148 PLANT PHYSIOLOGY 



force. If the shoots of plants are fixed in a vertical position on 

 the circumference of a wheel that is rotated horizontally at con- 

 siderable speed, it will be found that during further growth, the 

 seedlings will deviate from their vertical position. Their stems 

 will be directed toward the center, and the roots away from it. 

 By altering the speed of rotation, it is possible to change the 

 angle of deviation from the vertical line. By this means, it has 

 been found that the direction of the axial organs will always 

 correspond to the resultant between the force of gravity and 

 centrifugal force. In case of very rapid rotation, when the 

 centrifugal force is greater than that of gravitation, the shoots 

 will grow almost horizontally. And when the speed of rotation 

 is such that the centrifugal force is equal to the force of gravity, 

 then the shoots will grow at an angle of 45 deg. 



By subjecting the shoots to the effect of centrifugal force 

 during a short period and then transferring them to the clinostat, 

 it is possible to obtain the same curvatures as are secured after 

 a short exposure to the effect of gravity. It is also possible to 

 determine definitely the presentation time for each intensity of 

 centrifugal force. This time has been found to be in inverse 

 proportion to the magnitude of the force used. In this case, 

 also, one secures a full confirmation of the law of the quantity of 

 stimulus. 



Geotropic stimuli are not equally perceived by all parts of an 

 organ possessing geotropic sensitivity. The distribution of 

 geotropic sensitivity has been most studied in the root. Charles 

 Darw^in (1860) showed that the tip of the root, or the terminal 

 1 to 2 mm., is the region of sensitivity and that roots with the 

 tips cut off grow horizontally, having lost the capacity to perceive 

 the direction of gravity. In seedlings of grain, the geotropic 

 sensitivity is localized in the tip of the coleoptile; while in the 

 majority of dicot stems, it is more or less equally distributed 

 throughout the growing zone. 



The question as to exactly how plants perceive the direction 

 of gravity is of great interest but can by no means be regarded 

 as finally solved. 



The mechanism of this perception remained for a long time 

 unknown. Only the recent development of experiments on 

 growth hormones opened the possibility of a significant advance 

 in the solution of this difficult problem. The coincidence of the 



