more powerful than geotropism, when this acts obliquely on a radicle, which has been deflected from its perpendicular downward course.  The roots, moreover, of most plants are excited by light to bend either to or from it; but as roots are not naturally exposed to the light it is doubtful whether this sensitiveness, which is perhaps only the indirect result of the radicles being highly sensitive to other stimuli, is of any service to the plant.  The direction which the apex takes at each successive period of the growth of a root, ultimately determines its whole course; it is therefore highly important that the apex should pursue from the first the most advantageous direction; and we can thus understand why sensitiveness to geotropism, to contact and to moisture, all reside in the tip, and why the tip determines the upper growing part to bend either from or to the exciting cause.  A radicle may be compared with a burrowing animal such as a mole, which wishes to penetrate perpendicularly down into the ground.  By continually moving his head from side to side, or circumnutating, he will feel any stone [page 200] or other obstacle, as well as any difference in the hardness of the soil, and he will turn from that side; if the earth is damper on one than on the other side he will turn thitherward as a better hunting-ground.  Nevertheless, after each interruption, guided by the sense of gravity, he will be able to recover his downward course and to burrow to a greater depth. [page 201]

CHAPTER IV.

THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS.

Circumnutation of stems:  concluding remarks on—­Circumnutation of stolons:  aid thus afforded in winding amongst the stems of surrounding plants—­ Circumnutation of flower-stems—­Circumnutation of Dicotyledonous leaves—­ Singular oscillatory movement of leaves of Dionaea—­Leaves of Cannabis sink at night—­Leaves of Gymnosperms—­Of Monocotyledons—­Cryptogams—­Concluding remarks on the circumnutation of leaves; generally rise in the evening and sink in the morning.

We have seen in the first chapter that the stems of all seedlings, whether hypocotyls or epicotyls, as well as the cotyledons and the radicles, are continually circumnutating—­that is they grow first on one side and then on another, such growth being probably preceded by increased turgescence of the cells.  As it was unlikely that plants should change their manner of growth with advancing age, it seemed probable that the various organs of all plants at all ages, as long as they continued to grow, would be found to circumnutate, though perhaps to an extremely small extent.  As it was important for us to discover whether this was the case, we determined to observe carefully a certain number of plants which were growing vigorously, and which were not known to move in any manner.  We commenced with stems.  Observations of this kind are tedious, and it appeared to us that