THE VBGETATITE OEGANS OF PLANTS. 275 



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, especially when 

 growth 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 j)otato. 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 te- 

 nacity, which, from their position in the plant, are often 

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

 kinds, all united firmly by their sides, constitute the so- 

 called hast-fibers, which grow chiefly upon the interior of 

 the rind, in close proximity to the wood. With their 

 abundant development and "with age, the rind becomes 

 harJc 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. 



Bast-mats are made by weaving together strips of the 

 inner bark of the Linden (bass or bast-wood) tree ; and all 

 the textile materials employed in making cloth and cord- 

 age, with the exception of cotton, as flax,hemp, ISTew Zea- 

 land flax, etc., are bast-fibers. The leather-wood or moose- 

 wood bark often employed for tying flour-bags, has bast- 

 fibers of extraordinary tenacity. 



The external rind, like the interior pith, becomes sapless 

 and dead in perennial plants, and after a longer or shorter 

 period falls away. The outer bark of the grape separates 

 in long shreds a year or two after its formation. On most 

 forest trees the bark remains for several or many years. 

 The expansion o^ the tree furrows the bark with numerous 



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