516 



PHYSIOLOGY OF GROWTH. 



the light 

 must in this 

 case be re- 



/ r> garded as 



the deter- 

 mining fac- 

 tor, as I 

 also now 

 admit. 5 The 



movements executed by the laminae in 

 order to reach the most favourable light 

 position (this is, in general, one in which 

 they stand at right angles to the in- 

 cident rays of light) follow the shortest 

 and easiest course. These movements, 

 induced by the light, are, however, very 

 various in nature (photoepinastic nuta- 

 tions, torsions), and dependent on the 

 original position of the leaves themselves. 

 Geotropism, as will be shown, need play 

 no role in bringing about the movements 

 of orientation of the leaf-blades. Their 

 geotropic irritability, which, as we have 

 learned from preceding experiments, 

 certainly exists under certain circum- 

 stances (in the dark), may perhaps even 

 be inhibited by the action of light. 



That photoepinasty is concerned in 

 bringing about the normal light position 

 of the leaf -blades, is already clear from 

 the preceding experiments with Phase- 

 olus and Cucurbita plants. It causes, 

 e.g., the unfolding, on access of light, 

 of leaves which in many plants are in 

 the dark coiled shell-fashion. 

 Seedlings of Cucurbita which have been grown in the dark till 

 their hypocotyl has attained a fair length, and the cotyledons are 

 erect, are placed in a box fairly high at the back but low in 

 front, so that the cover, which is formed by a sheet of glass, may 

 be inclined at an angle of about 45. The box is papered inside 

 with dull black paper. To prevent heliotropic nutations, the hypo- 



FIG. 172. Shoot of Hedera 

 Helix subjected to unilateral 

 illumination. Its terminal por- 

 tion has curved away from the 

 source of light. 



