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 atime. Thus we 
