STEMS, ROOTS, AND LEAVES 



93 



vertical conduction of sugar through the stem. The inner phloem 

 as a whole is therefore primarily a food-conducting layer, as dis- 

 tinguished from the wood, which conducts water and soil salts. 



The wood, or xylem, is much more complex than the phloem 

 and is composed of a great variety of living and lifeless cells 

 which perform storage, conducting, and mechanical functions for 



Wood ray 



Duct 



Transverse watt 



Cell wall 

 Cytoplasm 



Sieve plate 



Sieve tube 



r i uets ^^-Ducts Cambium 



Wood fibers Wood Wood tubes 



parenchyma parenchyma 



FIG. 53. Microscopic structure of the wood of alder in radial long section 



a, a drawing of a radial section of the wood of the alder. The living tissues have the 

 protoplasm and nuclei dotted. The sieve tuhes have the sieve plates on the transverse 

 walls separating adjacent cells of the tubes, b, sectional views of a duct and of a 

 sieve tube. Compare the sectional views of the transverse walls of the duct and 

 sieve tube in 6 with the surface views of these same transverse walls hown in a. 



the plant. Only the more important of these tissue cells need 

 be described here. The water ducts, or trachece, are the most 

 conspicuous elements of the wood; these, as we have already 

 learned, are characteristic of the porous spring wood. In stems 

 like the alder the water ducts appear like large light holes in 

 thin sections of the wood (Fig. 51, a). Microscopic observation 

 will show, however, that they are greatly enlarged cells with 

 thickened walls and without living contents. In long sections 

 (Fig. 51, c and d, and Fig. 53), the ducts will be found to be 



