CHAP. IX. TRANSMITTED EFFECTS OF LIGHT. 475 



oxide of iron. A layer of this sand, moistened to the saino 

 degree as that over the seeds, was spread over a glass-plate ; and 

 when the layer was '05 of an inch in thickness (carefully mea- 

 sured) no light from a bright sky could be seen to pass through 

 it, unless it was viewed through a long blackened tube, and 

 then a trace of light could be detected, but probably much too 

 little to affect any plant. A layer ! of an inch in thickness was 

 quite impermeable to light, as judged by the eye aided by the tube. 

 It may be worth adding that the layer, when dried, remained 

 equally impermeable to light. This sand yielded to very slight 

 pressure whilst kept inoist, and in this state did not contract 

 or crack in the least. In a first trial, cotyledons which had 

 grown to a moderate height were exposed for 8 h. before a paraffin 

 iamp, and they became greatly bowed. At their bases on the 

 shaded side opposite to the light, well-defined, crescentic, open 

 furrows were formed, which (measured under a microscope with 

 a micrometer) were from '02 to -03 of an inch in breadth, and 

 these had evidently been left by the bending of the buried bases 

 of the cotyledons towards the light. On the side of the light 

 the cotyledons were iu close contact with the sand, which was a 

 very little heaped up. By removing with a sharp knife the 

 sand on one side of the cotyledons in the line of the light, the 

 bent portion and the open furrows were found to extend down 

 to a depth of about -1 of an inch, where no light could enter. 

 The chords of the short buried arcs formed in four cases angles 

 of 11, 13, 15, and 18, with the perpendicular. By the 

 following morning these short bowed portions had straightened 

 themselves through apogeo+ropism. 



In the next trial much younger cotyledons were similarly 

 treated, but were exposed to a rather obscure lateral light. 

 After some hours, a bowed cotyledon, '3 inch in height, had an 

 open furrow on the shaded side '01 inch in breadth; another 

 cotyledon, only '13 inch in height, had left a furrow '02 inch in 

 breadth. But the most curious case was that of a cotyledon which 

 had just protruded above the ground arid was only '03 inch in 

 height, and this was found to be bowed in the direction of the 

 light to a depth of '2 of an inch beneath the surface. From 

 what we know of the impermeability of this sand to light, the 

 upper illuminated part in these several cases must have deter- 

 mined the curvature of the lower buried portions. But an 

 apparent cause of doubt maybe suggested: as the cotyledons 

 lire continually circumnutating, they tend to form a minute 



