98 HEART- WOOD AND SAP-WOOD. 



sition in its tubes, of resinous and other matter, secreted by the 

 plant. The portion of the stem in which this has taken place, 

 thus acquires great toughness and durability, but it is no longer 

 fit to perform any office in the living system, save that of mecha- 

 nically supporting the rest ; since no fluid can pass, in any way, 

 through the now-filled-up channels. It is through the newer 

 layers, or sap-wood, therefore, that the sap entirely ascends ; 

 and these, in their turn, become inclosed by others ; and are 

 at last consolidated, like the more aged ones, into duramen. 

 The heart- wood alone is used by the artisan ; for the sap-wood 

 soon splits and decays. 



132. As the pith and the inner layers thus gradually lose 

 their original employment, and as in the outer part of the stem 

 alone any active processes of vegetation go on, the former may 

 be removed without injury to the latter ; and this is often natu- 

 rally accomplished by decay, which destroys the heart of an aged 

 tree, with some portion of the exterior of the stem, but leaves 

 the remainder a mere shell, still capable, however, of putting 

 forth buds and branches, and of adding to its own thickness. 



133. The chief important variety in the structure of the 

 Exogenous stem is that exhibited in the Pine and Fir tribe. No 

 ducts exist in their wood ; whilst the diameter of the tubes of 

 the wood itself is greater than in other cases ; so that a hori- 

 zontal section of the stem shows a series of openings of very 

 nearly the same size, arranged with beautiful regularity, the 

 division into annual layers being usually well-marked. As a 

 general rule it may be stated, that these are separated by the 

 most distinct line, in trees inhabiting temperate or cold climates, 

 whose vegetating processes are entirely suspended by the cold 

 after each layer is formed ; whilst in trees of warmer regions, 

 they pass into one another more gradually. In the former, too, 

 there is often a considerable difference in the thickness of the 

 respective layers, according as the seasons have been favourable, 

 or otherwise, to the formation of wood ; whilst in the latter, 

 their thickness is in general nearly uniform. 



134. These facts come to be of much interest, when we 

 examine the structure of the fossil plants, which are not unfre- 



