VEGETABLE ORGANISATION. 
429 
rarely bulbous j whereas, in many others of the monocotyledonous 
plants—-the tulip, for instance—it is bulbous. In every true root, 
however, a fibrous portion will be found to exist; and it is the ultimate 
fibrils or divisions of the fibrous part of the root, with the minute vas¬ 
cular bodies at their extremities, which constitute the essential part of 
this organ. It is by these little vascular terminations of the fibrils of 
the roots that plants obtain their nourishment from the soil in which 
they grow. They are found equally in the lofty forest-tree and the 
herb which grows beneath its shade, and are equally necessary to the 
existence of both ; for, if by accident or design the fibrils are materially 
injured, the plant withers and dies. In their internal structure, roots, 
with some exceptions, will be found to resemble the stem; in fact, in 
many instances they consist of a main descending trunk, branches, and 
twigs, the organisation of which is precisely conformable to the ascend¬ 
ing stem and its divisions, as it is found above ground. 
ts The stem, or trunk, varies much in different families of plants. 
In those among the acotyledonous plants in which it occurs, it consists 
of a thin epidermis or cuticle, surrounding a pulpy matter, the texture 
of both which parts is almost entirely cellular. A few fibres or vessels 
are, however, found in the stem of the fern tribe, forming an exception 
to the general internal organisation of this division of the vegetable 
kingdom. In the monocotyledonous plants, the stem consists also of 
a cuticular envelope and internal cellular substance; but the cuticular 
portion is more dense and firm, and the cellular texture of the internal 
part of the stem is traversed by longitudinal fibres or vessels, several of 
which are arranged together in bundles. In the dicotyledons, the stem 
is composed of bark, wood, and pith. The bark, which corresponds to 
the cuticular part of the stems of the acotyledonous and monocotyledo¬ 
nous plants, is formed of several layers of cellular and fibrous texture; 
the wood or fibrous portion of cellular texture and closely compacted 
vessels traversing the cells in a longitudinal direction, and symmetri¬ 
cally arranged, so as to form concentric and divergent or convergent 
layers; and the pith, which occupies the central portion of the stem, 
of a light spongy cellular tissue. 
tc The development of this apparently intricate organisation from the 
simple elements of cell and vessel, according to the principles already 
laid down, may be conceived to take place in the following manner: — 
Let us suppose a complete circular range of secondary vesicles becoming 
dilated by the process of growth in the interior of a primary or generat¬ 
ing cell; it is evident that the envelope of this cell, pressed at all points 
of the circumference by similar parts, which are growing at once in 
length and breadth, will take the form of a cylinder ; but it is also 
