Popular Science Monthly/Volume 28/March 1886/Durability of Resinous Woods
|DURABILITY OF RESINOUS WOODS.|
THERE can be no doubt that the resin in the wood derived from the different varieties of conifers, or pine-trees, is one of the most important factors which determine its quality, especially its durability and resistance against the influence of weather and the different forms of rot, all of which are now proved to be due to specific fungi. Just at present, timber from American conifers is highly valued in Europe, because of its richness in resin, although the amount of resin in wood is not the sole measure of its quality.
Until now an exact valuation of the importance of pitch in wood was impossible, because the accurate knowledge of the origin and the distribution of the resin, as well as of the arrangement of the organs producing it, was wanting.
At the experimental botanical station at Munich, I have made numerous experiments during a space of many years, and, as the results seem to contain many new points, I thought them worth presenting to the readers of the "Monthly." In face of the confusion prevailing in the nomenclature, it is necessary to state that the botanical names used are those of Carrière in his "Traité des Conifères," who separates Abies and Picea as distinct genera. The distribution of the resiniferous ducts is so characteristic within each of these genera as to serve as typical marks for them.
The species of Abies commonly called firs are characterized by the absence of resiniferous ducts within their woods; it is only in rare cases, as in Abies firma, or Japan fir, that we meet with them. Sometimes we find a cluster of parallel cells, often quite far apart from each other, filled with resin; these colonies of parallel cells are not to be considered as ducts, but as malformations due to the influence of different causes like cold and pressure; they are found also in other species of conifers exposed to the same causes, and occasionally attain the size of a man's hand.
The resin is produced only by the parallel cells of the medullary rays in the species Abies. Already in the first year's growth the cells are found to contain small drops of resin. The size of these drops increases with the age of the cells, the amount of amylum or starch in them decreasing in proportion.
Resin is composed of substances volatile at 100° C, and others which can not be distilled without decomposition; the latter form the solid residue, when resin or pitch is distilled with water. When the outer or sap wood (alburnum) becomes dry or heart wood (duramen), in which form it is that which is known commercially as wood, the cells are found to contain nothing but air with the resin coating the inside of the cell-walls; fresh pitch, as it oozes from the bark of the European Abies pectinata, contains 63 per cent of solid residue, and this is also the percentage of solid substances in the pitch of the sapwood of the genus Abies, but pitch from the heart, or from the dry, inner wood of the tree contains 70 per cent of solid substances.
During the life of a fir-tree the cells contain 50 per cent water, which, when the wood dries, disappears, and the pitch, which at first could not enter into the cell-walls, now permeates them, taking the place of the water.
The wood of Abies pectinata, which in Europe covers thousands of acres in dense, well-cultivated masses, contains the least resin of any fir cultivated, namely, only 72 per cent of the perfectly dry sapwood, while the innermost layers of heart-wood contain 11 per cent of pitch; it is therefore of inferior quality as far as richness in resin is concerned; only the very great heights and diameters which trees of this species rapidly attain make them valuable for cultivation.
The genus Picea (spruce) has the sap-wood of the same color as the heart-wood; it contains numerous ducts filled with resinous substances. These ducts run in all directions, the horizontal ones being branched off from those running perpendicularly, and communicating with others lying closer to the bark, running vertically. The inside of the ducts is made up of two kinds of cells, the one having thick walls and the same functions as the parenchymatic cells of medullary rays, the others having thin walls. The latter were formerly considered as mere cells of secretion producing resin; but there are many reasons which force me to consider them as merismatic cells, remaining without function sometimes for several years, until the sap-wood containing them becomes dry or heart wood, when they begin their 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 shallow ground it produces more but of less valuable quality. Two needles in one sheath characterize this species. The heart-wood of this pine contains 5·7 per cent of resin, the sap-wood proportionately less.
The sap-wood of this tree is quickly destroyed when the tree is cut; it assumes a dark-blue color and rots, through the agency of the mycelium of a fungus called Ceratostoma piliferum.
This tree is at present the most valuable and most cultivated timber-tree of Northern Germany.
The white pine, or common American pine (Pinus strobus), is now also extensively cultivated in Germany, where some forests can be found of trees about a hundred years old. Its wood has the lowest specific gravity of all coniferous wood. In spring, on account of the thinness of its bark, the tree is quickly warmed through, and the wood cells, formed in the beginning of the spring, are thin-walled; at the close of the period of vegetation in summer, the annual rings are finished by a few thick-walled narrow cells, thus giving only little thickness to the hard part of the annual layer. In amount of resin this pine stands at the head of all conifers, containing 6·9 per cent. The percentage increases up to the age of one hundred years, and with it the quality of the wood. It is of little value when young and exposed to moisture. Pinus cembra, a native of the Alps and Siberia, forms only small, dense rings every year during the short summers of these regions; the wood hence becomes heavier, and, although less resinous, more valuable.
A very valuable tree of Europe is the larch (Larix Europæa), which is native only in the colder regions of Europe and Asia; its wood consists of narrow annual rings, grown during the short summers of those countries. In the percentage of resin it stands between Pinus and Picea; its wood is more reddish, like that of the genus Pinus, and its resiniferous ducts are constructed like those of Picea; it contains 3·9 per cent of resin.
Just now, the different German governments are experimenting with the introduction of American trees which, it is expected, will stand the climate and furnish better wood, or, in proportion, a larger quantity, than the German forest-trees now cultivated. One of those of which much is expected is the Douglas spruce (Pseudotsuga Douglasii). Mr. J. Booth, of Hamburg, has cultivated this tree for a long time, and has already some very fine specimens in his park. I was enabled through his kindness to examine its wood carefully; the specific weight is very much above that of European conifers of the Pinus genus, but not as high as that of the larch; its quality increases in proportion to the width of its layers—a fact which, until a short time ago, was considered directly contrary to what is the case with wood from European pines.
Professor Sargeant, of Cambridge, has made the contribution to the United States census which embraces the researches and experiments with American trees. This report, one of the most variable of its kind from the great amount of statistical information contained in it, has been made with specially painstaking care; the distribution of American trees, the determination of their specific weights, their chemical composition in regard to mineral constituents (ashes), their commercial value, their strength, elasticity, and resistance, form the contents of a large volume. Compared with the figures obtained by Professor Sargeant, the wood of European forest-trees appears to be somewhat inferior in quality to that of American trees. The Bavarian Government, which on its 24,000 square miles of territory has over 7,000 square miles of forests, of which over 3,000 square miles are in solid masses, under a model administration of the Government, yielding about four per cent net profit, is wide awake for improvements, and has sent me to this country to investigate as fully as possible the facts about the growth of American forest-trees, their relation to the climate, and their yield of timber in quantity and quality. With their usual liberality and hospitality, the American authorities and learned men have lent a helping hand, truly worthy the spirit of a great nation not influenced by petty considerations of a possible rivalry.