DURABILITY OF RESINOUS WOODS. 681 



activity. They now increase in size, expanding like vesicles, and to- 

 tally obstructing the duct, so as to prevent the resin 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 Picea ranks second among conifers ; the species Picea 

 excelsa, common in Europe, contains 2'16 per cent in the sap-wood and 

 1"6 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 dif- 

 ference at a glance. During the process of transition of the sap-wood 

 into heart-wood, all these resiniferous ducts become closed by the ex- 

 pansion of the cells surrounding them, a process which can be dis- 

 cerned 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 cdl 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 Plnus the resiniferous canals are of different con- 

 struction, but agree in general arrangement with those of the genus 

 Picea ; their size is larger and they are inclosed by only thin- walled, 

 mei'ismatic 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 



