— Special Morphology and Classification: 339 
The stem in its young state is always provided with an 
epidermis, the cells of which often contain silica, as in 
Grasses and Palms, giving the surface a vitreous appearance. 
The simple cortex consists of polygonal or roundish paren- 
chymatous cells, among which are sometimes formed strongly 
thickened cubical sclerenchymatous cells, as also fibriform 
cells and bast-vessels. When very young the cortex is 
separated from the subjacent tissue by an annular layer of 
formative tissue, or cambium-sheath, in which the vascular 
bundles are formed; but the cambium possesses in general 
only a limited activity; and when this is exhausted, the stem 
no. longer continues to increase in diameter at that spot ; 
the cortical parenchyma then passes over gradually into fun- 
damental tissue, or 1s separated from it by the lgnified 
cambium-sheath (Fig. 460 1., v). The vascular bundles 
stand singly in the fundamental tissue ; and as they con- 
tinue to develope in the tissue in which they lie, they do 
not grow in a vertical direction, but turn towards the pith ; 
and from there, again bending outwards, pass into the 
leaves. In this course they must necessarily cross the 
cambium-sheath ; and at the place where this happens the 
bundle branches, one branch entering the leaf through the 
cortex, the other taking a similar course to the next older 
bundle. When a number of bundles have been formed in. 
this manner, every bundle which bends frcm the inner to 
the outer portion of the plant must cross all the younger 
ones in succession (Fig. 460 11.). The fibrovascular bundles 
are necessarily closed, in contrast to the open bundles of 
Dicotyledons and Gymnosperms. The term Ezdogens was 
previously applied to Monocotyledons [and still is com- 
monly in works on Descriptive Botany], because it was 
thought that the vascular bundles originated in the centre 
of the stem, and grew outwardly only in order to enter the 
leaves (see Fig. 95, p. 68). 
In many species the course of the vascular bundles is 
Z2 
