VEGETATIVE OBGAXS OF PLANTS. 



301 



ous 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 already described as 

 occurring in the maize-stalk. The deli- 

 cate cambial cells, c, are in the region of 

 most active growth. At this point new 

 cells rapidly develop, those to the right, 

 in the figure, remaining plain cells and 

 becoming loosely filled with starch ; 

 those to the left developing new ducts. 



In the slender, overground potato- 

 stem, as in all the stems of most agri- 

 cultural plants, the same relation of 

 parts is to be observed, although the 

 vascular and woody tissues often pre- 

 ponderate. 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. 



Pitted Wood-Cells of the Coni- 

 fers. In the wood of cone-bearing trees 

 there are no proper ducts, such as have 

 been described. The large wood-cells which constitute 

 the concentric rings of the wood are constructed in a spe- 

 cial manner, being provided laterally with pits, or, accord- 

 ing to Schacht, with visible pores, through which the 

 fluid contents of one cell may easily diffuse (by osmose), 

 or even pass directly into those of its neighbors. 



Fig. 53, B represents a portion of an isolated wood-cell 

 of the Scotch Fir (Pinussylvestris) magnified 200 diam- 

 eters. Upon it are seen nearly circular disks, x, y, the 



Fig. 53. 



