38 TIMBER AND TIMBER TREES. [chap. 



Oak trees which form their wood most rapidly under 

 ordinary conditions of growth are the best in quality. 

 In the Firs and Pines and Conifers generally the converse 

 'is usually true. 



Under ordinary conditions of growth, and with most 

 trees, the conversion or change of the alburnum into 

 duramen takes place with great' regularity (Fig. 4) -; but 

 to this rule there are exceptions in every species, a 

 variety of influences, such as temperature, aspect, soil, 

 and others less understood, apparently bearing upon 

 and tending to disturb this regularity. It is, indeed, often 

 found that outside the completed circles of duramen, 

 portions of the circumference of several successive layers 

 of alburnum (Fig. 5) have already been changed into 

 heart-wood, while the rest remain to be indurated in 

 the ordinary course ; the perfected segments generally 

 occurring earlier on the south side of trees of the 

 Northern Hemisphere, and on the north side of those 

 of the Southern Hemisphere. 



This is, perhaps, only to be accounted for by the 

 supposition that, being exposed to the most powerful 

 rays of the sun, especially during the summer months, 

 the indurating elements of the sap tend more that 

 particular side; while, on the reverse side, the action 

 is much slower, owing to the partially exhausted state 

 of the juices and the deadening effects of cold. 



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- 



