154 



PLANT PHYSIOLOGY 



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made of black paper or tin foil is placed over the tips of the 

 shoots so as to cover the whole of the coleoptile, no curvatures 

 are formed, notwithstanding the fact that the hypocotyl remains 

 exposed to light. On the other hand, if the hypocotyl is shaded 

 from light and only the coleoptile remains exposed to it, curva- 

 tures are formed as distinctly as when the whole of the shoot is 

 exposed. 



The mechanism of phototropic curvatures consists in a more 

 retarded growth of the exposed side of the stem as compared with 



the growth of the shaded side; 

 thus the lighted side becomes 

 shorter, and the entire organ will 

 curve. Comparing this fact with 

 the long-known phenomenon of 

 delayed growth caused by the 

 effect of light, many authors, 

 beginning with De Candolle, 

 attributed this bending of plants 

 toward light to the direct retard- 

 ing effect of light on the growth of 

 the exposed side. But consider- 

 ing the experiments that have 

 demonstrated that perception is 



Fig. 44. — Seedlings of ^'etona. On localized in the tip of the shoot, 

 the right, one bent toward light; on ^j^-^j^ -^ ^^^^ directly Subject tO 

 the left, one of the seedlings has re- _ "^ '' 



mained upright. Its plumule has curving, the foregoing explanation 



been covered with a non-transparent ^^^ ^^ ^^ modified aS follows: A 

 cap {after Holman and Kobbins). 



unilateral exposure to light causes 

 certain substances to appear in the organ of perception; these 

 descend to the growing zone and produce a different effect upon 

 the growth of the exposed and the shaded sides. 



If all the data concerning the physical nature and the manner 

 of the transmission of phototropic stimuli are correlated with 

 the dependence of the rate of growth (Art. 29) on the amount of 

 the growth hormone manufactured by the stem tip or the 

 coleoptile, the following mechanism of phototropic curvatures 

 may be observed (Cholodny, Went). Under the influence of 

 the stimulus of light, the total amount of the growth hormone 

 in the sensitive apex, as well as its distribution, is altered. As 

 the exact quantitative determinations by Went have shown, light 



