OF THE WOOD. 31 



of various vessels running for the most part 

 longitudinally; some having a spiral coat, 

 others not. Of these vessels, some in their 

 youngest state convey the sap from the 

 root to the extremities of the branches and 

 leaves ; others contain the various peculiar 

 or secreted juices ; others perhaps contain 

 air. The whole are joined together by the 

 cellular substance already described. 



Linnaeus and most writers believe that one 

 of the abovementioned circular layers of 

 wood is formed every year, the hard exter- 

 nal part being caused by the cold of winter ; 

 c onsequently, that the exact age of a sound 

 tree when felled may be known by counting 

 these rings. It has even been asserted that 

 the date of peculiarly severe winters may 

 be found in the harder more condensed rings 

 formed at those periods; and moreover, that 

 the north side of a tree may always be known 

 by the narrowness and density 6f the rings 

 on that side. All this is controverted by 

 Mirbel, chiefly on the authority of Du Hamel, 

 who nevertheless scarcely says enough to inva- 

 lidate the ancient opinion on the whole. It: 

 is very true that there may be occasional inter- 



