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 
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FIG. 47 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 
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