202 



PLANT LIFE. 



of contact now becomes negatively geotropic, and its growth 



on all sides is equally accel- 

 II _ 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 jiart in determining the 

 position of both aerial and sub- 

 terranean organs, it must be 

 remembered that it works con- 



E A 



,. , , , , jointly with many Other Stimuli. 



Fig. 199. — A, a bit of tlie stem of tlie- ^ 



iiop, showing the six angles, each 'phe position of the members 



caiT^^mg a row of emergences, crowned 



by a' branched rigid hair with very sharp jc; therefore, a resultant of the 



pomts. Magnified 3 diam. />, three ' 



emergences more highly magnified.- j-gjj^tions tO the VarioUS external 

 Alter 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 suriace. 

 Roots are particularly sensitive to the piresence of moisture. 

 If a cylinder of wire gauze be filled \\ith damp sawdust and a 

 number of seeds planted near its suriace they germinate and 

 the roots start to grow in tlie 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 liy the moisture, curve toward the damp sawdust. 

 Ui-ion entering it the stimulus ceases, and they start again to 

 grow downward, being positi\ely geotropic. Again the 

 stimulus of the moist suriace o\'crcomcs that of gra^itv, and 

 they turn bat k to it, often threading themselves in and out 



