activity. They now increase in size, expanding like vesicles, and totally obstructing the duct, so as to prevent the rosin from entering the heart-wood by way of the horizontal duct or sinking from a higher to a lower part of the tree. In the amount of resin contained in the wood, the genus Picca ranks second among conifers; the species Picca excelsa, common in Europe, contains 2·16 per cent in the sap-wood and 10 per cent in the heart-wood. The amount of pitch increases with the age of the tree.
I have found as a result of my investigation that there exists a very important law which will enable a microscopist to tell at a glance the difference between heart-wood and sap-wood: only the heart-wood is fit for building purposes and will stand the influence of weather; the sap-wood will decay rapidly, but is nevertheless used by unscrupulous builders. An examination of the resiniferous ducts will show the difference at a glance. During the process of transition of the sap-wood into heart-wood, all these resiniferous ducts become closed by the expansion of the cells surrounding them, a process which can be discerned unmistakably even in the smallest piece of any wood from a conifer; a similar process takes place in the growth of the bark.
Professor Hartig, of Munich, a famous botanist, proved by careful experiments the following law: The quality of the wood of all trees increases so long as the yearly growth shows a progressive course year after year. It has been thought until now that the quality of the wood of conifers is the better the closer the annual rings lie; this is but partly true. The older the tree the closer the annual rings, but the quality of the wood increases only as long as those rings represent an actual progress of growth; when once the annual amount of wood formed begins to diminish year by year, its quality becomes impaired, notwithstanding the rings become closer and narrower.
The amount of resin in the wood of a tree follows the same law: if we take, therefore, a splinter or a plug from any tree by means of a hollow auger, we can, by a simple calculation, determine whether the tree is still progressing, or already on the decline in growth, quantity of resin, and value.
In the genus Pinus the resiniferous canals are of different construction, but agree in general arrangement with those of the genus Picea; their size is larger and they are inclosed by only thin-walled, merismatic cells, which in the course of the transformation of the sap-wood into heart-wood enlarge and close the canals.
The heart-wood of the trees of the genus Pinus has a light-brown color, sometimes a little reddish, the coloring being due to a product of the oxidation of tannin, which is found in the cells and their walls.
Among the species of this genus several deserve a more elaborate mention:
The Scotch pine (Pinus silvestris), when growing on sandy soil, forms only a very small amount of sap-wood, whereas on gravelly and
