the apex takes at each successive period of the growth of a root, ultimately determines its whole course. It is therefore very important that the apex should follow from the first the most advantageous direction. We can thus understand why sensitiveness to geotropism, contact and moisture should all reside in the tip, and why it should determine the upper growing part to bend either from or to the exciting cause. Darwin has compared a radicle with a burrowing animal, such as a mole, which wishes to penetrate vertically into the ground. By a process of circumnutation, or the movement of his head from side to side, he is enabled to feel any stone or other obstacle, as well as any difference in hardness of soil that may exist, and will therefore turn from that side; but if damper on one side than on the other, will turn thither as a more suitable hunting-ground. Nevertheless, after each interruption, he, guided by the sense of gravity, will be able to recover his downward direction and to reach to a greater depth. Destruction of the tip of a radicle does not prevent the adjoining part from bending, if this part has already received some influence from the tip. As with a horizontally extended radicle, whose tip has been cut off or destroyed, the part which should bend most remains motionless for many days or hours, even though exposed at right angles to the full influence of gravity, we cannot do otherwise than conclude that the tip alone is sensitive to this power, and transmits some stimulus to the neighboring parts, thereby causing them to bend. Direct evidence of such transmission has been obtained. 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 some distance from the tip, and the tip was then cut off, the radicle subsequently became bent, although it was placed in a perpendicular position. Terminal portions of several radicles thus treated continued for some time to grow in the direction of their newly-acquired curvature, for being destitute of tips they were no longer acted upon by the power of gravity. New vegetative points, however, appeared, and being acted on by this influence coursed themselves perpendicularly downward as was their custom.