S PROPERTIES OF WOOD. 



wood, although at first sight it appears to he of manifold 

 variety. This is hecause the same structural or anatomical 

 data are characteristic of genera. For instance, all Euro- 

 pean, American or Asiatic deciduous oaks exhibit the sumo 

 structure and anatomy ; this is also true for all ashes, spruces, 

 or lirs, for all two-needle pines (Pinaster type) ; for all five- 

 needle pines (Cenihran type), and so on. The importance of 

 this law is not only positive, hy simplifying the study of the 

 characteristic structures of woody species, hut also negative, 

 as it thus becomes impossible, either by the naked eye, or 

 microscopically, to distinguish between different species of any 

 genus ; e.g., in oaks, to say of what species is any wood- 

 specimen, or from what locality it conies. Custom-house 

 experience of recent years has thoroughly confirmed this 

 statement. 



The same anatomical differences that exist in different 

 species of a genus, for instance between species of oak or 

 spruce, also exist in varieties of the same species, e.g., in the 

 sessile oak, or the European spruce. Such are : breadth of sap- 

 wood ; ratio of spring- wood to summer-wood ; number, height 

 and thickness of medullary rays ; mass of parenchyma, vessels, 

 tracheids, etc. Physical and technical characters, such as 

 lustre, odour, colour, durability, strength, are the only indivi- 

 dualities of woods of nearly allied species. The iirst three of 

 these characters change rapidly after a tree has been felled 

 and converted, so that they are of diagnostic value for a short 

 time only, while the two last can be decided only by technical 

 experiments ; their exact determination is therefore very diffi- 

 cult and not always practicable, nor satisfactory. 



Before describing the different components of woody tissue 

 it is necessary to give a " short description of its anatomical 

 elements, as seen under the microscope. 



The cells of the pith are nearly isodiametric and by their rapid 

 sub-division cause the longitudinal growth of shoots, in which 

 process the cells immediately round the pith also participate, 

 partly by division and partly by being stretched. The pith 

 cells often lose their contents early and then contain air only; 

 they often, however, retain their plasmic contents for a longer 

 period, during which they serve as reservoirs for starch and 



