NUTRITION. 87 



of the vessels, is encrusted, while in the latter it 

 is empty or only filled with juices scarcely so- 

 lidified. M. Dutrochet has proved that the 

 different degrees of hardness between divers 

 woods, and between the wood and the albur- 

 num, is owing to the nature of the juice con- 

 tained in their tissue, and not to the tissue it- 

 self, which is identical in both. The tissue of the 

 box and the poplar, though these woods differ so 

 much in density, become perfectly similar when 

 the matter they contain has been dissolved out 

 by nitric acid. The spaces which after mace- 

 ration appear to exist between the woody layers, 

 are not really such ; but were filled with cellu- 

 lar tissue, analagous, for each annual layer, 

 to the central pith of the first year's growth. 

 Each woody layer, being, in the Exogenous 

 trees of cold or temperate climates, the produce 

 of one year, the number of concentric zones in 

 a transverse cutting of a stem will show the 

 number of years during which that part of the 

 tree has existed. To know the entire age of 

 the tree itself, it must be cut exactly at the 

 crown, since of course the higher portions of the 

 stem were not in being when the deposits on 

 the lower were formed, An inscription graven 

 on the trunk of a tree, and penetrating to the 



