274 HOW CEOPS GEOW. 



the eijidermis. 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 ujpon 

 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. If a 

 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. 



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