THE MOVEMENTS OF PLANTS. 



m 



the stem near the bend is accelerated (whence the stem is said 

 to be laterally geotropic). The horizontal part is thus swung 

 around, twisting the stem and bringing a new flank under the 

 influence of the stimulus. If in its continued rotation the stem 

 comes in contact with a nearly erect support the free part con- 

 tinues to rotate, growing longer at the same time, and encircles 

 the support. The part below the 

 point of contact now becomes nega- 

 tively geotropic, and its growth on 

 all sides is equally accelerated. The 

 coils are thereby straightened until 

 the stem clasps the support very 

 closely, from which it is often pre- 

 vented from slipping by angles or 

 outgrowths of various kinds, which 

 roughen the surface (fig. 131). 



While gravity thus plays a large 

 part in determining the position 

 of both aerial and subterranean Y\a. 

 organs, it must be remembered 

 that it works conjointly with many 

 other stimuli. The position of the 

 members is, therefore, a resultant 

 of the reactions to the various external forces which stimu- 

 late them. 



250. (</) 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 132, the roots which pass into the air. 



a bit of the stem of 

 the hop, showing the six angles, 

 each carrying a row of emergences, 

 crowned by a branched rigid hair 

 with very sliarp points. Magni- 

 fied 3 diam. B, tliree emergences 

 more highly magnified. — After 

 Kemer. - 



