THE VEGETATIVE ORGANS OF PLANTS. 267 
Bulbs are the lower parts of stems, greatly thickened, 
the internodes being undeveloped, while the leaves—usu- 
ally scales or concentric coats—are in close contact with 
each other. The bulb is, in fact, a fleshy, permanent bud, 
usually in part or entirely subterranean. From its apex, 
the proper stem, the foliage, etc., proceed; while from 
its base, roots are sent out. The structural identity 
of the bulb with a bud is shown by the fact that the onion, 
which furnishes the commonest example of the bulb, often 
bears bulblets at the top of its stem, in place of flowers. 
In like manner, the axillary buds of the tiger-lily are 
thickened and fleshy, and fall off as bulblets to the ground, 
where they produce new plants. 
STRUCTURE OF THE Stem.—The stem is so complicated 
in its structural composition that to discuss it fully would 
occupy a volume. For our immediate purposes it is, 
however, only necessary to notice it very concisely. 
The rudimentary stem, as found in the seed, or the new- 
formed part of the maturer stem at the growing points 
just below the. terminal buds, consists of cellular tissue, 
i. e., of an aggregate of rounded and cohering cells, which 
rapidly multiply during the vigorous growth of the plant. 
In some of the lower orders of vegetation, as in mush- 
rooms and lichens, the stem, if any exist, always preserves 
a purely cellular character; but in all flowering plants the 
original cellular tissue of the stem, as well as of the root, 
is shortly penetrated by vascular tissue, consisting of ducts 
or tubes, which result from the obliteration of the hori- 
zontal partitions of cell-tissue, and by wood-cells, which are 
many times longer than wide, and the walls of which are 
much thickened by internal deposition. 
These ducts and wood-cells, together with some other 
forms of cells, are usually found in close connection, and 
are arranged in bundles, which constitute the fibers of the 
stem. They are always disposed lengthwise in the stem 
and branches, They are found to some extent in the soft- 
