348 



PLANT PHYSIOLOGY 



conclusion that the direction of the growth of the plant organs is 

 largely determined by gravity. That gravity is acting as a stimu- 

 lus and not as it might act upon dead, inert matter is shown by 

 the fact that the stems go up. If it were merely a question of 

 weight, the stems would go down towards the pull. 



The Perceptive and Responsive Regions. — The region sen- 

 sitive to the pull of gravity in the root, is located in the last two 

 millimeters including the rootcap, the inner portions of which 

 are believed to contain the cells most concerned in the reception 

 of the stimulus. That this is so can be shown by cutting off the 

 end two millimeters, when the root no longer responds to gravity. 

 It is also possible to place the tip in a small glass slipper as done 



Fig. 23. — Roots of horse bean (Vicia) in glass slippers. If the terminal 

 2 mm. are bent at right angles to the perpendicular as in a, the root will turn 

 as in b, so that the tip is again vertical. If the tip is vertical it makes no dif- 

 ference if the remainder of the root is not, as shown in c and d where no change 

 takes place in position. (After Czapek.) 



by Czapek and to turn the tip so that it alone is at right angles 

 to gravity. In this case the root bends in such a way as to bring 

 the tip parallel to the gravitational stimulus. If, on the other 

 hand, the tip is kept parallel to the force of gravity and the rest 

 of the root is bent at right angles to the pull, no effect is observed 

 (Fig. 23). The effective force acting upon a root placed at an angle, 

 A, with the vertical is equal to the sine of the angle. Since (see 

 above) the active force multiplied by the perception time is a 

 constant, the perception time varies inversely with the sine of 

 angle A (the larger the angle the greater is the pull of gravity) 

 (Fig. 24). 



The region which responds is located back some distance from 



