662 PLANT MOVEMENTS 



The same concentration of auxin which favors the elongation of stems 

 and coleoptiles retards the elongation of roots (Chap. XXXII). This fact 

 suggests that the positive (downward) geotropic reaction of root tips may be 

 caused by the same mechanism which invokes the negative (upward) curvature 

 of stems or coleoptiles. A greater concentration of auxin in the lower half 

 of horizontally placed roots would check elongation rather than increase it 

 and the growth of the upper side of the root would exceed that of the lower, 

 resulting in its downward curvature. Experimental tests have confirmed this 

 explanation. The lower halves of tips of horizontal roots have been found 

 to contain higher concentrations of auxins than the upper halves (Hawker, 

 1932). Since the growth rate of primary roots which are slowly rotated 

 about their own axis in a horizontal plane does not exceed that of vertical 

 roots {i.e. auxin concentrations are equal in the root tips in both positions) 

 the greater quantity of auxin in the lower half of the tips of horizontal 

 primary roots indicates that the auxin migrates to this region under the action 

 of gravity. The downward curvature of root tips is therefore found to be 

 correlated with the auxin concentration. 



Until the comparatively recent work upon auxins geotropic curvatures 

 were commonly explained by the statolith theory. According to this theory 

 the geotropic curvatures of stems and roots were caused by changes in position 

 of mobile solid bodies, such as starch grains, in certain "sensory cells" under 

 the action of gravity. The weight of these bodies upon the cytoplasm of 

 the sensory cells was believed to initiate a chain of events which ultimately 

 resulted in geotropic curvature. The statolith theory is now chiefly of his- 

 torical interest since it is possible to demonstrate experimentally a quantitative 

 relationship between auxins and geotropic curvatures in a number of different 

 plants. 



Thigmotropism. — The growth movements made by plants as a conse- 

 quence of contact with solid objects are known as thigmotropic reactions. 

 These movements are best illustrated in the growth of tendrils, though they 

 are also exhibited by petioles, stems and other organs of some plants. Tendrils 

 are slender cylindrical organs that structurally represent modified stems, leaves 

 or leaflets. Some common tendril-bearing species of plants are the grape vine, 

 greenbriers (Smilax), sweet pea and wild cucumber (Sicyos). As a result 

 of unequal rates of growth the tips of young tendrils exhibit the phenomenon 

 of nutation (see later) and make slow circular movements in space during 

 their elongation. As soon as a tendril comes in contact with a solid object, 

 rapid growth reactions are initiated. The cells on the side which makes 

 contact with the solid object shorten somewhat and the cells on the opposite 

 side quickly elongate with the result that the tendril is bent around the sup- 



