This explanation of the occasional irregular growth of radicles with amputated tips, is supported by the results of cauterising their tips; for often a greater length on one side than on the other was unavoidably injured or killed.  It should be remarked that in the following trials the tips were first dried with blotting-paper, and then slightly rubbed with a dry stick of nitrate of silver or lunar caustic.  A few touches with the caustic suffice to kill the root-cap and some of the upper layers of cells of the vegetative point.  Twenty-seven radicles, some young and very short, others of moderate length, were suspended vertically over water, after being thus cauterised.  Of these some entered the water immediately, and others on the second day.  The same number of uncauterised radicles of the same age were observed as controls.  After an interval of three or four days the contrast in appearance between the cauterised and control specimens was wonderfully great.  The controls had grown straight downwards, with the exception of the normal curvature, which we have called Sachs’ curvature.  Of the 27 cauterised radicles, 15 had become extremely distorted; 6 of them grew upwards and formed hoops, so that their tips sometimes came into contact with the bean above; 5 grew out rectangularly to one side; only a few of the remaining 12 were quite straight, and some of these towards the close of our observations became hooked at their extreme lower ends.  Radicles, extended horizontally instead of vertically, with their tips cauterised, also sometimes grew distorted, but not so commonly, as far as we could judge, as those suspended vertically; for this occurred with only 5 out of 19 radicles thus treated.

Instead of cutting off the tips, as in the first set of experiments, we next tried the effects of touching horizontally extended radicles with caustic in the manner just described.  But [page 530] some preliminary remarks must first be made.  It may be objected that the caustic would injure the radicles and prevent them from bending; but ample evidence was given in Chapter iii. that touching the tips of vertically suspended radicles with caustic on one side, does not stop their bending; on the contrary, it causes them to bend from the touched side.  We also tried touching both the upper and the lower sides of the tips of some radicles of the bean, extended horizontally in damp friable earth.  The tips of three were touched with caustic on their upper sides, and this would aid their geotropic bending; the tips of three were touched on their lower sides, which would tend to counteract the bending downwards; and three were left as controls.  After 24 h. an independent observer was asked to pick out of the nine radicles, the two which were most and the two which were least bent; he selected as the latter, two of those which had been touched on their lower sides, and as the most bent, two of those which had been touched on the upper side.  Hereafter analogous and more striking experiments with Pisum sativum and Cucurbita ovifera will be given.  We may therefore safely conclude that the mere application of caustic to the tip does not prevent the radicles from bending.