THE VEGETATIVE ORGANS 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 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. a 
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 Jas¢-cells. These, together with ducts of various 
kinds, all united: firmly by their sides, constitute the so- 
called bast-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 
bark 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, New 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 of the tree furrows the bark with numerous 
