VEGETATIVE OKGANS OF PLAlfTS. 297 



except where it runs out to the surface in the eyes or 

 huds, 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 ring becomes 

 strikingly apparent. In its active cambial cells, albu- 

 minoids are abundant, which assume a yellow tinge with 

 iodine. The starch of the cell-tissue, an 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 vas- 

 cular zones when they are multipled in number. 



Pith is the cell-tissue of the center of the stem. In 

 young stems it is charged with juices; in older ones it 

 often becomes dead and sapless. In many cases, espec- 

 ially when gi-owth is active, it becomes broken and nearly 

 obliterated, leaving a hollow stem, as in a rank pea-vine, 

 or clover-stalk, or in a hollow potato. In the potato 

 ■ tuber the pith-cells are occupied throughout with starch, 

 although, as the coloration by iodine makes evident, the 

 quantity of starch diminishes from the vascular zone 

 towards the center of the tuber. 



The Rind, which, at first, consists of mere epidermis, 

 or short, thick-walled cells, overlying soft cellular tissue, 

 becomes penetrated with cells of unusual length and 

 tenacity, which, from their position in the plant, are 

 termed bast-cells. These, together with ducts of various 

 kinds, constitute the so-called last, which grows chiefly 

 upon the interipr of the rind, in successive annual layers, 

 in close proximity to the wood. With their abundant 

 development and with age, the rind becomes larh as it 

 occurs on shrubs and trees. The bast-cells give to the 

 bark its peculiar toughness, and cause it to come off the 

 stem in long and pliant strips. 



