How far this oscillatory movement is general we do not know, as not many circumnutating plants were observed by us under the microscope; but no such movement could be detected in the case of Drosera with a 2-inch object-glass which we used.  The phenomenon is a remarkable one.  The whole hypocotyl of a cabbage or the whole leaf of a Dionaea could not jerk forwards unless a very large number of cells on one side were simultaneously affected.  Are we to suppose that these cells steadily become more and more turgescent on one side, until the part suddenly yields and bends, inducing what may be called a microscopically minute earthquake in the plant; or do the cells on one side suddenly become turgescent in an intermittent manner; each forward movement thus caused being opposed by the elasticity of the tissues?

Circumnutation is of paramount importance in the life of every plant; for it is through its modification that many highly beneficial or necessary movements have been acquired.  When light strikes one side of a plant, or light changes into darkness, or when gravitation acts on a displaced part, the plant is enabled in some unknown manner to increase the always varying turgescence of the cells on one side; so that the ordinary circumnutating movement is [page 548] modified, and the part bends either to or from the exciting cause; or it may occupy a new position, as in the so-called sleep of leaves.  The influence which modifies circumnutation may be transmitted from one part to another.  Innate or constitutional changes, independently of any external agency, often modify the circumnutating movements at particular periods of the life of the plant.  As circumnutation is universally present, we can understand how it is that movements of the same kind have been developed in the most distinct members of the vegetable series.  But it must not be supposed that all the movements of plants arise from modified circumnutation; for, as we shall presently see, there is reason to believe that this is not the case.

Having made these few preliminary remarks, we will in imagination take a germinating seed, and consider the part which the various movements play in the life-history of the plant.  The first change is the protrusion of the radicle, which begins at once to circumnutate.  This movement is immediately modified by the attraction of gravity and rendered geotropic.  The radicle, therefore, supposing the seed to be lying on the surface, quickly bends downwards, following a more or less spiral course, as was seen on the smoked glass-plates.  Sensitiveness to gravitation resides in the tip; and it is the tip which transmits some influence to the adjoining parts, causing them to bend.  As soon as the tip, protected by the root-cap, reaches the ground, it penetrates the surface, if this be soft or friable; and the act of penetration is apparently aided by the rocking or circumnutating movement of the whole end of the radicle.  If the surface is