192 
STUDIES IN GENERAL PHYSIOLOGY 
these animals are compelled to assume a definite orientation 
toward the center of the earth.’ If the growing stem of an 
Antennularian is put into any other than a vertical position, 
oP 
the growing tips bend until the stem again 
stands vertically, after which it grows straight 
upward. The root, upon the other hand, grows 
vertically downward, but not so directly as the 
stem. The stem is negatively, the root posi- 
tively, geotropic. The stem ab (Fig. 38) origin- 
ally stood vertically. I tilted it so that it 
formed an angle cba with the vertical; the newly 
formed piece be grew vertically upward after 
this change in position. I then put the stem 
back into its old position, after which growth 
continued in a vertical direction cd. Fig. 39 
also shows an upward bending of the stem. 
Roots growing downward are shown in W (Figs. 
40 and 41). While the tip of the growing 
stem does not weary of growing vertically 
upward whenever its position toward the 
horizon is slightly altered, the root is 
much more sensitive. It does not bear 
a 
FIG. 38 FIG, 39 
changes in position very well, and I have been able to 
demonstrate positive geotropism only in newly grown 
1 Part I, p. 176. 
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