202 



PLANT LIFE. 



of contact now becomes negatively geotropic, and its growth 



on all sides is equally accel- 

 j erated. The coils are thereby 



WJV straightened until the stem 



clasps the support very closely, 

 from which it is often prevented 

 from slipping by angles or out- 

 growths of various kinds, which 

 roughen the surface (fig. 199). 



While gravity thus plays a 

 large part in determining the 



(S^UI IH^IIli position of Dotn aerial and sub- 



^^» tKSSir terranean organs, it must be 



• 1 ^fcaas=^ remembered that it works con- 



B A 



,. . , , , jointly with many other stimuli. 



Fig. 199.-^4, a bit of the stem of the J J J 



hop, showing the six angles, each xhe position of the members 



carrying a row of emergences, crowned L 



by a branched rigid hair with very sharp j s therefore, 2, resultant of the 



points. Magnified 3 diam. />, three ' ' 



emergences more highly magnified.— reactions tO the Various external 



After Kerner. 



forces which stimulate it. 

 292. (d) Hydrotropism. — Hydrotropism is the state of a 

 plant or an organ when it is irritable to moisture. Hydro- 

 tropic organs may bend toward or away from a moist surface. 

 Roots are particularly sensitive to the presence of moisture. 

 If a cylinder of wire gauze be filled with damp sawdust and a 

 number of seeds planted near its surface they germinate and 

 the roots start to grow in the normal direction — i. e., directly 

 downward. If now the cylinder be suspended at an angle, 

 as shown in figure 200, the roots which pass into the air, 

 stimulated by the moisture, curve toward the damp sawdust. 

 Upon entering it the stimulus ceases, and they start again to 

 grow downward, being positively geotropic. Again the 

 stimulus of the moist surface overcomes that of gravity, and 

 they turn back to it, often threading themselves in and out 



