CHAP. IX. TRANSMITTED EFFECTS OF LIGHT. 477 



not directly in the line of 'the window, but obliquely towards 

 it ; one pointed at an angle of only 18, but the remaining 12 

 at angles varying between 45 and 62 from the line of the 

 window. At the commencement of the experiment, pins had 

 been laid on the earth in the direction towards which the slits in 

 the varnish faced ; and in this direction alone a small amount 

 of diffused light entered. At the close of the experiment, 7 of 

 the bowed cotyledons pointed exactly in the line of the pins, 

 and 6 of them in a line between that of the pins and that of the 

 window. This intermediate position is intelligible, for any light 

 from the sky which entered obliquely through the slits would 

 be much more efficient than the diffused light which entered 

 directly through them. After the 8 h. exposure, the contrast 

 in appearance between these 13 cotyledons and the many other 

 seedlings in the same pots, which were all (excepting the above 

 14 vertical ones) greatly bowed in straight and parallel lines 

 towards the window, was extremely remarkable. It is therefore 

 certain that a little weak light striking the upper halves of the 

 cotyledons of Phalaris, is far more potent in determining the 

 direction of the curvature of the lower halves, than the full 

 illumination of the latter during the whole time of exposure. 



In confirmation of the above results, the effect of thickly 

 painting with Indian ink one side of the upper part of three coty- 

 ledons of Phalaris, for a length of 2 inch from their tips, may be 

 worth giving. These were placed so that the unpainted surface 

 was directed not towards the window, but a little to one side ; 

 and they all became bent towards the unpainted side, and from 

 the line of the window by angles amounting to 31, 35, and 83. 

 The curvature in this direction extended down to their bases, 

 although the whole lower part was fully exposed to the light 

 from the window. 



Finally, although there can be no doubt that the illumination 

 of the upper part of the cotyledons of Phalaris greatly affects 

 the power and manner of bending of the lower part, yet some 

 observations seemed to render it probable that the simultaneous 

 stimulation of the lower part by light greatly favours, or is 

 almost necessary, for its well-marked curvature ; but our experi- 

 ments were not conclusive, owing to the difficulty of excluding 

 light from the lower halves without mechanically preventing 

 their curvature. 



Avena sativa. The cotyledons of this plant become quickly 

 bowed towards a lateral light, exactly like those of Phalaris. 



