C3h\p. IX TRANSMITTED EFFECTS OF LIGHT. 473 



in which the upper halves of 14 cotyledons were enclosed ic 

 tubes from which an extremely narrow stripe of the black 

 varnish had been scraped off. These cleared stripes were 

 not directed towards the window, but obliquely to one side 

 of the room, so that only a very little light could act on the 

 upper halves of the cotyledons. These 14 seedlings remained 

 during eight hours of exposure before a south-west window on 

 a hazy day quite upright; whereas all the other many free 

 seedlings in the same pots became greatly bowed towards the 

 light 



We will now turn to the trials with caps made of very thin 

 tin-foil. These were placed at different times on the summits of 

 24 cotyledons, and they extended down for a length of between 

 •15 and "2 of an inch. The seedHngs were exposed to a lateral 

 light for periods varying between 6 h. hO m. and 7 h. 45 m., 

 which sufficed to cause all the other seedlings in the same pots 

 to become almost rectangularly bent towards the light. They 

 varied i(i height from only '04 to 115 inch, but the greater 

 number were about -75 inch. Of the 24 cotyledons with their 

 summits thus protected, 3 became much bent, but not in the 

 direction of the light, and as they did not straighten themselves 

 through apogeotropism during the following night, either the 

 caps were too heavy or the plants themselves were in a weak 

 condition; and these three cases may be excluded. There 

 are left for consideration 21 cotyledons ; of these 17 remained 

 all the time quite upright ; the other 4 became slightly inclined 

 to the light, but not in a degree comparable with that of the 

 many free seedlings in the same pots. As the glass-tubes, when 

 unpainted, did not prevent the cotyledons from becominp; 

 greatly bowed, it cannot be supposed that the caps o*" very 

 thin tin-foil did so, except through the exclusion of the light 

 To prove that the plants had not been injured, the caps were 

 removed from 6 of the upright seedlings, and these were exposed 

 before a paraffin lamp for the same length of time as before 

 and they now all became greatly curved towards the light. 



As caps between '15 and -'2 of an inch in depth were thus 

 proved to be highly efficient in preventing the cotyledons from 

 bending towards the light, 8 other cotyledons were protected 

 with caps between only 06 and -12 in depth. Of these, two 

 remained vertical, one was considerably and five slightly curved 

 towards the light, but far less so than the free seedlings in the 

 same pots. 



