76 REGIONS OF THE STEM 



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 inter- 

 change 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 first 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, termed the cortex contains more or less 

 chlorenchyma — at least this is true of small stems — -and functions 

 as in the leaf; while the inner rather colorless portion of this 

 region serves to conduct foods manufactured by the leaves and 

 it also frequently serves as a storehouse. 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, con- 

 ducts the water and crude materials absorbed from the soil and 

 also serves as a storage organ for the reserve 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, the tissues will resemble the arrangement shown 

 in Fig. 40. It should be stated that the vascular bundles (Fig. 

 40, v) do not appear as separate strands in the earliest stages of 

 the development of the stem. This vascular tissue appears 

 rather as a more or less continuous circle of tissue surrounding 

 the pith. Very early in the development of the stem this circle 

 of vascular tissue is broken into segments or vascular bundles. 

 This separation of the ring of vascular tissue into segments is 



