THE STEM. 95 



lie between the cork cambium and stelar cambium. This 

 inner part contains a greater amount of water than the outer, 

 and always some living tissues. It may consist of a part of 

 the cortex and both primary and secondary bast. As the 

 tree grows older the bark may come to consist almost wholly 

 of secondary bast. It attains considerable thickness only 

 when the loss from weathering is slow. 



EXERCISE XVIII. 



Stems. — Cut cross-sections of the stem of (i) a seedling lieam and (2) 

 a young stem of asparagus and compare. Observe in (i) the three 

 regions, epidermis, cortex, and stele (T[ 107). In the stele observe (a) 

 the cut ends of the vascular strands and their arrangement. (Each pair 

 looks like a single strand except in very thin sections.) {h) The central 

 pith. In (2) observe the epidermis, very narrow cortex, and the stele 

 occupying the greater part of the section. In the latter observe the cut 

 ends of the strands, distributed throiighout the pith. 



Cut a cross-section of the three-year-old shoot of any shrub or tree. 

 Observe (a) the central pith, {b) the wood strands increased in number 

 and thickness until they form a cylinder of wood, in which three annual 

 layers can be observed (how marked ?) ; (c) the stelar cambium, a line of 

 weakness (young cells) outside the wood; (d) the bark, composed of the 

 bast strands on the inside, the cortex (in part) next, and the periderm 

 (brown) on the outside. Compare with the bean stem. How much is 

 the stele? (T^ 111-117.) 



118. Summary. — The stem shows nodes, i.e., the zones of 

 attachment of leaves, and internodes. The length of the 

 latter determines the distribution of the leaves. Stems may 

 be erect, prostrate, or climbing. They show three regions, 

 epidermis, cortex, and stele; each with great variety of struc- 

 ture in different plants. The stele consists of vascular strands 

 of two kinds, arranged in pairs, and embedded in pith. As 

 stems grow older they frequently increase in diameter by the 

 formation of concentric growing zones in the cortex and 

 stele. The outer one produces the periderm, the inner one 

 wood and bast. In trees and shrubs the wood and bast 

 receive annual additions. They separate readily at the stelar 

 cambium, the outer cylinder being the bark. 



