for when a radicle was left extended horizontally for an hour or an hour and a half, by which time the supposed influence will have travelled a little distance from the tip, and the tip was then cut off, the radicle afterwards became bent, although placed perpendicularly.  The terminal portions of several radicles thus treated continued for some time to grow in the direction of their newly-acquired curvature; for as they were destitute of tips, they were no longer acted on by geotropism.  But after three or four days when new vegetative points were formed, the radicles were again acted on by geotropism, and now they curved themselves perpendicularly downwards.  To see anything of the above kind in the animal kingdom, we should have to suppose than an animal whilst lying down determined to rise up in some particular direction; and that after its head had been cut off, an impulse continued to travel very slowly along the nerves to the proper muscles; so that after several hours the headless animal rose up in the predetermined direction.

As the tip of the radicle has been found to be the [page 544] part which is sensitive to geotropism in the members of such distinct families as the Leguminosae, Malvaceae, Cucurbitaceae and Gramineae, we may infer that this character is common to the roots of most seedling plants.  Whilst a root is penetrating the ground, the tip must travel first; and we can see the advantage of its being sensitive to geotropism, as it has to determine the course of the whole root.  Whenever the tip is deflected by any subterranean obstacle, it will also be an advantage that a considerable length of the root should be able to bend, more especially as the tip itself grows slowly and bends but little, so that the proper downward course may be soon recovered.  But it appears at first sight immaterial whether this were effected by the whole growing part being sensitive to geotropism, or by an influence transmitted exclusively from the tip.  We should, however, remember that it is the tip which is sensitive to the contact of hard objects, causing the radicle to bend away from them, thus guiding it along the lines of least resistance in the soil.  It is again the tip which is alone sensitive, at least in some cases, to moisture, causing the radicle to bend towards its source.  These two kinds of sensitiveness conquer for a time the sensitiveness to geotropism, which, however, ultimately prevails.  Therefore, the three kinds of sensitiveness must often come into antagonism; first one prevailing, and then another; and it would be an advantage, perhaps a necessity, for the interweighing and reconciling of these three kinds of sensitiveness, that they should be all localised in the same group of cells which have to transmit the command to the adjoining parts of the radicle, causing it to bend to or from the source of irritation.