514 PHYSIOLOGY OF GROWTH. 



for three to five hours, no photoepinastic movement will yet have 

 taken place. If, however, we now put them back into the dark, 

 the leaves spread out in the course of six to twelve hours. Here, 

 then, we have an example of photoepinastic after-effect. 



The seedlings, grown in the dark and not too old, always 

 exhibit photoepinastic nutations when illuminated, whether the 

 light strikes them from above or in some other direction. The 

 leaves strive to place themselves approximately at right angles to 

 the incident rays of light, and in experiments with Cucurbita it 

 is easy also to prove the fact that here and the same thing 

 obtains in other cases also one organ (viz. the hypocotyl) exerts 

 a material influence on the final orientation towards the light of 

 other organs (the cotyledons). The intense positive heliotropism 

 of the hypocotyl plays an important part in determining the light 

 position of the leaves, when the seedlings are illuminated only 

 from one side. 



If growing leaves are brought out of their normal position as 

 regards the light into an abnormal one, they endeavour to return 

 to their former position. I have made a series of experiments to 

 prove this, and it will be instructive to repeat them. We culti- 

 vate a few plants of Cucurbita in flower-pots. The experiments 

 begin when the plants have formed several leaves. Some of the 

 leaves are raised so that the petiole is vertical, and the tip of the 

 lamina points upwards. The leaves are bound to small sticks, 

 the thread being brought close below the lamina at the end of 

 the petiole. If we arrange the plants so that with unilateral 

 illumination the under sides of the leaves are directed towards the 

 light, then the leaf-blades, owing to photoepinastic movements, 

 will speedily assume a normal inclination to the light. If, with 

 unilateral illumination, we arrange a flower-pot, in which plants 

 of Cucurbita are growing, in such a way that the free leaf-stalks, 

 as also the laminae with their under sides directed towards the 

 light, are vertical, then heliotropic and photoepinastic movements 

 take place, and the organs return to the normal light position. If 

 we take into the dark plants of Cucurbita which have developed 

 under normal conditions, taking care that some of their leaves 

 (stalk and blade) are vertical, the blades perform photoepinastic 

 after-effect movements until they reach a horizontal position. 



It is of special interest in connection with questions relating to 

 the natural orientation of plant structures, and their anisotropy, 

 to make a few observations and experiments on the growth of the 



