PHYS:OLOGY. 265 



bark, down to the wood, and finding it several years after- 

 wards in the decayed bark. 



THE WOOD. 



If any portion of most trees be sawn transversely, several 

 cylinders of wood may be seen enclosing each other, or form- 

 ing concentric layers around the pith, and occupying the 

 space between it and the bark. The layer which encloses all 

 the others, and is in contact with the bark, is of a lighter co- 

 lor ; is more succulent, arid less durable than those within it. 

 This portion of the wood is called Alburnum, or white wood, 

 and it is in this that the vitality of some old trees chiefly re- 

 sides, the interior being decayed and wasted away. 



The heart wood, as it is termed, is in most trees of a darker 

 color than the alburnum, or sap wood, and is also of a firmer 

 consistence, arid as we have already said, is not so liable to 

 decay by exposure. 



The Concentric layers are found only in Dicotyledonous 

 trees, as we have already explained. The vessels in each 

 layer are always largest in that part of it which is nearest the 

 centre of the stem, owing to their being formed in the spring 

 of the year,when vegetation is most active, and the pores are 

 distended with their appropriate fluids. The situation of 

 these large pores might, in some cases, lead to the belief that 

 they were formed between the annual layers ; but on close 

 inspection it will be seen that they gradually diminish in size 

 to the outer margin of each ring, which part has grown du- 

 ring the autumn, when vegetation was ceasing, and therefore 

 when small vessels only were required for the conveyance of 

 the sap. The concentric rings are very apparent to the na- 

 ked eye in the Oak and Chestnut, but in some woods, as the 

 Andromeda, and Kalmia latifolia, they are seen with difficulty 

 without a microscope. 



The Medullary rays are flattened masses of woody sub- 

 stance, which run between the fibres of the concentric lay- 

 ers. They extend from the pith to the bark, and are quite 

 apparent in many kinds of wood, as the Beech and Oak. on 

 splitting which, they often present a shining surface on the 

 longitudinal fibres of the wood. 



The Medullary rays, as well as the concentric rings, and the 

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