22 GROWTH AND WORK OF PLANTS 



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 turns downward. 

 It has been found that this perceptive zone, which perceives the 

 stimulus, is in the root tip. While the root tip receives the stimu- 

 lus, 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. 



36. 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 upon 

 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, or by clock arrange- 

 ment. 



37. Geotropism. The direction of growth of stems and 

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

 paragraphs, is caused by the stimulus from the influence of 

 gravity. This stimulus causes a turning of the stem or root 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 

 the gravitation influence. The turning of the root towards the 



