32 GENERAL BOTANY 



roots and stems took up an intermediate position which was a 

 resultant of the response of "the plant to the two forces, gravity 

 and centrifugal force, acting separately. 



Knight concluded, therefore, that roots and stems respond to 

 centrifugal force acting as a stimulus. His simple appliances 

 are now replaced by more perfect pieces of apparatus, on which 

 disks can be rotated with extreme rapidity and accuracy. If 

 kernels of germinating seeds of corn are placed on such a disk 



FIG. 17. Diagram illustrating the principle of an experiment by Thomas 

 Andrew Knight (1806) 



a, position of seeds, roots, and plumule (stem) at the beginning of the experiment ; 

 6, position of root and plumule (stem) after rapid rotation. Further discussion in 

 explanation of a and b in the text 



in the positions indicated in Fig. 17, a, and the disk is then 

 rotated for twenty-four hours, the elongating root and stem will 

 gradually assume the positions indicated in b. The roots will 

 all grow toward the circumference of the wheels, while the 

 stems will all grow toward its center. 



By employing a force which he could control and modify, 

 Knight was thus able to show that the root and the stem could 

 be caused to curve and to take up various positions as a result 

 of their response to this force acting as a stimulus. Since seeds 

 germinating in the soil seemed to behave toward gravity as they 

 did toward centrifugal force in his experiment, he concluded that 

 gravity, acting as a stimulus, directed the growth of the root 

 toward the earth's center and the stem away from it, and that 



