i 
STRUCTURE OF CONIFERZ. 57 
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 (jig. 2, b.) ; the 
two being connected by thin vertical plates, termed medud- 
lary rays, which are also formed of cells (fig. 2, ¢, ¢.); 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, there- 
fore, 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 medul- 
lary rays (Lign. 1, fig. 2, ¢: see also Plate V. jig. 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. jig. 1, 1%): 
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 ConIFER® (cone-bearing).—The remains of 
a numerous family of dicotyledonous trees, termed Conifere, 
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 
