CHAPTER IX 



THE SIGNIFICANCE OF GROWTH SUBSTANCES FOR 



GEOTROPISM 



The bending of the organs of a plant toward or away from the 

 earth is a well-known phenomenon. This response to gravity 

 eventually brings about a position of equilibrium with respect to 

 the earth. Perception of the gravitational stimulus and the 

 course of the processes which lead to the geocurvature have long 

 been the subject of scientific inquiries. Much of the older litera- 

 ture pertinent to the question of geotropism has been discussed 

 in reviews by Schober (1899), Christiansen (1917), Zimmermann 

 (1927), and Rawitscher (1932). Since much that is known about 

 growth substances was learned in the study of tropisms, a few of 

 the more outstanding original contributions will be discussed for 

 purposes of orientation in this field. 



THE EARLY INVESTIGATIONS 



Knight (1806, 1803-1812) demonstrated by experiments in 

 centrifuging that the force of gravity caused an upward bending 

 of stems placed in a horizontal position and a downward curva- 

 ture in horizontally placed main roots. He attempted to explain 

 how the same stimulus could produce these opposite responses in 

 the shoot and the root on the basis of a purely mechanical effect of 

 gravity. Growth in the root takes place by the continual laying 

 down of new tissues at the vegetative tip. In seedlings, the food 

 for growth comes in solution from the cotyledons. It was 

 assumed that gravity affected this fluid and the tender, flexible 

 fibers and bundles in such a way as to cause bending of the root 

 tip in a downward direction. The curvature was supposed to be a 

 passive, downward movement of the tip, due to its weight. Such 

 a hypothesis cannot be used as an explanation for the upward 

 curvature of the stem, for Knight pointed out that it grows by the 

 stretching of the region back of the tip, the amount of extension 

 being in proportion to the food supply. It was held that an 



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