126 PLANT PHYSIOLOGY 
ful experiments have shown that it is with reference to 
the direction of the force of gravity that these different 
plant parts orient themselves. 
188. Experiments have shown that by attaching 
plants to a rapidly whirling wheel the centrifugal force 
has the same effect as gravity and stimulates the main 
root growth away from the center of the wheel, while 
the growth of the main stem becomes directed toward 
the center and that of the lateral roots at right angles 
to the radius. On the other hand, if the wheel to which 
a plant is attached be rotated very slowly with its axis 
horizontal so that all sides of the plant are successively 
exposed to the stimulus of gravity, the rotation being so 
slow that the centrifugal force is negligible, the different 
parts of the plant continue to grow in any direction they 
may have happened to start. It is thus apparent that 
the general form of the plant is largely controlled by 
the stimulus of gravity as well as by the stimulus of 
light. 
189. The zone of curvature is that of most rapid 
growth. The perceptive region may, however, be dis- 
tant some millimeters. Thus in the root it has been 
shown that the root cap is the region of greatest percep- 
tion. It has been suggested that the cells there contain- 
ing starch grains are the perceptive cells, the different 
position in the cell assumed by these starch grains in 
response to gravity as the root is pointed in various 
directions furnishing the stimulus which is communi- 
cated from cell to cell to the growing zone. Here cer- 
tain cells on one side are stimulated to grow more rapidly 
than those on the opposite side until the root has assumed 
its proper position, when the starch grains (statoliths) 
will resume their normal position in the perceptive cells. 
The similar starch-bearing cells in the perceptive regions | 
Pry SD ett ¢ 
