From these several sets of experiments, including those with the glass-tubes, and those when the tips were cut off, we may infer that the exclusion of light from the upper part of the cotyledons of Phalaris prevents the lower part, though fully exposed to a lateral light, from becoming curved.  The summit for a length of .04 or .05 of an inch, though it is itself sensitive and curves towards the light, has only a slight power of causing the lower part to bend.  Nor has the exclusion of light from the summit for a length of .1 of an inch a strong influence on the curvature of the lower part.  On the other hand, an exclusion for a length of between .15 and .2 of an inch, or of the whole upper half, plainly prevents the lower and fully illuminated part from becoming curved in the manner (see Fig. 181) which invariably occurs when a free cotyledon is exposed to a lateral light.  With very young seedlings the sensitive zone seems to extend rather lower down relatively to their height than in older seedlings.  We must therefore conclude that when seedlings are freely exposed to a lateral light some influence is transmitted from the upper to the lower part, causing the latter to bend.

This conclusion is supported by what may be seen to occur on a small scale, especially with young cotyledons, without any artificial exclusion of the light; for they bend beneath the earth where no light can enter.  Seeds of Phalaris were covered with a layer one-fourth of an inch in thickness of very fine sand, consisting of extremely minute grains of silex coated with [page 475] oxide of iron.  A layer of this sand, moistened to the same degree as that over the seeds, was spread over a glass-plate; and when the layer was .05 of an inch in thickness (carefully measured) 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 .1 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 moist, 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 lamp, 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 in 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 11o, 13o, 15o, and 18o, with the perpendicular.  By the following morning these short bowed portions had straightened themselves through apogeotropism.