has also been formed between these outer protective layers and the cambium.
We have now to obtain some ideas as to these curious processes of increase in thickness of the stems and branches.
The first thing to insure this is to understand the constitution and behavior of the cambium cylinder, for it is principally this tissue which brings about the changes we have to study.
We saw in Chapter IV that the xylem of each primary vascular bundle is separated from the phloem of the same bundle by a thin strand of cambium (Figs. 9 and 12); we also saw that the bundles are arranged in a closed ring round the pith, and are in their turn surrounded by the primary cortex, each being separated laterally from its neighbors by a primary medullary ray. The next point to bear in mind is that these medullary rays (like the pith and cortex) are merely parts of the general cell-tissue, or fundamental tissue, through which the vascular bundles run upwards and downwards with a tangentially sinuous course from the leaves. The primary medullary rays, therefore, are merely spokes, as it were, joining the pith and cortex; and if we could remove, the whole of the vascular bundles and epidermis from the young stem we should have left a solid cylinder of cell (pith) in the center, a hollow cylinder (cortex) concentric to this, and a space between the two bridged over at numerous places by cellular spokes (medullary rays) radiating from the pith to the cortex. Each spoke