I.] ALBURNUM AND DURAMEN. 39 



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 is more strikingly exemplified in 

 the Firs and Pines, and occurs with greater frequency 

 in trees of this kind than in any others. Accidental 

 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 induration 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 

 spread over a larger surface. The sap-wood is, as a rule, 

 darker in the white-wood class than the heart-wood, 

 whether seasoned or unseasoned, but is paler in colour 

 in most hardwood trees which have had time to season. 

 In some of the white, or softer woods, when fresh cut, 

 the difference is scarcely perceptible ; but exposure to 

 the air quickly gives to the outer layers a greenish tinge, 

 due to a species of mould fungi which attack them, and 



