66o 



PLANT MOVEMENTS 



light has a direct effect upon growth independent of its influence upon the 

 auxin distributing mechanism of the plant (van Overbeelc, 1936a, 1936b). 



The relative importance of this "light-grow^th" reaction in phototropic 

 movements is somewhat uncertain. It seems clear that phototropic curvatures 

 are not simple light-growth reactions as was widely believed at one time, 

 for it has been demonstrated that the light-growth effect is inadequate to 

 account for the differences in growth rates that are responsible for most photo- 

 tropic curvatures. The effect of unilateral illumination upon the distribu- 

 tion of auxin appears to be the primary cause of phototropic curvatures and 

 the direct influence of light upon the growth of cells seems to be of secondary 

 importance. 



Geotropism. — If a potted plant be placed in a horizontal position for a 

 few days the stems no longer lie prostrate but begin to turn upward away from 



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Fig. 147. Diagram illustrating geotropic curvature of root and hypocotyl of a 

 mustard plant. (.1?) plant just after having been placed horizontally, (B) one day 

 later. 



the direction of gravitational attraction. This change in position first appears 

 in the region of elongation just back of the stem tip and with time may 

 extend backward toward the older portions of the stem. If the primary 

 root tips of the plant are examined they will also be found to have altered 

 their position, but in exactly the opposite direction, by growing downward 

 toward the center of the earth. The behavior of roots can be more easily 

 observed in germinating seeds and as in stems the change in position first 

 appears in the region of elongation just back of the root tip (Fig. 147). 



If horizontally placed potted plants are rotated slowly about the stem 

 as an axis, so that every vertical segment of the stem becomes successively 

 the upper and then the lower side, no geotropic curvatures appear. No seg- 

 ment of the stem or root remains long enough in any one position for growth 

 curvatures to occur. Similarly the roots fail to curve when germinating 

 seeds are fastened to the rim of a wheel which is rotated rapidly in a horizontal 

 plane. Growth is more strongly influenced by the centrifugal force generated 

 by the rapid rotation than by the force of gravity. The roots react positively 



