GEOTROPISM IN ANIMALS 177 
complicate the phenomena of geotropism. Recently, how- 
ever, I have discovered geotropic bendings in the growing 
portions of a different Hydroid (Antennularia antennina) 
which are not masked by any secondary phenomena. 
Antennularia antennina consists of a main stem about 
1mm. in diameter, and often 20cm. long, which usually 
arises perfectly perpendicularly from a felt-like mass of very 
fine rootlets. From the main stem spring in regular order 
very delicate, short, unbranching lateral twigs, upon the 
upper surface of which are found polyps and nematophores. 
If such a stem is placed in any position deviating from the 
vertical, the tip of the stem, if it grows at all, bends sharply 
back toward the vertical and continues to grow vertically 
upward. Only the newly growing portion of the tip is able 
thus to change its orientation. If the orientation of the 
animal is not again altered, the stem grows absolutely ver- 
tically upward; it is negatively geotropic. The roots, upon 
the other hand, grow vertically downward; they are positively 
geotropic; yet the direction of their downward growth is not 
so perfectly straight as that of the upward-growing stem. 
As often as the orientation of the stem with reference to the 
vertical is changed, if any new growth whatsoever occurs, 
the stem bends toward the vertical and grows upward in this 
direction. 
But not only the orientation of the organs, but also the 
place where new organs originate, is dependent to a large 
extent upon gravitation. But I will speak of these facts at 
another place, and publish therewith the pictures necessary 
to illustrate them. 
These curvatures during growth are independent of the 
light. They occur equally well whether the stems are grown 
in the dark or the light room; when cultivated in the light, 
so far as I have been able to see, their orientation is not 
affected in the least by the direction of the rays of light. 
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