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 conversion 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.
