THE TREE—ITS SHOOT-SYSTEM.
layer may be called the initial layer. This layer behaves essentially like the cambium of a fibro-vascular bundle, except that its daughter-cells become cork and phelloderm instead of phlöem and xylem.
The first event to notice is that each of the initial cells grows radially, and divides by a tangential wall into an inner cell nearest the axis of the branch and an outer cell nearer the epidermis; the outer cell becomes forthwith a cork-cell — i. e., its contents die and mostly disappear, and the cellulose cell-wall becomes suberized — the inner cell remains capable of repeating the process. But this is not the only case. After the division, as before, of the initial cell, it may happen that the inner cell becomes transformed into a collenchymatous cortical cell containing chlorophyll, and it is the outer of the daughter-cells which retains the meristem character and acts again as a phellogen cell, cutting off daughter-cells sometimes on one side and at others on the other. Thus, in the oak, the phellogen gives rise to permanent tissue on both side, of the initial layer: those cells which lie on the inside become phelloderm (cortical cells), those on the outside become transformed into phellem (cork). The three tissues, phelloderm, phellogen, and phellem, are called the periderm.It is obvious that the cork-cambium, by thus adding to the cortical parenchyma, is gradually driven radially outwards from the center of the stem. This means that it obtains room to extend tangentially, and it does this by its cells occasionally dividing by walls perpendicular