DISPERSAL AND GERMINATION OF SEEDS 179 



sensitiveness to stimuli is called irritability. The hypocotyl 

 is a very irritable structure, and conspicuous among the 

 stimuli to which it responds are gravity and moisture. 



111. Geotropism. This word means " earth-turning," 

 and it implies that there is a turning (curving) in response 

 to the influence of gravity. 

 It should be understood that 

 geotropism is not the " influence 

 of gravity," as is sometimes 

 stated, but it is the response 

 of the plant to the stimulus cf 

 gravity. If the hypocotyl, 

 when it emerges from the seed, 

 is directed upwards or horizon- 

 tally, gravity acts as a stimulus 

 and the irritable hypocotyl re- 

 sponds by developing a curva- 

 ture that directs it downwards. 

 The stimulus of gravity, there- 

 fore, results in directing the 

 tip of the hypocotyl towards 

 the soil and in keeping it in 

 that direction (Fig. 145). 



This is only one way of re- 

 sponding to the stimulus of 

 gravity, for a stem usually re- 

 sponds by curving away from 

 the earth and by maintain- 

 ing this direction. These two 

 kinds of response are distinguished by speaking of the hypo- 

 cotyl as positively geotropic (its direction being towards the 

 source of the stimulus), and of the stem as negatively geotropic 

 (its direction being directly away from the -source of the 

 stimulus). Even these two directions do not include all of 

 the responses to gravity, for branches of roots and of stems 



FIG. 147. Germination of scarlet-run- 

 ner bean : first stage of series shown 

 in Fig. 148 ; one cotyledon removed 

 to show the arch developed by the 

 first joint of the stem, the stem-tip 

 and leaves thus being freed, and the 

 cotyledons remaining within the 

 testa. 



