THE VEGETATIVE OEGAJSTS 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. 



Stexjctuee 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 maturcr 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 ^isswe, 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- 



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