In height until the lower parts are well thickened, when it may be allowed to acquire the allotted height by moderate annual increments.
In relation to its characteristic function of protection, the epidermis, which, as above defined, consists of a single layer of cells has typically thickened and cuticularized outer walls.
Isolated celh (idioblasts), thickened in various ways, are not uncommonly founc supporting the tissues of the leaf.
This consists of elongated cells with cellulose walls, which are locall~ thickened along the original corners of the cells, reducing the lumer to a cylinder, so that a number of vertical pillars of cellulose con nected by comparatively thin walls form the framework of th~ tissue.
Thus the structure of an old thickened root approximates to that of an old thickened stem, and so far as the vascular tissue is concerned can often only be distinguished from the latter by the position and orientation of the primary xylems. The cambium of the primary root, together with the tissues which it forms, is always directly continuous with that of the primary stem, just in the same way as the tissues of the primary stele.