42 



PLANTS AND MAN 



instead of being in scattered groups, form a continuous ring 

 just inside the cortex; in them most of the circulation of sugar 

 and other soluble plant foods take place. The water conducting 

 channels form a second ring of cells on the inside of the phloem 

 zone; they are continuous with the xylem of the root and that of 

 the leaf veins. Between the two is a narrow zone of growth 

 cells — the cambium — which keep dividing as long as the tree 

 lives, bringing about increase in diameter of the trunk by 

 forming new phloem cells and new xylem cells. The remainder 



CORTEX 



PHLOEM (^ooj> j>ocTs) 

 CAMB/UM 



XYLEM ft^-^T-^^ 3>UCTS) 



Fig. 20. — Inside the bark and cortex tissues of a tree trunk lies the sap wood, 

 a zone of living cells which act as the conductive and growing portion of the 

 stem. 



of the maple tree trunk consists of wood. Wood makes up most of 

 the tissue within the cambium ring since it is produced in associa- 

 tion with the water-conducting channels. There are two kinds of 

 wood cells. Long tubes, made up of cylindrical cells placed end on 

 end and known as vessels, conduct the water; scattered among 

 the vessels are the more slender cells with thickened walls known 

 as fibers. These form the skeletal rather than the conductive 

 elements of the trunk. Both the vessels and fibers which make 

 up most of old trunks and branches are dead cells. Living water- 

 conductive cells are found only in the immediate vicinity of the 



