MACROSCOPIC STRUCTURE. 37 



are very numerous. Several coniferous genera have resin- 

 ducts, which are readily distinguished from vessels owing to 

 their constitution and their position in the wood. Unlike 

 vessels, they have no proper walls, and are not only vertical, 

 or parallel to the wood-fibres, but also horizontal, in the 

 medullary rays. Resin-ducts are full of turpentine from the 

 commencement of their formation, as intercellular passages ; 

 as long as the passage widens and the surrounding cells 

 increase in number, turpentine Hows from these cells into 

 the resin-duct.* The presence or absence of resin-ducts, their 

 size, colour, etc., or their local swellings as resin-galls, supply 

 an important aid in the identification of genera. AH species 

 that have resin-ducts, when freshly felled exude turpentine 

 from the sapwood of the sections. Owing to their arrange- 

 ment in the wood, resin-ducts appear in a transverse section 

 either as dot-like cross-sections of the vertical ducts, or as fine 

 radial lines, the longitudinal sections of the horizontal ducts. 

 On the radial section the ducts appear as lines running verti- 

 cally or horizontally ; on the tangential section, the horizontal 

 ducts are fine dots, and the vertical ducts are lines. The 

 horizontal or medullary -ray ducts are always finer than the 

 vertical ducts. Eesin-ducts of coniferous woods are always 

 seen most clearly, if light falls over the observer's shoulder 

 on to a piece of wood held nearly horizontally. When the 

 annual zones of wood are narrow, there is usually less 

 reduction in the summer- wood than in the spring- wood, so 

 that in general narrow-zoned wood is harder and heavier limn 

 wide-zoned wood (ride p. 57). 



80. SprtK-e (Picea), Pines (Pinus, sections Taeda ami Pinaster), 

 Larcli (Larid 1 ), Douglas-fir (Pseiidotswja). 



The genus Pi<-<' includes all species of spruce ; the 

 sections Taeda and Pinaster include certain species of pine ; 

 Larix includes all larches ; Pseudotsuga four species of 

 Douglas-fir from America and Asia only, the other genera 

 being also European. 



Transverse Section. Medullary rays are scarcely visible ; 



* H. Mayr, u Das Htirx tier Nadelhol/.er." Berlin, l.s'JI. 



