52 



THE AMERICAN MONTHLY 



[March, 



altered form of cells are those which make up the bulk of the heart 

 wood of a tree, being elongated, thick-walled, tough cells of various 

 forms, pointed at both ends and nearly solid. Some are divided into 

 two or three parts vs^ith cavities in them ; others, termed ducts, are hol- 

 low tubes, sometimes of considerable length, filled with air and with 

 their walls curiously marked with lines, spirals, or dots. These lines 

 or dots generally denote thin places in the walls, through which the 

 sap can pass. In all the fully-formed tissue of wood the bulk of the 

 cells will be found to be nearly empty, containing only air or a little 

 water, but no protoplasm. Experience has shown that such cells do 

 not grow after the cavities become empty, that no further progressive 

 changes are made. Physiologically such cells are dead, but they are 

 still of very great importance in the tree, because so long as they do 

 not decay they form the frame-work which supports the branches and 

 leaves in the living tree, and constitute the most valuable part of it 

 when worked into lumber. If the cells decayed as soon as they ceased 

 performing their functions, a tree would be only a hollow tube, with 

 bark and two or three inches of sap wood. Precisely this occurs in 

 old trees, where the pressure of succeeding cycles of growth has so 

 crushed the earliest layers of wood as to deprive them of all nourish- 

 ment and kill them completely, so that they begin decaying, leaving a 

 hollow butt. Moreover, these cells are strongest and least liable to 

 change under varying circumstances just after they have reached ma- 

 turity, and hence the best lumber comes from fully-matured trees. 



Fig. 6. — White pine {Pinus strvlius). 



Undoubtedly growing trees in their lusty youth furnish excellent lum- 

 ber, but a critical, practical comparison between the two kinds shows 

 that the latter, from the presence of an undue amount of water and cel- 

 lulose elements, decays quicker, and shrinks and warps more in drying, 



