180 



ELEMENTARY STUDIES IN BOTANY 



are very commonly directed horizontally, so that they are 

 neither positively nor negatively geotropic. 



112. Hydrotropism. This word means " water-turning," 

 and it implies that there may be a curvature in response to 

 the influence of moisture. The hypocotyl is sensitive to 

 moisture and is positively hydrotropic. Since ordinarily the 

 stimuli of moisture and gravity act upon the hypocotyl 



FIG. 148. Germination of scarlet-runner bean, the series following the stage shown in 

 Fig. 147; the elongating first joint of the stem frees the stem-tip and leaves and 

 finally straightens ; the cotyledons remain within the seed coat. 



from the same general direction, they cooperate in the 

 result, but experiments can be devised to make them 

 contradictory. 



113. Development of roots. When the hypocotyl pene- 

 trates the soil under the joint directive influence of gravity 

 and water, the root system begins to develop from its tip 

 (Fig. 146). In some cases the root continues the direction 

 of the hypocotyl, being positively geotropic, resulting in what 

 is called a tap-root; in other cases it branches at once in every 

 direction, and is not at all positively geotropic in its responses. 

 In any event, so far as seed-germination goes, the root an- 



