As the plant gains in height, and as the mass of its foliage accumulates, the strains thrown upon its axis, and especially the lower part of its axis, rapidly increase. Supposing the forms to remain similar, the strains must increase in the ratio of the cubes of the dimensions; or even in a somewhat higher ratio. One consequence must be that the compressions to which the vessels at the lower part of the incipient stem are subject, become greater as fast as the height to which the sap has to be raised becomes greater; and another consequence must be that the local exudation of sap produced by the pressure is proportionately augmented. Hence the materials for interstitial nutrition being there supplied more abundantly, we may expect thickening of the surrounding tissues to show itself there first: in other words, wood will be formed round the vessels of the lower part of the incipient stem. The resulting greater ability of this lower part of the stem to bear strains, renders possible an increase of height; and while after an increase of height the lowest part becomes still further strained, and still further thickens, the part above it, exposed to like actions, undergoes a like thickening. This induration, while it spreads upwards, also spreads outwards. As fast as the rude cylinder of dense matter formed in this way, begins to inclose the original vessels, it begins to play the part of a resistant mass, which more and more prevents the contained vessels from being squeezed; while between it and the outer layers the greatest compression occurs at each bend. Thus at the same time that the original vessels become useless, the peripheral cells of the developing wood become those which have their liquid contents squeezed out longitudinally and laterally with increasing force; and, consequently, amid them are formed new sap-channels, from which there is the most active local exudation, producing the greatest deposit of dense matter. Thus fusing together, as it were, the individualities of successive generations of plants, and recognizing as all-important that facilitation of the process which natural selection has all along given, we are enabled to interpret the chief internal differentiations of plants as consequent on an equilibration between inner and outer forces. Here, indeed, we see illustrated in a way more than usually easy to follow, the eventual balancing of outer actions by inner reactions. The relation between the demand for liquid and the formation of channels that supply liquid, as well as that between the incidence of strains and the deposit of substance which resists strains, are among the clearest special examples of the general truth that the moving equilibrium of an organism, if not overthrown by an incident force, must eventually be adjusted to it.