318 



TEXTBOOK OF PLANT PHYSIOLOGY 



The seedlings of some cereals, particularly those of sorghum 

 and millet (Setaria), are most convenient objects for such experi- 

 ments. These seedlings consist of a rather long hypocotyl bearing 

 a comparatively small plumule (cotyledon) which contains the 

 first true leaf. If the shoot is placed in a phototropic chamber, the 

 hypocotyl will form a distinct curve and the tip of the plumule 

 will be directed toward the light (Fig. 128). At first, the cur- 

 vature is formed directly beneath 

 |\ the plumule, but, later, as further 



growth of the hypocotyl takes 

 place, the curvature is somewhat 

 lowered. By means of very simple 

 experiments it maybe shown easily 

 that the plumule is the organ 

 which perceives light, while the 

 hypocotyl merely responds to the 

 excitation transmitted from the 

 plumule. If an opaque hood made 

 of black paper or tinfoil is placed 

 over the tips of the shoots, so as 

 to cover the whole of the plumule, 

 no curvatures are formed, not- 

 Fig. 128. — Seedlings of Setaria. On withstanding the fact that the 



S: 2fr~**SZmXi - hyp™"* 1 remainsexposed to light. 



mained upright, its plumule has On the other hand, if the hypocotyl 



be Sp C «"w SlST is s haded from light while only 



the plumule remains exposed to 

 it, curvatures 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 to 

 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, attrib- 

 uted this bending of plants towards light to the direct retarding 

 effect of light on the growth of the exposed side. But considering 

 the experiments which have demonstrated that perception is 

 localized in the tip of the shoot, which, however, is not directly 

 subjected to curving, the foregoing explanation has to be modified 



