i.] SAP-WOOD. ii 



Such indurations of portions of the layers occur more 

 frequently in the Firs and Pines than in the wood of 

 trees of harder and more compact texture. In Dantzic 

 Fir, for example, I have noticed parts of twenty or 

 more concentric rings changed from alburnum into 

 duramen, or heart-wood, while the remaining portions of 

 the circles retained their sap-like or alburnum character, 

 and greater or less deviations in this respect are fre- 

 quently met with in other species. It may be that these 

 can only be accounted for by the exceptional influences 

 before mentioned, for it seems quite possible that, when- 

 ever a tree is suddenly thrown open and -exposed by the 

 clearing away of others from its vicinity, the hardening 

 process will go on with unusual rapidity. 



In such Firs and Pines as have been sheltered in the 

 depths of a forest, we do not find the wood of this 

 variable character, as the perfecting of the duramen 

 takes place then with much greater regularity and uni- 

 formity, if somewhat less rapidly, than in more exposed 

 situations. 



This peculiarity of growth is more strikingly exem- 

 plified in the Firs and Pines, and occurs with greater 

 frequency in trees of this kind than in any others. Acci- 

 dental circumstances no doubt affect the sap-wood of 

 many other kinds to a greater or less degree ; but in 

 trees of a close texture the want of vigour in the sap is 

 generally found to affect the whole circumference of a 

 layer rather than several distinct portions of it. 



The proportion of sap-wood, or alburnum, to heart- 

 wood, or duramen, in trees in which it occurs, is exces- 

 sive in the young, but decreases rapidly as they advance 

 in age, the difference being in some measure attributable 

 to the fact that, as the circumference of the tree increases, 

 the tissues of each successive layer, or annual ring, are 



