456 SENSITIVENESS TO LIGHT. Chap. IX. 



by a tin-vessel, having on one side a circular hole 

 1'23 mm. in diameter (i.e. a little less than the -^Lth of 

 an inch) ; and the box was placed in front of a paraffin 

 lamp and on another occasion in front of a window ; 

 and both times the seedlings were manifestly bent 

 after a few hours towards the little hole. 



A more severe trial was now made ; little tubes of 

 very thin glass, closed at their upper ends and coated 

 with black varnish, were slipped over the cotyledons 

 of Phalaris (which had germinated in darkness) and 

 just fitted them. Narrow stripes of the varnish had 

 been previously scraped off one side, through which 

 alone light could enter ; and their dimensions were 

 afterwards measured under the microscope. As a 

 control experiment, similar unvarnished and trans- 

 parent tubes were tried, and they did not prevent the 

 cotyledons bending towards the light. Two cotyledons 

 were placed before a south-west window, one of which 

 was illuminated by a stripe in the varnish, only '004 

 inch (O'l mm.) in breadth and 'OIG inch (0*4 mm.) in 

 length ; and the other by a stripe '008 inch in breadth 

 and "06 inch in length. The seedlings were examined 

 after an exposure of 7 h. 40 m., and were found to be 

 manifestly bowed towards the light. Some other coty- 

 ledons were at the same time treated similarly, ex- 

 cepting that the little stripes were directed not to the 

 sky, but in such a manner that they received only the 

 diffused light from the room ; and these cotyledons did 

 not become at all bowed. Seven other cotyledons were 

 illuminated through narrow, but comparatively long, 

 cleared stripes in the varnish — namely, in breadth 

 between -01 and -026 inch, and in length between -15 

 and '3 inch ; and these all became bowed to the side, 

 by which light entered through the stripes, whether 

 these were directed towards the sky or to one side of 



