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 medulla, 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. 



