STRUCTURE 317 



stomata identical with leaf stomata, though they are 

 usually much fewer in number. Below this there is in 

 the green aerial stems of the great majority of plants 

 a living tissue (cortex) containing chlorophyll, though 

 generally not highly specialised like the mesophyll of a 

 foliage leaf. Very frequently, especially in the primary 

 shoots of woody plants, the outer cortex is collen- 

 chymatous. In some few cases, especially when the 

 leaves are reduced and functionless, the outer cortex, 

 or portions of it, may be differentiated as palisade tissue. 

 Below the cortex, as in the root, the vascular cylinder 

 occupies the centre of the stem. But the vascular 

 cylinder of the stem is very different in character from 

 that of the root. In the first place it is generally very 

 much wider (the cortex being correspondingly narrower) 

 in proportion to the whole width of the axis. Secondly, 

 the vascular tissue is arranged like that of the leaf in 

 bundles, each consisting of a strand of xylem and a 

 strand of phloem. Each of these bundles has the 

 xylem towards the centre of the stem, and the phloem 

 outside, towards the cortex ; and all the bundles (in 

 the plants called dicotyledons) are arranged side by side, 

 separated by strips of parenchymatous tissue, the rays, 

 and forming a hollow cylinder enclosing the usually 

 parenchymatous pith, which fills up the centre of the 

 stem. Each bundle, like those of the larger leaf veins, 

 nearly always possesses a strand of fibres outside the 

 phloem (Fig. 52, A, B, per.). These fibrous strands may 

 be separated laterally by parenchyma, or they may be 

 joined laterally to form a complete fibrous cylinder. 

 This fibrous cylinder, or the fibrous strands together 

 with the intervening parenchyma, forms the pericycle 

 or outermost layer of the vascular cylinder, and 

 immediately outside it comes the endodermis, whose 



