But radicles of seedlings, as well as those of more vigorous plants, would pass over stones, roots and other obstacles, which they must necessarily encounter in the soil. This they are abundantly able to do, for they are exceedingly sensitive just above their apices, and bend like a tendril _towards_ the touching object. When, however, one side of the apex is pressed by any object, the growing part bends _away_ from that object, and this seems a beautiful adaptation for avoiding obstacles in the soil, and for following the lines of least resistance. [Illustration: SEEDLING OF WINTER GRAPE. Earth Cut Away to Show Directions Taken by Tip of Radicle in Avoiding a Stone.] So feeble is the circumnutating movement of the terminal growing part, both of the primary and secondary radicles, that it can assist them but little in penetrating the ground, excepting when the superficial layer is very soft and moist. But it must aid them materially when they chance to break obliquely into cracks, or into burrows that have been made by earth-worms or larvæ. Moreover, combined as it is with the sensitiveness of the tip of the radicle to contact, it can hardly fail to be of the highest importance, for as the tip is always endeavoring to bend to all sides, it will press on all sides, and will thus be able to discriminate between the harder and softer adjoining surfaces. Consequently, it will tend to bend from the harder soil, and will thus take the directions of the least resistance. So it will act if it meet with a stone or the root of another plant in the soil, as must incessantly occur. If the tip were not sensitive, and did not excite the upper part of the radicle to bend away, whenever obstacles were encountered at right angles to its growing direction, it would undoubtedly be liable to be doubled up into a contorted mass. But with radicles growing down inclined plates of glass, as shown by experiment, it has been observed that as soon as the tip merely touched a slip of wood cemented across the plate, the entire terminal growing point curved away, so that the tip soon stood at right angles to its former direction; and thus, as far as the pressure of the surrounding soil would permit, would it be with an obstacle encountered in the ground. Thick and strong radicles, like those of the horse-chestnut, are endowed with less sensitiveness than more delicate ones, and would therefore be the better able by the force of their growth to overcome any slight impediment to their progress. Further, as radicles perceive an excess of moisture in the air on one side and bend towards this side, it is reasonable to infer that they will act in a similar manner with respect to moisture in the earth, for the sensitiveness of moisture resides in the tip, which determines the bending of the upper part. May not this capacity partly account for the extent to which drain-pipes often become choked with roots? The direction which