THE STEM 105 



cambium and pith, which includes the medullary rays so con- 

 spicuous in perennial stems, are composed of live paren- 

 chyma, cells, from which alone growth can take place ; they 

 are the active part of the stem. The xylem contains the 

 large vessels, t and s, that convey water up the stem, together 

 with the wood fibers, h. These are the permanent tissues. 

 After completing their growth the cells of the xylem gradu- 

 ally lose their protoplasm, and all vitality ceases. Even the 

 cell sap disappears, and sometimes the walls of the ducts are 

 disintegrated, leaving a mere air space like that shown at I in 

 Figs. 115 and 116. The dead cells and tissues, however, are 

 by no means useless. They constitute the heartwood that 

 is so valuable for timber, and serve an important purpose as 

 a mechanical support for the stem. The phloem contains 

 on its outer face a mass of hard fibers, b, called bast, and 

 toward the interior, the sieve tubes, sb, with a number of 

 smaller vessels that convey down the stem the sap containing 

 the food made in the leaves. It is separated from the cortex 

 by the bundle sheath, e, and on its other side, from the ex- 

 terior face of the xylem by the cambium, C. In this position 

 the growing cambium adds new cells to the inner side of the 

 phloem, and to the outer side of the xylem, so that the former 

 grows on its inner face and the latter on its outer. In peren- 

 nial plants, as new rings are added to the xylem from season 

 to season, the older ones die and are changed into heartwood, 

 which thus gradually increases in thickness till in some of the 

 giant redwoods and eucalypti, it may attain a diameter of 

 thirty-five or forty feet. In the phloem, on the other hand, 

 as new cells are added from within, the older ones are 

 gradually changed into hard bast, 6, then into bark, and 

 are finally sloughed off and fall to the ground. It is this 

 free line of communication with the active cambium that 

 enables dicotyl stems to grow on indefinitely, the sheath, e, 

 being formed on the exterior face of the bundles only, leav- 

 ing the other free, whence they are said to be open. 



Make drawings of cross and vertical sections of a dicotyl 



