250 



THE MAINTENANCE OF THE INDIVIDUAL 



instead of being located in a ring as in the dicotyledons, are scattered 

 through the pith although more concentrated toward the outer edge of 

 the stem. Examination of this outer edge or rind shows that there 

 is no true bark, but that this outer area is made up of these same 

 woody bundles closely massed together. Under high power, the 

 bundles are seen to have outer strengthened walls of wood cells 



enclosing tubelike cells of 

 different diameters of 

 which the larger have 

 pitted surfaces. The area 

 containing these tubes is 

 the xylem. Other elon- 

 gated tubular cells having 

 their ends perforated with 

 small holes like a sieve, 

 form the sieve tubes, 

 w^hich are the conducting 

 tissues of the phloem. In 

 the phloem, the tubes pass 

 foods down from the 

 leaves, while the xylem 



A cross section through a closed monocotyle- carries water up from the 

 donous bundle. Note that the thick-walled roots to the leaves. The 

 xylem cells completely enclose the cells of the entire WOody bundle is en- 



^ °^™' closed w^th a tough wall of 



sclerenchyma which gives strength and resiliency to the stem. Since 

 this hard tissue binds the entire bundle, it is called a closed bundle. 

 Monocotyledonous stems grow, then, through an increase and 

 lengthening of closed bundles in the parenchyma of the stem. 



The end result in both monocotyledon and dicotyledon stems is the 

 same. The vascular bundles put the root, stem, and leaves in direct 

 communication. The root hairs at one end and the cells of the leaf 

 at the other end are the opposite terminals of long communicating 

 woody tubes. These tubes carry water and solutes up from the soil 

 to the cells of the leaf, and, as will be shown presently, carry elaborated 

 food materials down from the leaves to various parts of the plant, 

 where they may be stored for future consumption or used immediately 

 to liberate the energy needed in growth and in destructive metabolic 

 changes. The vascular bundles which leave the stem to enter the 

 leaves do so by way of the petiole or leaf stalk. As they enter the blade 



