OP THE tVOOl)* 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 ; 

 consequently, 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 of 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- 



