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, ¢ 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 phloém 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 
phloém, 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 phloém, on the other hand, 
as new cells are added from within, the older ones are 
gradually changed into hard bast, b, 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 
