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- 

 A 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. 



FIG. 116. A 

 wood fiber, con- 

 sisting of a much 

 elongated cell 

 with thick 

 woody walls. 



FIG. 117. Very much enlarged lengthwise section 

 through an Alfalfa stem, showing the conductive and food- 

 making tissues of the stem. I, tracheae (commonly called 

 xylem), which constitute the water-conducting tissue; 

 p, the conductive tissue (commonly called phloem), which 

 conducts the food made by the leaves; c, the food-making 

 and storage tissue (cortex) just under the epidermis (e). 

 The cells of the cortex contain chloroplasts (ch). a, 

 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 



