

GROWTH OF ROOT AND STEM 27 



The stimulus which, causes the root to turn downward is ini- 

 tiated by gravity. We must be careful, however, not to confuse 

 this stimulus from gravity with the pull which gravity exerts on all 

 bodies. For example, if we lift a seedling and then "let go" of 

 it, it will fall to the ground, or until it meets some object which 

 intercepts it. It is not this force of gravity which causes the roots 

 of plants to grow in a downward direction. Gravity merely in- 

 itiates a stimulus in response to which the root grows in the down- 

 ward direction. It has been found that this perceptive zone, 

 which perceives the stimulus, is in the root tip. While the root tip 

 receives the stimulus, this is not the part of the root which curves. 

 The curvature takes place in the motor zone, or region of growth 

 by elongation. The stimulus received by the sense organ in the 

 root tip is carried back to the growth region, and the cells on the 

 upper side of the horizontal root elongate faster than those on the 

 under side, and the curving results. 



43. How it is determined that gravity stimulates the root 

 to grow in a downward direction. This is shown by con- 

 stantly changing the position of the root, so that the stimulating 

 influence of gravity is neutralized and does not act for a long 

 enough time in one direction upon the sense organ in the root tip 

 to accumulate a sufficient amount of excitation from the stimu- 

 lus. This is done by pinning seedlings in various positions near 

 the margin of an upright revolving disk or wheel. The wheel is 

 revolved slowly, and dripping water is used to keep the roots 

 moist. The roots and stems will continue to elongate in the same 

 direction in which they were placed. The wheel can be revolved 

 by a slow small stream of water from the hydrant falling on little 

 paddles arranged on the margin, or by clock arrangement. 



44. Geotropism. The direction of growth of stems and 

 roots, when uninfluenced by light, as shown in the preceding 

 paragraphs, is in response to the stimulus from the influence of 

 gravity. In response to this stimulus the stem or root turns in a 

 definite direction in relation to the earth. This phenomenon is 

 called, therefore, geotropism, which name means literally earth 

 turning, since the earth in this case is the body which provides 



