498 



Plant Physiology 



I j -1 i- 



seed in a moist chamber pinned in a horizontal posi- 

 tion will respond to the stimulus of gravity by growth 

 curvatures in the same manner. 

 Secondary roots and branches take 

 up plagiotropic positions, but in the 

 remote branches of the root little geo- 

 tropic response is manifest. Shoots 

 from a fallen trunk assume the ver- 

 tical position. If the terminal shoot 

 of spruce is cut off, one or more lateral 

 shoots of the first whorl may be 

 raised into the vertical position. 

 The erection of the jointed stems of 

 grasses is effected by curvatures in 

 the nodes, and these stems are par- 

 ticularly interesting for study. 



FIG. 142. Geotropic ~ 



curvature of root of Geotropic response is not a ques- 

 Vicia Faba; horizontal tion of weight, and this is shown by 



position (I), after 7 hrs. ,. j. ,. /. ' ,v 



(ii), and after 23 hrs. the diverse reactions of the mam 

 (ill). [After Sachs and axis and branches. Furthermore, 

 there is no geotropic response when 

 gravity is eliminated, as by revolving seedlings in a ver- 

 tical plane on a klinostat geared to make one revolution 

 in about fifteen minutes. On a klinostat rotated hori- 

 zontally at a low rate of speed the usual stimulus of 

 gravity is felt; but when rotated at a higher rate of 

 speed the root grows outward and toward the horizontal, 

 and the shoot inward and toward the horizontal, de- 

 pending upon the rate of rotation. 



In the case of the root the perceptive region is usually 

 confined to about one millimeter, or less, at the very tip, 



