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. 
