70 BOTANY 
through the cortex or bast or through both, since these 
are the tissues removed. Careful experiment has shown 
that the proteins travel down the bast and the carbo- 
hydrates down the cortex. The bast then conducts 
proteins and the cortex carbohydrates. This translo- 
cation current or flow of organic matter built in the 
leaf is directed towards such parts of the plant as 
require these materials for storage or growth. 
The epidermis, as we have already seen, is partly 
protective and partly absorptive. The pith may at 
times contain a store of reserve material, but more 
often its cells lose their protoplasm and hence cease 
to live. 
In the bean and other dicotyledons, as the root 
develops, there is formed a ring of secondary meristem 
called cambium, which, pushing the bast outwards, 
forms a ring between wood and bast, so that in the 
older root these tissues instead of being arranged 
alternately are so placed that the latter lies outside 
the former. The cambium, by constant division, adds 
to the bast on the outside and to the wood on the 
inside, and in this way the root is thickened. At the 
same time the endodermis, cortex, and epidermis 
disappear, being replaced by a corky bark (Fig. 46). 
In its early stages the root structure of a 
monocotyledon is practically the same as that of a 
dicotyledon, though there will as a rule be more 
wood groups than in the latter. The most important 
difference, however, is that no cambium is formed, and 
hence there is no provision for thickening. This 
accounts for the thin fibrous nature of the roots of 
most monocotyledons. | | 
TYPES oF Roors, 
We have seen that there are two main types of 
root systems, the tap-root and the adventitious. Tap- 
root systems are best adapted to plants that require 
