io TIMBER AND TIMBER TREES. [CHAP. 







description of timber, and that such trees are not only 

 the best, but the most durable. 



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 principal strength of the indurating elements of the 

 sap is drawn to 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. 



This hypothesis is supported by the fact that, in the 

 Firs and Pines, we frequently find patches of indurated 

 wood, somewhat darker in colour than the rest, and 

 brittle in character. The brittleness is easily accounted 

 for by supposing the first strength of the sap to have 

 been drawn to the other side of the tree, and the con- 

 version of these portions of sap-wood into heart-wood 

 to have been much slower than would have been the 

 case had the action of the sap been uniform. 



