274 HOW CROPS GROW. 
the epidermis. They occur at first separately, as in the 
endogens, but instead of being scattered throughout the 
cell-tissue, are disposed in a circle. As they grow, they 
usually close up to a ring or zone of wood, which, within, 
incloses unaltered cell-tissue—the pith—and without, in 
shrubs and trees, is covered by rind. 
As the stem enlarges, new rings of fibers may be form- 
ed, but always outside of the older ones. In hard stems 
of slow growth the rings are close together and chiefly 
consist of very firm wood-cells. In the soft stems of herbs 
the cell-tissue preponderates, and the ducts and cells of 
the vascular zones are delicate. The hardening of herba- 
ceous stems which takes place as they become mature, is 
due to the increase and induration of the wood-cells 
and ducts. 
The circular disposition of the fibers in the exogenous 
stem may be readily seen in a multitude of common 
plants. 
The potato tuber is a form of stem always accessible 
for observation. If a potato be cut across near the stem- 
end with a sharp knife, it is usually easy to identify upon 
the section a ring of vascular tissue, the general course of 
which is parallel to the circumference of the tuber except 
where it runs out to the surface in the eyes or buds, and 
in the narrow stem at whose extremity it grows. Ifa 
slice across a potato be soaked in solution of iodine for a 
few minutes, the vascular rings become strikingly apparent. 
In its active cambial cells, albuminoids are abundant, which 
assume a yellow tinge with iodine. The starch of the cell 
tissue, on the other hand, becomes intensely blue, making 
the vascular tissue all the more evident. 
Since the structure of the root is quite similar to that 
of the stem, a section of the common beet as well as one 
of a branch from any tree of temperate latitudes may 
serve to illustrate the concentric arrangement of the 
vascular zones when they are multiplied in number. 
