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 72100 per cent of the perfectly dry sapwood, while the innermost layers of heart-wood contain 113 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
