470 TEANSMITTED EFFECTS OF LIGHT. CHAP. IX. 



curvature could not have been aided in the least by the weight 

 of the upper part, which acted at right angles to the plane of 

 curvature. 



It will be shown that when the upper halves of the coty- 

 ledons of Phalaris and Avena were enclosed in little pipes of 

 tin-foil or of blackened glass, in which case the upper part was 

 mechanically prevented from bending, the lower and unenclosed 

 part did not bend when exposed to a lateral light; and it 

 occurred to us that this fact might be due, not to the exclusion 

 of the light from the upper part, but to some necessity of the 

 bending gradually travelling down the cotyledons, so that 

 unless the upper part first became bent, the lower could not 

 bend, however much it might be stimulated. It was necessary 

 for our purpose to ascertain whether this notion was true, and it 

 was proved false ; for the lower halves of several cotyledons 

 became bowed to the light, although their upper halves were 

 enclosed in little glass tubes (not blackened), which prevented, 

 as far as we could judge, their bending. Nevertheless, as the 

 part within the tube might possibly bend a very little, fine rigid 

 rods or flat splinters of thin glass were cemented with shellac to 

 one side of the upper part of 15 cotyledons ; and in six cases 

 they were in addition tied on with threads. They were thus 

 forced to remain quite straight. The result was that the lower 

 halves of all became bowed to the light, but generally not in so 

 great a degree as the corresponding part of the free seedlings 

 in the same pots ; and this may perhaps be accounted for by 

 some slight degree of injury having been caused by a consider- 

 able surface having been smeared with shellac. It may be 

 added, that when the cotyledons of Phalaris and Avena are 

 acted on by apogeotropism, it is the upper part which begins 

 first to bend; and when this part was rendered rigid in the 

 manner just described, the upward curvature of the basal part 

 was not thus prevented. 



To test our belief that the upper part of the cotyledons of 

 Phalaris, when exposed to a lateral light, regulates the bending 

 of the lower part, many experiments were tried ; but most of our 

 first attempts proved useless from various causes not worth 

 specifying. Seven cotyledons had their tips cut off for lengths 

 varying between '1 and '16 of an inch, and these, when left 

 exposed all day to a lateral light, remained upright. In another 

 set of 7 cotyledons, the tips were cut off for a length of only 

 about *05 of an inch (1*27 mm.) and these became bowed towards 



