92 BOTANY 
medullary rays, while that in contact with their inner 
side is known as pith. The pith cavity seen in the bean 
stem is absent from most plants (Fig. 57). In each of 
two opposite corners of the bean stem there is an extra. 
vascular bundle. These form the leaf traces and give 
off vascular strands to the leaves. 
Functions.—The epidermis, in the case of the stem, 
is purely protective. The wood, it has been seen, by 
means of its long tubes or vessels, conducts, from the 
soil, water containing dissolved minerals. In this con-- 
nection it is instructive to place a number of bean 
plants with their roots dipping into eosin, and, by 
taking sections from time to time, to note the rate at 
which the solution travels. The bast and cortex carry 
respectively the proteins and carbohydrates formed in 
the leaves to the parts that require them, and the 
ecambium provides for the thickening of the stem. Cells: 
eut off from the outside of the cambium are pushed 
outwards to form bast, while cells cut from the inside: 
develop to form the wood. By means of their cambium, 
too, the medullary rays keep pace with the development: 
of the bundles. 
The bean is an annual, so that it is not possible to: 
trace the development of the stem from year to year. 
In the older stems of trees and other perennials certain 
new points must be noted (Figs. 58-59). The wood 
which now comprises nearly the whole stem is arranged 
in rings, each ring representing one year’s growth.. 
This ringed formation arises from the variation in: 
growth due to the changing of the seasons. The wood 
produced in spring shows large and well-formed vessels, 
while that which originates in the autumn contains: 
smaller vessels with much thickened walls. The: 
autumn wood is therefore more compact, and thus, by 
its regular alternation with the spring wood marks. 
out in rings the annual layers. The pith, as a rule,, 
entirely disappears, while the cortex and bast are: 
