i74 TRANSMITTED EFFECTS OF LIGHT. Chap. IX 



Another trial was made in a diflfercnt manner, namely, bj 

 tmndaging with strips of tin-foil, about -2 in breadth, the upper 

 part, but not the actual summit, of eight moderately young 

 seedlings a little over half an inch in height. The summits and 

 the basal parts were thus left fully exposed to a lateral light 

 during 8 h. ; an upper intermediate zone being protected. 

 With four of these seedlings the summits were exposed for 

 a length of '05 inch, and in two of them this part became 

 curved towards the light, but the whole lower part remained 

 quite upright; whereas the entire length of the other two 

 seedlings became slightly curved towards the light. The 

 summits of the four other seedlings were exposed for a length 

 of -04 inch, and of these one remained almost upright, whilst 

 the other three became considerably curved towards the light. 

 The many free seedlings in the same pots were all greatly 

 curved towards the light. 



From these several sets of experiments, including those with 

 the glass-tubes, and those when the tips were cut off, we may 

 infer that the exclusion of light from the upper part of the 

 cotyledons of Phalaris prevents the lower part, though fully 

 exposed to a lateral light, from becoming curved. The summit 

 for a length of -04 or -05 of an inch, though it is itself sensitive 

 and curves towards the light, has only a slight power of causing 

 the lower part to bend. Nor has the exclusion of light from the 

 summit for a length of '1 of an inch a strong influence on the 

 curvature of the lower part. On the other hand, an exclusion 

 for a length of between "IS and '2 of an inch, or of the whole 

 upper half, plainly prevents the lower and fully illuminated 

 part from becoming curved in the manner (see Fig. 181) which 

 invariably occurs when a free cotyledon is exposed to a lateral 

 light. With very young seedlings the sensitive zone seems to 

 extend rather lower down relatively to their height than in older 

 seedlings. We must therefore conclude that when seedlings 

 are freely exposed to a lateral light some influence is trans- 

 mitted from the upper to the lower part, causing the latter to 

 bend. 



This conclusion is supported by what may be seen to occur 

 on a small scale, especially with young cotyledons, without any 

 artificial exclusion of the light ; for they bend beneath the earth 

 where no light can enter. Seeds of Phalaris were covered 

 with a layer one-fourth of an inch in thickness of very fine 

 sand, consisting of extremely minute grains of silex coated with 



