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 
geotromic. 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. 
