148 T^^ Stem — Strictures. [April, 



up oxygen liberated by the decomposition of carbonic acid, or of 

 water, conducting it into the leaves. 



During the first year's growth of an exogen the vascular cylin- 

 der, as before observed, consists of an internal layer of spiral ves- 

 sels, forming the medullary sheath and external bundles of porous 

 and ligneous vessels. Subsequently, the layer of spiral vessels is 

 not repeated, but concentric zones of porous vessels in conjunction 

 with the parenchyma,^ or cellular tissue, are formed, constituting in 

 the tree the woody circles. Not only the size, but the texture of the 

 woody layers, varies in different parts of the cross section of the stem, 

 which on examination will often be found occasioned by a large and 

 healthy root, for some reason originating on that side of the tree. 

 After a tree has been growing a number of years, we have the dis- 

 tinctions in the wood, duramen and alburnum — or hartwood and sap- 

 wood. The vessels being at first open, admitting a ready passage of 

 the juices, ultimately become filled by the deposite of hard fibrous 

 matter, and when thus thickened and compressed, it forms heart- 

 wood, which, in consequence of its compactness, is stronger and 

 more durable. Next is the cambium layer, consisting of a semi- 

 fluid, which marks the separation between the wood and the bark. 

 This is an organ zable mucilage, and from it new elementary portions 

 are formed, both of vascular and cellular tissues. Next is the cor- 

 tex or bark external to the wood, which is a protection to these for- 

 matory processes going on between the wood and itself, and consists of 

 several layers. And first there is a cellular tissue which subsequent- 

 ly undergoes changes as successive deposites are made, which thus, 

 by these deposites, passes into a vascular layer. Here we have then 

 connected with the bark as with the wood-zones, a cellular and vas- 

 cular system, the position and relative proportions of the elements 

 varying in the two. In the bark the cellular system is external and 

 much developed ; in the wood it is internal and restricted. The in- 

 crease of bark takes place in a manner directly opposite to that of 

 the wood. In the wood the successive layers are developed on the 

 outside, in the bark on the inside. Hence we see, as the tree in- 

 creases in size, the outer layers of bark often become so distended 

 as to crack, and often peal off ; in other cases, as in the beech, they 

 retain their position, but become extremely porous. 



*TIu8 ccllnlar tissue hiis a fine prccu color in the bark of the sicm and branches, but 

 is coloi'lcss ill the bark of the I'uot. 



