CHAP. IX. TRANSMITTED EFFECTS OF LIGHT. 469 



soon as the upper convex surface of the bowed- 

 down portion received more light than the lower 

 concave surface. The position ultimately assumed by 

 young and upright cotyledons, exposed to light enter- 

 ing obliquely from above through a window, is shown 

 in the accompanying figure (Fig. 181) ; and here it 

 may be seen that the whole upper part has become 

 very nearly straight. When the cotyledons were 

 exposed before a bright lamp, standing on the same 

 level with them, the upper part, which was at first 



Fig. 181. 



Phalaris Canariensis : cotyledons after exposure in a box open on one side 

 in front of a south-west window during 8 h. Curvature towards the 

 light accurately traced. The short horizontal lines show the level of 

 the ground. 



greatly arched towards the light, became straight and 

 strictly parallel with the surface of the soil in the 

 pots ; the basal part being now rectangularly bent. 

 All this great amount of curvature, together with the 

 subsequent straightening of the upper part, was often 

 effected in a .few hours. 



After the uppermost part has become bowed a little to the 

 light, its overhanging weight must tend to increase the curva- 

 ture of the lower part ; but any such effect was shown in several 

 ways to be quite insignificant. When little caps of tin-foil 

 (hereafter to be described) were placed on the summits of the 

 cotyledons, though this must have added considerably to their 

 weight, the rate or amount of bending was not thus increased. 

 But the best evidence was afforded by placing pots with seedlings 

 of Phalaris before a lamp in such a position, that the cotyledons 

 were horizontally extended and projected at right angles to the 

 line of light. In the course of 5i h. they were directed towards 

 the light with their bases bent at right angles ; and this abrupt 



