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 



