358 
THE 
CA^^ADIAN 
NATURALIST. 
supplied by the outer layers of the liber, so that however 
diverse in appearance, it appears that the structure of these 
two is alike. In the beech^ fir, &c. I conceive the epidermis 
is not thrown off. Beneath the bark, we perceive the wood, 
which likewise consists of two parts ; the outer is the albur- 
num, or soft wood ; the interior, the duramen, or heart- 
wood ; the latter is usually darker in tint, and more compact 
in texture. These parts are composed also of strata or 
layers, which are very visible, as each layer exhibits two 
structures, the inner side of it being composed of cellular 
tissue, the outer of ducts and woody fibre ; they do not 
increase in size, but a new one is formed every year, between 
the liber and the alburnum. If we strip the liber from a 
tree in June, we find that it readily separates, and that a 
mucilaginous fibrous substance is deposited between it and 
the trunk, which appears to form a layer of alburnum on one 
side, and a layer of liber on the other. In the centre of the 
tree we see a ring, or a section of a tube, called the medul- 
lary sheath, or corona, whence proceed radiating lines to all 
parts of the circumference, forming, when the tree is split, 
those shining satiny bands, so beautiful and conspicuous in 
the maple, beech, and birch. These are medullary rays, and 
serve to connect the medullary sheath with the leaves ; the 
water taken up by the roots passing up the spiral vessels 
and ducts of the sheath ^ through the rays into the leaves. 
Within the corona is the medidla, or pith, which is composed 
of cellular tissue slightly compressed : it never increases in 
diameter after it is formed, and therefore its bulk depends 
not on the size of the tree, but in some measure on the size 
of the twigs ; the ash, butternut, elder, &c. which have 
thick large twigs, having more pith than the maple, beech, 
or elm, whose twigs are slender. In some full grown trees, 
its presence is indeed scarcely to be distinguished, the cells 
having been gradually filled up by the juices of the tree. 
