GERMINATION OF SEEDS 



93 



appearing in the word "photograph." The term helio- 

 tropism is often used, meaning "directed by the influence 

 of the sun "; but while the sun is the usual source of light, 

 it is not the only one. It should be noted that it is not 

 light in general that acts as the stimulus, but the direction 

 of the rays of light. The response 

 of the stem to this stimulus is to 

 turn directly toward the source of 

 the light rays; that is, the stem is 

 positively phototropic. Fig. 86 shows 

 a bean seedling that was placed in a 

 horizontal position and two hours 

 afterward photographed. Fig. 87 

 shows the same plant completely 

 inverted, allowed to grow for two 

 days, and then photographed. In 

 both cases the strong curvature 

 developed in response to the stimu- 

 lus of light is very evident, the tip 

 of the stem in both experiments 

 being directed toward the source 

 of light. 



It should be remembered that 

 these stimuli that influence direc- 

 tion call forth a response only when 

 the organ is out of line, and the 

 response or reaction is a curve that 

 brings it back into line. It is also 

 important to note that the sensi- 

 tive or irritable region of an organ 

 is not necessarily the region in which the reaction occurs; 

 and this means that the stimulus has been transmitted in 

 some way from the irritable cells to those that respond, for 

 example, by developing a curvature. Nor does the reac- 

 tion follow the stimulation immediately; for there is an 



FIG. 87. The same seedling 

 shown in Fig. 86, com- 

 pletely inverted, and after 

 two days photographed. 



