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 oblique 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 solid 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 oblique 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 isnot 
improbable that the result may be caused by the rearrangement 
