That these dissimilarities arose by degrees, as fast as the units of which the phænogamic axis is composed became integrated, is a conclusion in harmony with the truth that in every shoot of every plant, they are at first slight and become gradually marked. Already, in briefly tracing the contrasts between the outer and inner tissues of plants, some facts have been named showing, by implication, how the cessation of the leaf-function in axes is due to that change of conditions entailed by the discharge of other functions. Here we have to consider more closely facts of this class, together with others immediately to the point. On pulling off from a stem of grass the successive sheaths of its leaves, the more-inclosed parts of which are of a fainter green than the outer parts, it will be found that the tubular axis eventually reached is of a still fainter green; but when the axis eventually shoots up into a flowering stem, its exposed part acquires as bright a green as the leaves. In other Monocotyledons, the leaf-sheaths of which are successively burst and exfoliated by the swelling axis, it may be observed that where the dead sheaths do not much obstruct the light and air, the surface of the axis underneath is full of chlorophyll. _Dendrobium_ is an example. But when the dead sheaths accumulate into an opaque envelope, the chlorophyll is absent, and also, we may infer, the function which its presence habitually implies. Carrying with us this evidence, we shall recognize a like relation in Dicotyledons. While its outer layer remains tolerably transparent, an exogenous stem or branch continues to show, by the formation of chlorophyll, that it shares in the duties of the leaves; but in proportion as a bark which the light cannot penetrate is produced by the adherent flakes of dead skin, or by the actual deposit of a protective substance, the differentiation of duties becomes more decided. Cactuses and Euphorbias supply us with converse facts having the same implication. The succulent axes so strangely combined in these plants, maintain for a long time the translucency of their outer layers and their greenness; and they so efficiently perform the offices of leaves that leaves are not produced. In some cases, axes that are not succulent participate largely in the leaf-function, or entirely usurp it--still, however, by fulfilling the same essential conditions. Occasionally, as in _Statice brassicæfolia_, stems become fringed; and the fringes they bear assume, along with the thinness of leaves, their darker green and general aspect. In the genus _Ruscus_, the flattened axis simulates so closely the leaf-structure, that were it not for the flower borne on its mid-rib, or edge, its axial nature would hardly be suspected. And let us not omit to note that where axes usurp the characters of leaves, in their attitudes as well as in their shapes and thicknesses, there are contrasts between their under and