An introduction to physiological and systematical botany/Chapter 4
CHAPTER IV.
OF THE CELLULAR INTEGUMENT.
Immediately under the Cuticle we find a succulent cellular substance, for the most part of a green colour, at least in the leaves and branches, which is called by Du Hamel the Enveloppe cellulaire, and by Mirbel the Tissu herbacé. This is in general the seat of colour, and in that respect analogous to the rete mucosum, or pulpy substance situated under the human cuticle, which is red in the European, and black in the Negro; but we must carry the analogy no further, for these two parts perform no functions in common. Du Hamel supposed this pulp to form the cuticle; but this is improbable, as his experiments show, when that membrane is removed, that the Cellular Integument exfoliates, at least in trees, or is thrown off in consequence of the injury it has sustained, and a new cuticle, covering a new layer of the same succulent matter, is formed under the old one. Annual stems or branches have not the same power, any more than leaves. But little attention has been paid to this organ till lately, though it is very universal, even, as Mirbel observes, in Mosses and Ferns. The same writer remarks that "leaves consist almost entirely of a plate of thissubstance, covered on each side by thecuticle. The stems and branches of bothannual and perennial plants are investedwith it; but in woody parts it is dried upand reproduced continually, such partsonly having that reproductive power. Theold layers remain, are pushed outwardby the new ones, and form at length therugged dry dead covering of the old trunksof trees."
When we come to consider the curious functions of leaves, we shall find this part to be of the very first importance. In it the principal changes operated upon the juices of plants by light and air, and the consequent elaboration of all their peculiar secretions, take place.