224 THE MOVEMENT OF WATER THROUGH THE PLANT 



with water, while in others (ash) it becomes relatively dry. The water in 

 the heartwood of such species as apple and elm appears to be largely static 

 and is not directly involved in translocation. 



Coincident with the development of secondary xylem, secondary phloem 

 tissues also develop from the cambium (Chap. XXVHI). Cork cambiums 

 are also initiated in the bark which produce cork laj^ers. Profound modifica- 

 tions therefore occur in the outer tissues as well as in the xylem of woody 

 stems as they grow older. 



A somewhat more detailed concept of the structure of the cells and ele- 

 ments of the stems of angiosperms through which the movement of water 

 occurs is presented in Fig. 56 and Fig. 57, which represent longitudinal tan- 

 gential and radial sections from the xylem of a tulip tree which may be 

 taken as representative for this group of plants. Fig. 58 and Fig. 59 illus- 

 trate in a similar way the structure of the xylem tissues of the white pine — 

 a representative gymnosperm. The following discussion merely amplifies 

 somewhat the facts depicted graphically in these two figures. 



The xylem tissues of the wood of angiosperms are composed of vessels, 

 tracheids, fibers of several types, wood parenchyma, and xylem ray cells. 

 There is a tremendous variability in the proportional distribution and arrange- 

 ment of these tissues according to species. 



The most characteristic elements of the xylem tissue of angiosperms are 

 the vessels. These are, in general, more or less tubular structures which may 

 extend through many feet of the xylem. In some species cross walls, usually 

 perforated, are of frequent occurrence in vessels, in others such cross walls are 

 infrequent or lacking. In diameter they may range in trees from about 20 fx. 

 to about 400 /x. In vines they may be as much as 700 jx in diameter. The 

 vessels branch extensively in certain regions of the plant, especially at nodes, 

 within the leaf lamina, and at the points in the root systems where root! 

 branching occurs. 



The vessels of the protoxylcm (the first cells of the xylem to mature 

 during the ontogeny of a growing stem or root tip) have cellulose walls which 

 are reenforced by distinctive lignified thickenings which appear as rings, spirals, 

 or other characteristic patterns. The vessels that develop later in the onto- 

 geny of any growing tip have lignified, usually pitted walls which lack any of 

 the thickenings which distinguish the walls of the vessels of the protoxylem. 



Pits normally occur in all parts of vessel walls which are contiguous 

 with other vessels or cells. These pits consist essentially of thin areas in the 

 walls. The walls of the vast majority of plant cells are pitted. The archi- 

 tecturally distinct type known as the bordered pit is characteristic of most 



