NATURE OF PLANTS 



81 



the wood cells of the stem remain alive for only a few years. 

 This fact is usually apparent in any log or stump where it will 

 be seen that the outer annular rings are of a lighter color, the 

 sap wood, and the older parts are of a darker color, the heart 

 wood. The sap wood is active in the transport of water and 

 contains many living cells. For this reason it is not so valuable 

 for lumber. The heart wood is composed of dead cells and 

 while the most serviceable for building purposes, it is of little 

 use to the tree as we often see a vigorous tree whose heart wood 

 has been largely destroyed by decay. 



40. Other Types of Stems. — The cone bearing trees, such as 

 the pines and spruces, have essentially the same arrangement 

 of tissues and mode of growth as noted above in the dicotyledons. 

 The xylem, however, consists entirely of tracheids (Fig. 58) 



Fig. 60. Cross-section of a single vascular bundle of corn stem: ph. 

 phloem ; x. small cells of the xylem ; v. vessels of xylem ; st, stereome that 

 forms a sheath about the bundle; p, parenchyma of the stem. 



7 



