Two possible misinterpretations must be guarded against. It is not to be supposed that this force-pump action causes movement of the sap towards one point rather than another: it is simply an aid to its movement. From the stock of sap distributed through the plant, more or less is everywhere being abstracted--here by evaporation, here by the unfolding of the parts into their typical shapes, here by both. The result is a tension on the contained liquid columns, which is greatest now in this direction and now in that. This tension it is which must be regarded as the force that determines the current upwards or downwards; and all which the mechanical actions do is to facilitate the transfer to the places of greatest demand. Hence it happens that in a plant prevented from oscillating, but having a typical tendency to assume a certain height and bulk, the demands set up by its unfolding parts will still cause currents; and there will still be alternate ascents and descents, according as the varying conditions change the direction of greatest demand--the only difference being that, in the absence of oscillations, the growth will be less vigorous. Similarly, it must not be supposed that mechanical actions are here alleged to be the sole causes of wood formation in the individual plant. The tendency of the individual plant to form wood at places where wood has been habitually formed by ancestral plants, is manifestly a cause, and, indeed, the chief cause. In this, as in all other cases, inherited structures repeat themselves irrespective of the circumstances of the individual: absence of the appropriate conditions resulting simply in imperfect repetition of the structures. Hence the fact that in trained trees and hothouse shrubs, dense substance is still largely deposited; though not so largely as where the normal mechanical strains have acted. Hence, too, the fact, that in such plants as the Elephant’s-foot or the _Welwitschia mirabilis_, which for untold generations can have undergone no oscillations, there is an extensive formation of wood (though not to any considerable height above the ground), in repetition of an ancestral type: natural selection having here maintained the habit as securing some other advantage than that of support. Still, it must be borne in mind that though intermittent mechanical strains cannot be assigned as the direct causes of these internal differentiations in plants that are artificially sheltered or supported, they are assignable as the indirect causes; since the inherited structures, repeated apart from such strains, are themselves interpretable as accumulated results of such strains acting on successive generations of ancestral plants. This will become clear on combining the several threads of the argument and bringing it to a close, which we may now do.