THE PLANT BODY 



33 



the opposite positions assumed by these organs on the earth's 

 surface were due to gravity acting as the stimulating agent. 



It is a matter of common experience that if germinating seeds 

 are placed horizontally in soil or with the root pointing upward 

 and the stem downward, the growing organs turn and adjust 

 themselves to grav- 

 ity, as they do to 

 centrifugal force on 

 rotating disks. 



Lateral organs such 

 as branches and lat- 

 eral roots have also 

 been found to be gov- 

 erned very largely, 

 in the position which 

 they finally assume, 

 by gravity, although 

 other forces are often 

 influential in the ulti- 

 mate adjustment. 



Knight's early con- FlG< lg> Di agrams illustrating the principle of an 

 elusions were proved experiment by Julius von Sachs (1879) 



to be correct by the 

 great German botan- 

 ist Julius von Sachs 

 in 1879. Sachs used 

 a different method 

 for proving that grav- 



a, the position of germinating seeds of corn on a disk, 

 which is then slowly rotated for several hours in a 

 vertical position ; b, the positions assumed by roots and 

 plumule (stem) after twenty-four hours of rotation ; 

 c, the position of germinating seeds of corn on a station- 

 ary disk at the beginning of the experiment; d, the 

 positions assumed by roots and plumule (stem) after 

 twenty-four hours without rotation. Consult the text 

 for a discussion of this experiment 



ity acts upon plants 

 as a directive stimulating force. He placed growing seedlings 

 on slowly rotating vertical wheels or disks (Fig. 18). As the 

 wheels revolved, the stimulus of gravity continued, but the 

 effect of gravity on the growing stem and root was practically 

 eliminated, since it acted for too short an interval of time on 

 any given side of these organs to secure a reaction in the form of 

 a curvature. Plant organs usually have to remain in a position 

 of stimulation from thirty minutes to several hours in order to 



