266 Experimental Zoology 



Influence of Gravity on Growth 



It is well known that gravity has an important influence in 

 determining the direction of growth in plants : roots turn and 

 grow downward, stems upward. In most animals, on the con- 

 trary, gravity appears to have no determining influence on 

 growth, although an important influence in the orientation of 

 some freely moving forms. In animals that are fixed we might 

 expect to meet with a response to gravity similar to that in plants, 

 and this has been found to occur in a few cases. It is a matter 

 of general observation that most fixed forms grow at right an- 

 gles to the surface to which they are attached, but in many cases 

 the direction of their growth cannot be due to gravity, for the 

 surface of attachment may be obhque or even vertical. In such 

 cases contact reaction, i.e. stereotropism, or some tropism other 

 than geotropism, must determine the direction of growth. In 

 several species of hydroids, however, it has been shown that 

 gravity determines the direction of growth. Loeb has shown 

 that stolons of Aglaophenia, if they do not come in contact with 

 a soHd body, grow out at first horizontally and then downward. 

 Another hydroid, Antennularia antennina, also responds to 

 gravity. Pieces of this hydroid produce new stems that grow 

 upward, and stolons that turn downward. This is strikingly 

 seen when a piece is put into an obhque position. New stems 

 arise from the upper parts of the old one and stolons from be- 

 neath. Even an inverted piece was found to produce a root 

 from its lower or distal end, and a stem from its upper or basal 

 end. Stevens has shown for A. ramosum that the level at which 

 the piece is cut off is a more potent factor in the result than grav- 

 ity. Driesch observed in a species of Sertularia that whenever 

 he altered the position of the piece the new growth changed its 

 position so that the new part turned away from the center of 

 the earth. 



These are the only cases in which a response to gravity has been 

 recorded. How the response is affected is not known, but it is not 

 improbable that the result may be caused by the rearrangement 



