ORGANIZATION AND GROWTH 



197 



about one or two weeks. But they do not grow in the old 

 direction obliquely toward the tip of the stem, but instead 

 vertically downward. Nothing of this sort happens to the 

 branches situated upon the upper surface. The newly formed 

 organs which grow vertically downward look exactly like 



FIG. 48 



roots, and that they are indeed such can be proved physio- 

 logically. If they are brought in contact with a solid body, 

 they attach themselves to it and grow over its surface. Be- 

 sides being positively geotropic, they are also positively 

 stereotropic a form of irritability possessed only by the 

 roots, but not by the branches, of Antennularia. Such an 

 Antennularian stem is shown in Fig. 40, in which all the 

 branches of the lower surface have been transformed into 



