THE STEM AND ITS FUNCTIONS 105 



stem (Fig. 57). The individual Inindlcs are very distinctive 

 in appearance (Fig. 58), each possessing a large air-space or 

 lacuna, two large vessels, and a patch of very regularly arranged 

 sieve tubes and companion cells. In such a stem no distinction 

 between pith and cortex now remains. The departure of the 

 leaf-traces here is very complex, a large number of bundles 

 moving outward from the center of the stem and entering the 

 sheathing leaf-base. 



The Structure of Wood. — In shrubs and trees* the great bulk of 

 the stem, particularly in its older portions, consists of but 



^' -""^ xSap-Wood 



i 



iiSfm0 >Heart-W/ood 



Fig. 59. — Heart-wood and sap-wood. Transversely-cut end of an oak log, 

 showing the darkly-colored heart-wood at the center of the stem, surrounded by 

 the lighter sap-wood. 



one tissue, the wood. Wood is so important in the economy 

 of the plant and of such great significance to man that we are 

 justified in studying it a little more closely than we have the 

 other tissues. 



Through the activity of the cambium (a fuller account of whicli 

 we shall reserve for the chapter on growth) a new concentric 

 layer of wood cells is added each year to the outside of the woody 

 cylinder. The tracheids and ducts produced at the beginning 

 of growth in the spring are usually of large diameter and are 

 accompanied by comparatively few fibers, and it is apparently 



* Conifers and dicotyledons alone are discussed here. Woody mono- 

 cotyledons are rare and their woody tissues are very complex. 



