FINENESS OF GRAIN. 83 



corn as wood-bread, and thus may partly replace other fodder, 

 Such an experiment has been made with beechwood. 



Tannin appears to play a varied part in wood, its most 

 important role being that it precedes and includes the colour- 

 ing-matter that gives its colour and durability to hearlwoud. 



Among the ethereal oils in wood, turpentine and camphor 

 may be mentioned. Turpentine is contained partly in resin- 

 ducts, partly in pareiichymatous cells. If the resin-ducts are 

 severed turpentine exudes; it is pressed out by the turgor 

 of the sapwood. The cell-walls saturated with water are im- 

 permeable by turpentine. When wood is wounded and exposed, 

 resin instead of water passes into the cell-walls (resinosis). 



Camphor is found chieily in Lauraceaj (Cainplioni) ; the 

 species yielding it are given on p. 17. It is a highly refrac- 

 tive substance formed like tannin in enlarged pareiichymatous 

 cells. 



Betulin occurs in the wood and bark of birches, and in- 

 creases their combustibility. 



D. MKC IIANU AI, I'uoi'KirriKs OF WOOD. 



This group of those properties uf wood, which are based on 

 its anatomical and physical conditions, is dealt with under a 

 separate heading. Long technical experience is here more 

 decisive than physical and anatomical science, which is not 

 yet developed sui'liciently to all'ord a clear explanation of 

 the subject from a consideration of the separate physical 

 and anatomical factors. 



1. L'iuencss oj 



The term "line-grained" is not equivalent ta ''narrow- 

 zoned," nor to " anatomically of simple structure." Wood that 

 can be worked easily is line-grained, whether or not it appears 

 so to the eye. Oak wood as well as spruce wood may be line- 

 grained or coarse-grained. The woods of old Wey mouth pines, 

 of walnut, box, horse-chestnut and mahogany are specially 

 line-grained. One condition for fineness of grain is uniformity 

 in the annual woody zones, both in width and in the ratios of 

 the widths of spring and summer wood within the annual zones. 



This uniformity of the tissues depends chieliy on the age 



