278 HOW CROPS GROW. 
proaches the corky-layer or the vascular ring, the cells are 
smaller, and contain smaller starch grains; either side of 
these are noticed cells containing no starch, but having 
nuclei, c, y. These nucleated cells are capable of multi- 
plication, and they are situated where the growth of the 
tuber takes place. The rind, which 
makes a large part of the flesh of the 
potato, increases in thickness by the 
formation of new cells within and with- 
out. Without, where it joins the corky 
skin, the latter likewise grows. Within, 
contiguous to the vascular zone, new 
ducts are formed. In a similar manner, 
the pith expands by formation of new 
cells, where it joins the vascular tissue, 
The latter consists, in our figure, of ring, 
spiral, and dotted ducts, like those al- 
ready described as occurring in the 
maize-stalk. The delicate cambial cells, 
c, are in the region of most active 
growth. At this point new cells rap- 
idly develope, those to the right, in the 
figure, remaining plain cells and becom- 
ing loosely filled with starch; those to 
the left developing new ducts. 
In the slender, overground potato- 
stem, as in all the stems of most agricul- 
tural plants, the same relation of parts 
is to be observed, although the vascular 
and woody tissues often preponderate. 
Wood-cells are especially abundant in 
those stems that need strength for the fulfilment of their 
offices, and in them, especially in those of our trees, the 
structure is commonly more complicated. 
Perforation of Wood-Cells in the Conifers.—In the 
wood of cone-bearing trees there are no proper ducts, such 
Fig. 53. 
