 

 greater amount of the energy of the sunlight. The turning is 

 due to one side of the stem growing more slowly than the 

 opposite side, and it is true, in general, that plants grow 

 faster at night than in the daylight. Very bright light will 

 in some cases actually stop all growth for a time. Thus we 



