THE PATH OF LIQUIDS IN THE PLANT 65 
distributed quite regularly in a circle, surrounding the 
central pith (medulla). This central tissue extends out 
between the nbro-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 ot 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 
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