PLANT LIFE. 



Fig. 199.- 



a bit of the stem of the - 



of contact now becomes negatively geotropic, and its growth 

 on all sides is equally accel- 

 erated. The coils are thereby 

 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 



(-^?H\ 'fffl'''ll^ position of both aerial and sub- 



'^""i Tpfoj^'f terranean organs, it must be 



remembered that it works con- 

 jointly with many other stimuli, 

 hop, .'showing the six angles, each The position of the members 



carrying a row ot emergences, crowned 1 



by a branched rigid hair with very sharp jg therefore, a resultant of the 



points. Magnified 3 diam. />, three 



emergences more highly magnified.- reactions to the various external 



After Kemer. 



forces which stimulate it. 

 292. ((/) 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 surfa< e 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 



