8 INTRODUCTION 



The heartwood, as it is called, of tlie Laburnum, 

 for instance, is so dark as to be known to the French 

 as Green Ebony, Avhilst the outer or sapwood is light 

 yellow. In the Locust dark heartwood makes its 

 appearance in a five-year-old shoot, so that the sap- 

 wood is always a narrow outer band: in other trees, 

 such as the Elms, the sapwood is wide, and in others, 

 again, such as the Maple, there is little or no 

 distinction of colour between the two regions. 



In each annual ring there is often a well-marked 

 difference between the wood formed in spring and 

 the later, summer or autumn wood. In a piece of 

 Oak, for example, we can see with the naked eye 

 that the inner part of each ring is crowded with 

 relatively large holes, or " pores," as they are termed ; 

 but the wood formed later in the season shows few 

 or none of them, though far smaller ones can be 

 detected with a lens. 



In Pines, and most other coniferous trees, these 

 pores are absent, though a few similar large holes 

 appear scattered over the section of the rings. These 

 will prove, on examination, to be sections of large 

 ducts containing resin, whilst the pores of the wood 

 of broad-leaved trees are water-conducting vessels. 



Apart from other characters, these pores give us 

 a basis for a threefold classification of woods, into 

 non-porous, which is practically equivalent to conif- 

 erous; ring-porous, as in Oak, Chestnut, Elm, etc., 

 where the large pores in the spring wood define the 

 inner margin of each ring ; and diffuse-porous, where 

 the pores are usually small and are scattered evenly 

 throughout the annual ring. 



