90 A TEXT-BOOK OF BOTANY 



ences, a condition that is called irritability. The outside 

 influences that affect irritable organs are called stimuli; 

 for example, among animals light is a stimulus to the 

 eye. 



52. Geotropism. The young root, developing at the 

 end of the hypocotyl, is very sensitive to gravity, a con- 

 dition that is called geotropism, the root being said to be 

 geotropic. The word means "directed by the influence of 

 the earth," what is commonly called gravity acting as a 

 stimulus. If the root-tip, when it pushes out of the testa, 

 is directed upward or horizontally, gravity acts as a 

 stimulus and the irritable root responds by developing a 

 curvature that directs it downward (Fig. 83). This is 

 only one way of responding to the stimulus of gravity; 

 and since this way directs the organ toward the source of 

 the stimulus, the organ is said to be positively geotropic. 

 If the same stimulus and response that directs the root-tip 

 toward the soil continues to direct it within the soil, it 

 continues to grow directly downward and becomes a tap- 

 root (Figs. 68 and 89). When such a root, having en- 

 tered the soil, begins to send out branches, these do not 

 respond to the stimulus of gravity as does the tap-root, 

 for they extend through the soil in every direction, and 

 are evidently not positively geotropic. 



53. Hydrotropism. The root is very sensitive also to 

 the presence of moisture, a condition that is called 

 hydrotropism, the root being said to be hydrotropic. The 

 word means "directed by the influence of moisture," the 

 moisture acting as a stimulus, and the root being positively 

 hydrotropic. Since ordinarily the stimuli of moisture and 

 gravity act from the same general direction upon the root, 

 the responses are not contradictory. It is of interest, 

 therefore, to arrange an experiment that will make them 

 contradictory. An erect support, shaped as shown in Fig. 

 84, is covered with bibulous paper which is kept moist. 



