130 CELLS AND TISSUES 
only much thickened but also made woody — a feature in which 
they differ from collenchyma and bast fibers, where the thicken- 
. ings are mainly of cellulose. (Fig. 116.) Where 
the wood fibers are abundant, as in Oaks, the wood 
is compact. Likewise, due to a greater number of 
wood fibers, fall wood is more compact than spring 
wood. 
Fre. 117.— Very much enlarged lengthwise section 
through an Alfalfa stem, showing the conductive and food- 
making tissues of the stem. {, tracheae (commonly called 
Fic. 116.—A xylem), which constitute the water-conducting tissue; 
wood fiber, con- p, the conductive tissue (commonly called phloem), which 
sisting of amuch conducts the food made by the leaves; c, the food-making 
elongated cell and storage tissue (cortex) just under the epidermis (e). 
with thick The cells of the cortex contain chloroplasts (ch). a, 
woody walls. cambium. 
Conductive Tissues. — The conductive tissues of plants are of 
two kinds, xylem and phloem, which occurring together form the 
vascular bundles through which water, mineral salts, and foods 
are distributed to all parts of the plant. (Fig. 117.) The xylem 
is devoted chiefly to carrying water with what it may have in 
solution and the phloem to carrying foods. Furthermore, the 
xylem and phloem differ in that the conductive cells of the former 
are empty while the conductive cells of the latter retain their pro- 
toplasm. In Conifers, such as Pines, Firs, etc., the water-conduct- 
ing cells have tapering ends and do not form a continuous series. 
They have peculiar pits in their walls, known as bordered pits, 
through which the liquids pass from cell to cell. They are com- 
monly known as tracheids, meaning ‘‘trachea-like.” Other plants 
have tracheids, but tracheids with bordered pits are characteristic 
of Conifers. The tracheids are also important strengthening as 
