upper surfaces, answering to the contrasts between the relations of these surfaces to the light. Of this _Ruscus androgynus_ furnishes a striking example. In it the difference which the unaided eye perceives is much less conspicuous than that disclosed by the microscope; for I find that while the face of the pseudo-leaf has no stomata, the back is abundantly supplied with them. One more illustration must be added. Equally for the morphological and physiological truths which it enforces, the _Mühlenbeckia platyclada_ is one of the most instructive of plants. In it the simulation of forms and usurpation of functions, are carried out in a much more marvellous way than among the _Cactaceæ_. Imagine a growth resembling in outline a very long willow-leaf, but without a mid-rib, and having its two surfaces alike. Imagine that across this thin, green, semi-transparent structure, there are from ten to thirty divisions, which prove to be the successive nodes of an axis. Imagine that along the edges of this leaf-shaped aggregate of internodes, there arise axillary buds, some of which unfold into flowers, and others of which shoot up vertically into growths like the one which bears them. Imagine a whole plant thus seemingly composed of jointed willow-leaves growing from one another’s edges, and some conception will be formed of the _Mühlenbeckia_. The two facts which have meaning for us here are--first, that the performance of leaf-functions by these axes goes along with the assumption of a leaf-like translucency; and, second, that these flattened axes, retaining their upright attitudes, and therefore keeping their two faces similarly conditioned, have these two faces alike in colour and texture. That physiological differentiation of the surface which arises in Phænogams between axial organs and foliar organs, is thus traceable with tolerable clearness to those differences between their conditions which integration has entailed--partly in the way above described and partly in other ways still to be named. By its relative position, as being shaded by the leaves, the axis is less-favourably circumstanced for performing those assimilative actions effected by the aid of light. Further, that relatively-small ratio of surface to mass in the axis, which is necessitated by its functions as a support and a channel for circulation, prevents it from taking in, with the same facility as the leaves, those surrounding gases from which matter is to be assimilated. Both these special causes, however, in common with that previously assigned, fall within the general cause. And in the fact that where the differential conditions do not exist, the physiological differentiation does not arise, or is obliterated, we have clear proof that it is determined by unlikenesses in the relations of the parts to the environment.