278 



HOW CROPS GROW. 



proaches the corky-layer or the vascular ring, the cells Sire 

 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 

 jDotato, increases m 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, 

 y IJ II f]f^'y 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 developc, 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 comm.only more complicated. 



Perforation of Wood-Cells in the Conifers.— In the 



wood of cone-bearing trees there are no proper ducts, such 



Fi"-. 53. 



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