68o THE POPULAR SCIENCE MONTHLY. 



cases, as in Abies Jirma, or Japan fir, that we meet with them. Some- 

 times 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 {alburniQn) becomes dry or heart wood {cluramen)y 

 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 G3 per cent of solid residue, and 

 this is also the percentage of solid substances in the pitch of the sap- 

 wood 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 ^^ per cent of the perfectly dry sap- 

 wood, while the innermost layers of heart-wood contain 1^ 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 sub- 

 stances. 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 consid- 

 ered as mere cells of secretion producing resin ; but there are many 

 reasons which force me to consider them as merismatic cells, remain- 

 ing without function sometimes for several years, until the sap-wood 

 containing them becomes dry or heart wood, when they begin their 



