GEOTROPISM. 



117 



fluences are, as is well known to you, gravitation, light, heat, 

 moisture, and there are many others of lesser importance.* 



A. Geotropism. 



It is a familiar fact that most land plants in growing bring 

 their main roots to point downwards, their main stems to point 



FIG. 30. GERMINATION-BOXES. One-fourth the true size. 



upwards, and their side roots and stems into intermediate posi- 

 tions. We must now seek to find out what determines those 

 positions. 



k Upon Geotropism, as well as upon other phases of Irritability, an immense 

 amount of valuable experimentation is practicable, enough to fill the entire year. It 

 is needful, therefore, in order to keep the subject within its proper place and propor- 

 tion in this course, rigidly to choose only the most important and illustrative topics. 

 Accordingly those are here selected which seem to demonstrate best the true nature 

 of Irritability and its leading manifestations, especially those of ecological import- 

 ance. Geotropism is much more fully treated than the others, because, by means of 

 it, the different factors concerned in irritable responses may be so well differentiated 

 and determined. 



