DISPERSAL AND GERMINATION OF SEEDS 185 



way, all structures of the stem are not positively phototropic, 

 for many branches grow horizontally, and leaf blades are very 

 commonly horizontal. When it is remembered, however, 

 that groups of stimuli act on organs, and that not all of them 

 may be influencing the organ in the same direction, it will 

 be understood that the actual direction is a resultant and 

 not necessarily the direction that would be determined by 

 any stimulus acting alone. 



It should be kept in mind that stimuli which influence direc- 

 tion call forth an evident response only when the organ is out 

 of line, and the response (reaction) is a curve that brings it 

 back into line. The sensitive or irritable region of an organ 

 is not necessarily the region where the reaction occurs ; 

 for example, the root tip is the sensitive region that " per- 

 ceives " the stimulus, but the reaction appears in a curve 

 at some distance from the tip. Nor does the reaction follow 

 the stimulation immediately, for there is an interval, known 

 as reaction time, which is generally much longer in plants 

 than in animals. The reaction may be several hours, but 

 in some cases it may be very short, as the movement of the 

 leaves of the sensitive plant ( 125, p. 213), and the snap- 

 ping shut of the leaves of Dioncea ( 127, p. 222). 



116. The hypocotyl. Any study of the germination of 

 the seed impresses the fact that the hypocotyl is the most 

 important organ. It is distinctly an organ of the embryo, 

 for its work is over when germination has been completed. 

 Its importance lies in the fact that it relates the seedling 

 effectively to its surroundings, that is, it " orients " the seed- 

 ling. It puts the root in the soil and it often liberates the 

 plumule so that the stem may begin to develop its succes- 

 sion of leaves in relation to air and light. When the seedling 

 has thus been started in the right directions, the hypocotyl 

 disappears as a distinct structure, and the plant body con- 

 sists of root, stem, and leaves. 



117. Summary. The chances of the successful germina- 



