STRUCTURE OF VEGETABLES.
Every plant is essentially an aggregation of cells;[35] and the most simple forms of vegetation consist of a congeries of cells (cellular tissue) of the same kind, and have no visible fructification; such are the sea-weeds (algæ, conferæ, &c.), mosses, and lichens. In the more complex tribes the cells become variously modified, are elongated into tubes or vessels (vascular tissue), some of which possess a spiral structure, and others have their sides studded with little glands. The vascular tissue consists of two kinds of vessels. 1. The spiral vessels or tracheæ: these are membraneous tubes, with conical extremities, having within a coil of elastic fibre spirally twisted, and capable of being unrolled ([Lign. 1, b.]). 2. The ducts; which are a modification of the structure of the spiral vessel; their extremities are rounded or conical, and their sides marked with transverse lines, rings, or bars. Their functions appear to be different from those of the spiral vessels, and they are found in situations where the latter never occur.
[35] "A cell in botanical language, means a little bag composed of membrane, and containing a living substance capable of spontaneous growth by multiplication, or division of its parts. Of such little bodies, millions of which may be contained within the space of a cubic inch, all the soft parts of vegetables are composed; in sea-weeds they are often of large size."—Dr. Harvey's Sea-side Book, with which the reader is doubtless familiar.
The organization of the stem in the whole class of flowering plants, possesses characters so evident, as to afford the most important aid in the investigation of their fossil remains. Without dwelling on minor modifications, they are separable into two divisions, namely, the Endogenous (signifying to grow from within), and the Exogenous (to grow from without). Both possess vascular tissue, but so differently arranged in the two classes, as to constitute distinctive characters which are seldom obliterated, although what was once a flexible stem, is now a mass of flint.
Endogenous Stems.—As the seeds of the plants belonging to this division have but one cotyledon, or seed-lobe, as the Lily, they are also termed monocotyledonous; the reader therefore must remember that these terms are synonymous. These stems consist of an uniform mass of cellular tissue, in which bundles of vascular or woody fibre are imbedded; a transverse section presents a surface dotted over with spots, produced by the division of these groups of vessels, pretty uniformly distributed, but more densely arranged towards the circumference ([Lign. 1, fig. 4]). A slice of cane affords an illustration of this structure.
The increase of these stems is effected by the formation of new cells and bundles of vessels in the central axis, which force their way among the old tissue, and occasion the condensation of the latter towards the outer edge. These plants have neither pith, concentric circles of woody fibre, nor true bark; negative characters of the highest importance in the determination of fossil stems.
Exogenous Stems.—The seeds have two cotyledons, or seed-lobes, as in the Bean, hence the plants of this class are also called dicotyledonous. In these stems the cellular tissue forms a central column, or pith ([Lign. 1, fig. 2, a.]), and an external band, or cylinder, called the bark ([fig. 2, b.]); the two being connected by thin vertical plates, termed medullary rays, which are also formed of cells ([fig. 2, c, c.]); the diagram, [Lign. 1], exhibits this arrangement. The interval between the pith and the bark, and the interspaces of the vertical radiating plates ([fig. 2, d.]), are filled up by woody fibre or vascular tissue, consisting of spiral and other vessels. The ligneous structure of exogenous stems consists, therefore, of a cylinder formed of wedge-shaped processes, that extend between the medullary rays to the pith, and is surrounded by the bark; a new zone of woody fibre is added annually between the bark and the former cylinder, and from this mode of increase the term exogenous is derived: a transverse section of a branch of oak or ash will show this structure. The rings, or concentric circles, are the annual zones of wood; the fine lines radiating from the centre, or pith, to the circumference, or bark, are the medullary rays ([Lign. 1, fig. 2, c]: see also [Plate V. fig. 4]).
The organization above described, will be found more or less manifest in fossil wood, stems, and branches. The monocotyledonous structure is beautifully displayed in the silicified stems of palms from Antigua ([Plate V. fig. 1, 1a.]): and the dicotyledonous, in petrified trees from Egypt. The pith, medullary rays, vascular tissue, and circles of growth, are preserved in the siliceous and calcareous wood found in many parts of England.
STRUCTURE OF CONIFERÆ.
Structure of Coniferæ (cone-bearing).—The remains of a numerous family of dicotyledonous trees, termed Coniferæ, as the pine, fir, larch, &c., are so abundant in the stratified rocks, that it is necessary to describe in more detail the peculiarity of structure by which their stems and branches may be recognised. The most delicate woody tissue, as we have above stated, consists of elongated cells or tubes, of two kinds: in the one, the membrane of which they are composed is smooth: in the other, the walls of the tubes are covered by little oval or circular bodies called glands ([Lign. 1, fig. 1, c.]). A branch of larch or pine, split longitudinally, and viewed by a powerful lens, will exhibit the appearance here described. This glandular structure is so constantly and largely developed in the coniferæ, that although it is also possessed by other aromatic trees, we shall rarely err in referring fossil wood in which this organization is apparent, to this family of vegetables (see [Plate V. figs. 2, 3]). These glands in the pines and firs, are supposed to be the cells which secrete a colourless volatile oil, that exudes in the state of turpentine.