Although the cells of the higher plants may be all very much alike when they begin life, they vary immensely in size, shape, and structure by the time they reach full development, their ultimate construction being dependent upon the functions they have to perform for the wellbeing of the plant. The most common change is the thickening of the cell wall internally, by successive layers of cellulose, till the internal cavity is nearly filled up, and the cellulose gets converted into wood (fig. 5). The cells of the pith remain thin-walled (fig. 6). Those on the outside of the trunk of the Cork tree, Elm, Ash, etc, get thickened like those of the wood; but in this case the material is converted into cork, which is very light and almost impermeable by water. A thin layer on the outer face of all leathery leaves, like those of the India Rubber and Palms, forms the cuticle, and is also of the nature of cork.