The tip is more sensitive to prolonged contact with an object than to gravitation when this acts obliquely on the radicle, and sometimes even when it acts in the most favourable direction at right angles to the radicle.  The tip was excited by an attached bead of shellac weighing less than 1/200th of a grain (0.33 mg.); it is therefore more sensitive than the most delicate tendril, namely, that of Passiflora gracilis, which was barely acted on by a bit of wire weighing 1/50th of a grain.  But this degree of sensitiveness is as nothing compared with that of the glands of Drosera, for these are excited by particles weighing only 1/78740 of a grain.  The sensitiveness of the tip cannot be accounted for by its being covered by a thinner layer of tissue than the other parts, for it is protected by the relatively thick root-cap.  It is remarkable that although the radicle bends away, when one side of the tip is slightly touched [page 551] with caustic, yet if the side be much cauterised the injury is too great, and the power of transmitting some influence to the adjoining parts causing them to bend, is lost.  Other analogous cases are known to occur.

After a radicle has been deflected by some obstacle, geotropism directs the tip again to grow perpendicularly downwards; but geotropism is a feeble power, and here, as Sachs has shown, another interesting adaptive movement comes into play; for radicles at a distance of a few millimeters from the tip are sensitive to prolonged contact in such a manner that they bend towards the touching object, instead of from it as occurs when an object touches one side of the tip.  Moreover, the curvature thus caused is abrupt; the pressed part alone bending.  Even slight pressure suffices, such as a bit of card cemented to one side. therefore a radicle, as it passes over the edge of any obstacle in the ground, will through the action of geotropism press against it; and this pressure will cause the radicle to endeavour to bend abruptly over the edge.  It will thus recover as quickly as possible its normal downward course.

Radicles are also sensitive to air which contains more moisture on one side than the other, and they bend towards its source.  It is therefore probable that they are in like manner sensitive to dampness in the soil.  It was ascertained in several cases that this sensitiveness resides in the tip, which transmits an influence causing the adjoining upper part to bend in opposition to geotropism towards the moist object.  We may therefore infer that roots will be deflected from their downward course towards any source of moisture in the soil.

Again, most or all radicles are slightly sensitive to light, and according to Wiesner, generally bend a little [page 552] from it.  Whether this can be of any service to them is very doubtful, but with seeds germinating on the surface it will slightly aid geotropism in directing the radicles to the ground.* We ascertained in one instance that such sensitiveness resided in the tip, and caused the adjoining parts to bend from the light.  The sub-aërial roots observed by Wiesner were all apheliotropic, and this, no doubt, is of use in bringing them into contact with trunks of trees or surfaces of rock, as is their habit.