46 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



ticularly sharp, owing to the contrast between the 

 porous and the dense wood, e.g. oak, ash, plum, teak, 

 &c. In the other of these chief types the line of 

 demarcation is due to similar differences in the density 

 of the fibrous and other elements of the wood rather 

 than to contrast between porous (i.e. very vascular) and 

 dense wood ; the autumn-wood has very thick walls and 

 small lumina, the spring-wood thinner walls and 

 larger lumina, without special reference to the vessels, 

 which are usually small and nearly evenly dispersed 

 through the whole. As examples I may refer to the 

 wood of birch, maple, horse-chestnut, Shorea robusta, 

 &c. That difficulties in deciding must occur in some 

 cases is only too evident, and it is well known that in 

 individual cases departures from the type are produced 

 by local disturbances e.g. the formation of two so- 

 called " annual rings " in one year, and (at least it is 

 suspected and needs investigation) the suppression of 

 the demarcation lines by changes due to climatic and 

 other local influences. Nevertheless the characters 

 usually work well in practice. It should be remarked 

 that whereas the course of the annual rings is normally 

 concentric and regular, it is wavy in some cases, e.g. 

 barberry, where the crests of the waves project out- 

 wards at the medullary rays, whereas they project 

 inwards in Kalmia latifolia y hornbeam, beech, &c. 



