Structure and Function of Endodermis in Abietinece 277 
At the same time the vascular tissue, within which are numerous 
medullary rays, undergoes concentration, many of the rays dis¬ 
appearing. There is, at this stage, a central pith consisting of thick- 
walled cellulose elements (Fig. 5 , a). 
The next stage in development consists in the separation, in a 
radial direction, of the xylem into two groups, thus defining the 
Fig. 5. Pinus sylvestris L. (a) —(/). Diagrams showing the evolution of the 
endodermis of the leaf from the dwarf shoot, x., xylem; ph., phloem; 
c., cortex; pi., pith; scl., sclerenchyma; en., endodermis; per., pericycle; 
hyp., hypoderm. 
future plane of division of the two needles, which grow laterally on 
the dwarf shoot (Fig. 5, b). Subsequently indentation of the cortex 
in this plane begins, together with a further separation of the parts 
of the xylem on each side into two groups corresponding with the 
two vascular bundles of each leaf (Fig. 5, c). 
