Is this unlikeness between the outer tissues of the attached ends and those of the free ends in plants, determined by their converse with the unlike parts of the environment? That they result from an equilibration partly arising in the individual and partly arising by the survival of individuals in which it has been carried furthest, is inferable _à priori_; and this _à priori_ argument may be adequately enforced by arguments of the inductive order. A few typical ones must here suffice. The gemmules of the _Marchantia_ are little disc-shaped masses of cells composed of two or more layers. Their sides being alike, there is nothing to determine which side falls lowermost when one of them is detached. Whichever side falls lowermost, however, presently begins to send out rootlets, while the uppermost side begins to assume those characters which distinguish the face of the frond. When this differentiation has commenced, the tendency to its complete establishment becomes more and more decided; as is proved by the fact that if the positions of the surfaces be altered, the gemmule bends itself so as to re-adjust them: the change towards equilibrium with environing forces having been once set up, there is acquired, as it were, an increasing momentum which resists any counter-change. But the evidence shows that at the outset, the relations to earth and air alone determine the differentiation of the under surface from the upper. The experiences of the gardener, multiplying his plants by cuttings and layers, constitute another class of evidences not to be omitted: they are commonplace but instructive examples of physiological differentiation. While circumstanced as it usually is, the meristematic tissue of each branch in a Phænogam continues to perform its ordinary function--regularly producing on its outer side the cortical substances, and on its inner side the vascular and woody tissues. But change the conditions to those which the underground part of the plant is exposed to, and there begins another differentiation resulting in underground structures. Contact with water often suffices alone to produce this result, as in the branches of some trees when they droop into a pool, or as occasionally with a cutting placed in a bottle of water; and when the light is excluded by imbedding the end of the cutting, or the middle of the still-attached branch, in the earth, this production of tissues adapted to the function of absorbing moisture and mineral constituents proceeds still more readily. With such cases may be grouped those in which this development of underground organs by an above-ground tissue, is not exceptional but habitual. Creeping plants furnish good illustrations. From the shoots of the Ground-Ivy, rootlets are put out into the soil in a manner differing but little from that in which they are put out by an imbedded layer; save that the process follows naturally-induced conditions instead of following artificially-induced conditions.