GEOTROPISM IN ANIMALS 177 



complicate the phenomena of geotropism. Kecently, 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. 



