THE PATH OF LIQUIDS IN THE PLANT 65 



distributed quite regularly in a circle, surrounding the 

 i:entral pith {medulla). This central tissue extends out 

 between the fibro- vascular bundles, forming the pith-rays, 

 or medullary rays. The tissue outside the zone of bundles 

 is the cortex. 



In older stems of this type the bundles increase in 

 number until a nearly continuous cylinder of vascular 

 tissue results (Fig. 47), broken only by numerous thin 

 medullary rays. In cross-sectional view this cylinder, of 

 course, appears as a circle. Stems having this arrange- 

 ment of tissues grow by the formation of cylinders of new 

 tissue, outside of, and surrounding the older woody tissue. 

 On this account they are called exogenous (outside growing) 

 stems, or, preferably, dicotyledonous stems. 



63. Extension of Vascular Tissue into the Roots and 

 Leaves. — As shown in Fig. 48, the vascular bundles of 

 the stem continue down into the roots, branching out into 

 the smallest rootlets, and connecting with the tissue which 

 lies next to the epidermis with its root-hairs. Thus it is 

 seen that there is an unbroken connection of vascular tissue 

 from the roots to the leaves. Root-hairs and leaf-veins 

 (Figs. 49 and 50) are the opposite extremities of this sys- 

 tem, which serves for the conduction of liquids through 

 the plant. 



64. Structure of the Fibro-vascular Bundles.— When 

 cross-sections of the bundles of the type shown in the 

 castor-oil plant are examined under the microscope, 

 they appear somewhat wedge-shaped, with the smaller 

 end pointing toward the center of the stem (Fig. 51). 

 Three well-defined regions may be clearly distinguished, 

 as follows: 



I. The xylem, at the pointed end of the bundle, and 

 s 



