178 



ELEMENTARY STUDIES IN BOTANY 



which is still called micropyle in the seed. It is no longer 

 an open " little gate/' for it has been closed up during the 

 formation of the testa, but it remains the easiest point of 

 exit through the testa. Growth consists of cell-division 

 followed by cell-enlargement. If the cells of the hypocotyl 

 all divide and then each new cell enlarges to the size of the 

 parent cell, the result would be a hypocotyl twice as long and 

 twice as thick. Growth, however, does not proceed so uni- 



t 



FIG. 146. A germination series of the garden bean, showing roots developing from 

 the tip of the hypocotyl, the hypocotyl arch (pulling out cotyledons and plumule), 

 and the straightening of the hypocotyl. 



formly as this throughout a whole structure, and the most 

 obvious result of this early growth of the hypocotyl is its 

 rapid elongation. This elongation speedily brings the tip of 

 the hypocotyl against the micropyle, and then through it, 

 so that the first sign of the embryo outside of the seed is the 

 emerging tip of the hypocotyl. 



As the hypocotyl continues to elongate, it will be observed 

 to curve towards the earth, unless it emerged from the seed 

 in that direction (Fig. 145). This curvature is a response 

 by the hypocotyl to surrounding influences (stimuli), and 



