OUTLINES OF BOTANY. 
XV 
the deposition of nutritive matter (20) commencing while in the state 
of bud, and usually involving a number of axillary buds (eyes), as in the 
Potato and Jerusalem Artichoke (p. 228). It differs from a rhizoma in 
being borne on a slender stalk. 
42. A Bulb is a thickened subterranean bud, formed of an extremely abbre¬ 
viated stem (the plate), which is generally much shorter than broad, clothed 
externally with the more or less persistent bases of former leaves in the 
form of scales, inclosing an undeveloped part borne on its upper surface, 
and emitting roots from the lower. In the scaly bulb, the scales are sepa¬ 
rate and thickened, as in the Lily (p. 494) ; in the coated or tunicated bulb, 
they invest each other in concentric layers, as in the Onion (p. 493). 
43. Bulbi.ets are small aerial bulbs, like buds with fleshy scales, borne 
in the axils of leaves, as in the Bulb-bearing Lily, or on the flower-stalks of 
some Onions (p. 493), from which they separate spontaneously and fall to 
the ground to strike root and grow. 
44. The Cohm or solid bulb is a simple and globular thickened subterra¬ 
nean stem, either quite naked, like that of the Indian Turnip (p. 446), or 
sometimes barely invested with a scaly or membranous coating derived from 
the bases of former leaves, as in the Crocus and Colchicum, where it ap¬ 
proaches the proper bulb. 
* * Its Internal Structure. 
45. The stem of a Pheenogamous Plant (2) is composed of a cellular sys¬ 
tem (cellular tissue, 4), into which a woody system (consisting of woody 
tissue and vessels, 7-9) is longitudinally introduced, more or less sparingly 
in herbs, and largely in shrubs and trees, the quantity increasing as the 
stem and branches grow. 
46. There are two principal plans according to which the woody matter 
is arranged in the stem, and which characterize the two great classes of 
Phaenogamous Plants. 
47. In the first, — to which all the woody plants of the Northern United 
States (excepting the Greenbrier, p. 485), as well as a majority of the 
herbs, belong, — the woody matter is so arranged as to form a layer or hol¬ 
low cylinder of wood, interposed between a central part of the original cel¬ 
lular system (the pith), and an outer portion of it (the green bark). 
That part of the cellular tissue in which the woody matter is imbedded 
forms plates or rays (the medullary rays or silver-grain), more and 
more compressed and condensed as the woody matter increases in quantity, 
which run horizontally and connect the pith with the bark. The woody 
tissue thus introduced belongs chiefly to the wood proper, but partly to the 
bark, forming an inner fibrous layer of that substance, the liber or Fibrous 
bark. The section of a yearling stem of the kind accordingly exhibits,— 
1. on the surface, the Epidermis or skin, which invests the whole plant, and 
consists merely of the outermost layer or layers of cells : 2. the Green bark , 
composed wholly of cellular tissue, the soft cells of which contain, like the 
same part in the leaves, the green matter of vegetation (chlorophyll, 54), 
which is generally produced in parts naturally exposed to the light) : 
3. the Liber or Jihrous inner bark, composed partly of tough threads of woody 
tissue (the material of linen, hemp, &c.) : 4. the proper Woody Layer of the 
stem, traversed by its cellular Medullary Rays : and 5. the entirely cellular 
Pith in the centre. (There are two parts more, which would demand notice 
in a detailed account, viz. the Medullarij Sheath , which consists of an ex¬ 
tremely delicate ring of spiral vessels (9), the earliest-formed part of the 
woody system, and which therefore lies immediately in contact with the 
pith, between it and the wood •, and the Corky Layer of the bark, which is 
formed immediately under the epidermis, and which, soon becoming 
opaque and of a gray, ashen, or brownish color, covers and conceals from 
view the subjacent green layer on shrubs and trees.) 
