But in the common Ivy which, instead of running along the surface of the earth, runs up inclined or vertical surfaces, we see the process interestingly modified without being essentially changed. The rootlets, here differentiated by their conditions into organs of attachment much more than organs of absorption, still develop on that side of the shoot next the supporting surface, and do not develop where the shoot, growing away from the tree or wall, is surrounded equally on all sides by light and air: thus showing, undeniably, that the production of the rootlets is determined by the differential incidence of forces. Though survival of the fittest doubtless furthered this transition yet it clearly did not initiate it. That greenness which may be observed in these Ivy-branch rootlets while they are quite young, soft, and unshaded, introduces us to facts which are the converse of the foregoing facts; and proves that the parts ordinarily imbedded in the soil and adapted to its actions, acquire, often in very marked degrees, the superficial structures of the aërial parts, when they are exposed to light and air. This may be witnessed in Maize, which, when luxuriant, sends out from its nodes near the ground, clusters of roots that are thick, succulent, and of the same colour as the leaves. Examples more familiar to us in England occur in every field of turnips. On noting how green is the uncovered part of a turnip-root, and how manifestly the area over which the greenness extends varies with the area exposed to light, as well as with the degree of the exposure, it will be seen that beyond question, root-tissue assumes to a considerable extent the appearances and function of leaf tissue, when subject to the same agencies. Let us not forget, too, that where exposed roots do not approach in superficial character towards leaves, they approach in superficial character towards stems: becoming clothed with a thick, fissured bark, like that of the trunk and branches. But the most conclusive evidence is furnished by the actual substitutions of surface-structures and functions, that occur in aërial organs which have taken to growing permanently under ground, and in underground organs which have taken to growing permanently in the air. On the one hand, there is the rhizome exemplified by Ginger--a stem which, instead of shooting up vertically, runs horizontally below the surface of the soil, and assumes the character of a root, alike in colour, texture, and production of rootlets; and there is that kind of swollen underground axis, bearing axillary buds, which the Potato exemplifies--a structure which, though homologically an axis, simulates a tuberous root in surface-character, and when exposed to the air, manifests no greater readiness to develop chlorophyll than a tuberous root does. On the other hand, there are the aërial roots of certain Orchids which, habitually green at their tips, continue green throughout their whole lengths when kept moist; which