5o8 



THE STRUCTURE OF ECONOMIC PLANTS 



ground portion of adventitious stems which arise from the fleshy- 

 roots, Casparian strips are well developed. 



The pericycle is one to three cells in width and a discontinuous 

 cylinder of pericyclic fibers is differentiated in it, some of which 

 are very much thickened. (Fig. 167.) The outer phloem, in 

 addition to sieve tubes and companion cells, consists of parenchyma 

 which frequently contains crystals of calcium oxalate, and a few 



Fig. 167. Transection of sector of stele of mature stem showing outer and inner cambiums : 

 i ca, inner cambium ; i ph, inner phloem ; Ix ves, latex vessel ; o ca, outer cambium ; o ph, 

 outer phloem; pel, pericyclic fiber; pi, pith; xy i, primary xylem; xj t, large secondary 

 xylem vessel. 



latex canals may occur adjacent to the pericycle. The cambium 

 forms a continuous cylinder early in ontogeny as a result of the 

 development of an active interfascicular cambium. In the stele, 

 there are commonly two or three bundles which have larger vessels 

 than the intervening ones; and it is characteristic of the stem of 

 Ipomoea, as well as of many other representatives of the Convol- 

 vulaceae, that these bundles are located in two groups which lie 

 approximately opposite each other in the vascular cylinder. The 



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