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, 1^a._): 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. [Sidenote: 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.