Fig. 181.  Phalaris Canariensis:  cotyledons after exposure in a box open on one side in front of a south-west window during 8 h.  Curvature towards the light accurately traced.  The short horizontal lines show the level of the ground.

greatly arched towards the light, became straight and strictly parallel with the surface of the soil in the pots; the basal part being now rectangularly bent.  All this great amount of curvature, together with the subsequent straightening of the upper part, was often effected in a few hours.

[After the uppermost part has become bowed a little to the light, its overhanging weight must tend to increase the curvature of the lower part; but any such effect was shown in several ways to be quite insignificant.  When little caps of tin-foil (hereafter to be described) were placed on the summits of the cotyledons, though this must have added considerably to their weight, the rate or amount of bending was not thus increased.  But the best evidence was afforded by placing pots with seedlings of Phalaris before a lamp in such a position, that the cotyledons were horizontally extended and projected at right angles to the line of light.  In the course of 5 ½ h. they were directed towards the light with their bases bent at right angles; and this abrupt [page 470] 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 cotyledons 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 considerable 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.