NATURE OF PLANTS 71 



quently the scars formed by the fall of these protective scales 

 form a ring (Fig. 39, r) each year about the stem which marks 

 the position of each successive bud. These scars are therefore 

 known as annual rings and by counting the number of these rings 

 on a twig you can ascertain its age and observe the extent of 

 elongation during each season. The minute dots or lenticels 

 consist of loosely arranged cells that permit an interchange of 

 gases between the living cells within the stem and the atmosphere, 

 functioning in much the same way as do the stomata of the 

 leaves. 



We are now interested to learn something of the internal char- 

 acter of the stem. By cutting with a sharp knife across the stem 

 between the terminal bud and the fiist annual ring we see that 

 there are several regions in the stem. On the outside is a very 

 thin brown layer, largely composed of cork cells, which prevents 

 loss of water and which in old stems becomes quite thick and 

 variously split. Within is a zone of rather delicate tissue. The 

 outer part of this region, known as the cortex, contains more or 

 less chlorenchyma and functions as in the leaf; while the inner, 

 rather colorless portion serves to conduct the foods manufactured 

 by the leaves and also frequently serves as a store house. The 

 third zone is made up of thick-walled, compact cells, regularly 

 arranged. This is the wood or xylem portion of the stem. It 

 gives stability, conducts the water and crude materials absorbed 

 from the soil and also may serve as a storage organ for the re- 

 serve foods of the plant. The center of the stem, known as the 

 pith, consists of delicate cells which conduct water for a short 

 time and soon die. 



33. The Anatomy of the Stem. — We are now desirous of seeing 

 how this complex structure of»the stem comes about and of 

 studying in more detail the character and operations of the 

 apparatus. 



If thin sections are made across the stems of seedlings, as of 

 the castor bean, or of shoots just emerging from the buds, the 

 tissues will resemble the arrangement shown in Fig. 40. All 

 very young stems of dicotyledons, i. e., plants whose seedlings 

 have two leaves, as the squash, the bean, etc., show more or 



