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 eases 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 ee ; 
brings it back into line. It is also ee ae eerie 
important to note that the sensi- pletely inverted, and after 
wo days photographed. 
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 
