16 A TEXT-BOOK OF BOTANY 
in diameter (§ 24). The new wood, however, is not formed 
in connection with the old wood, but just within the bast, 
that is, farther in between the “spokes” of old wood, 
resulting in bundles like those of the stem (Fig. 73). In 
this way a thickening vascular cylinder is formed, like that 
of stems that increase in diameter; and presently the cross- 
section of the root resembles that of the stem. It is evident 
(Fig. 73) that the principal pith rays separating the vas- 
cular bundles of such a root extend inward to the original 
radiating strands of wood that alternate 
with the original strands of bast. The vas- 
cular bundles of the root connect with those 
of the stem, and these in turn with those of 
the leaves, so that throughout the whole 
plant there is a continuous vascular system. 
The origin of the branches of roots is very 
different from that of stems. In a stem 
the branch begins at the outer part of the 
cortex, but in the root it begins at the sur- 
face of the vascular cylinder and breaks 
through the cortex (Fig. 74). If the cor- 
tex of a root be stripped off, the branches 
7 : will be found attached to the central axis, 
Fic. 74.—Longitu- ( 
dinal section of and the perforations made by the branches 
ee ee through the cortex can be seen. 
branches start- 38. Growth in length.—The clongating 
108 hove ot region of the root is much more restricted 
penetrating the than that of the stem. It was stated (§ 26) 
cortex. S : 
that the elongating region of a stem may 
extend ten to twenty inches from the tip, or even more; 
but the elongating region of a root is hardly ever more than 
two-fifths of an inch, and often not more than half of that. 
The region of elongation and of greatest elongation should 
be determined by using such seedlings as those of peas, 
beans, and corn. When the young roots have become a 
