228 THE MOVEMENT OF WATER THROUGH THE PLANT 



produced often extend for a long distance in a vertical direction through the 

 wood. The distribution of the wood parenchyma strands throughout an 

 annual ring of xylem tissue varies with the species. In some they are more 

 or less scattered through the xylem, in others they occur only in the last layer 

 or two of cells which are produced in the summer wood — in other words at 

 the termination of the season's growth — and in others only in contact with 

 the vessels, or in contact with other wood parenchyma cells which are them- 

 selves in contact with the vessels. 



All of the xylem elements previously described are oriented with their 

 long axes in a vertical direction. In addition to this vertical system there 

 is also present in the xylem a transverse, radiating system in which the long 

 axis of the cells is at right angles to the long axis of the stem. These trans- 

 versely oriented tissue units are known as vascular rays. In the stems of 

 most species they are continuous from the outer extremity of the phloem 

 through the cambium into the xylem, which they penetrate to a greater or 

 lesser distance. The portion of the vascular ray found in the xylem is termed 

 the xylem ray; that found in the phloem the phloem ray. Xylem rays may 

 vary from one to many cells in thickness and likewise in height, certain types 

 usually being characteristic of any one species. The cells of the xylem rays, 

 like those of the wood parenchyma, usually remain alive until the woody tissue 

 is converted into heartwood. The cells of the xylem ray are typically elongate 

 and more or less angular in cross section. The xylem rays probably serve as 

 routes along which lateral movement of water occurs from the xylem to the 

 cambium and phloem, and along which translocation of soluble foods takes 

 place from the phloem to the living cells of the xylem. 



The living ray cells are in contact at various points with the strands of 

 living wood parenchyma cells. The vertically oriented wood parenchyma 

 strands and transversely oriented xylem ray strands thus form a unit system 

 of living cells within the woody cylinder. Hence there is present a continu- 

 ous intermcshing network of living cells throughout the greater mass of non- 

 living vessels, tracheids, and fibers in the younger portions of any woody an- 

 giosperm stem. There are probably few if any of the conducting elements — 

 vessels and tracheids — which are not in contact at one or more points with 

 this continuous sj'stem of living cells. 



The wood of the gymnosperms is simpler in its structure than that of the 

 angiosperms, this group of woody species also showing, in general, a greater 

 uniformity in stem structure than the latter group. The only cell types uni- 

 versally present in the wood of coniferous trees are the tracheids and wood 

 ray cells. Wood parenchyma cells are also present in the wood of most 

 species of conifers, while in many species tracheid-like fiber cells are also 



