SPECIFIC GRAVITY. '25 



2. Differences due to Anatomical Structure. 



The specific weight of a M-ood depends chiefly on the character 

 of the lumina of the woody elements — the more abundant they 

 are, the greater their dimensions — and the thicker their walls, 

 the less will be the woody substance and the lighter the 

 particular kind of wood. Hence the greater or lesser quantity 

 of woody substance contained in a given volume of wood is the 

 chief factor in its comparative weight. 



In most woods this woody substance is unequally distributed, 

 there being more substance in the summer-wood and less in the 

 spring-wood. It therefore follows that the specific gra\ity of a 

 wood depends on the ratio of the mass of the summer- to the 

 spring-zones, and wood is so much the heavier, the broader the 

 summer portion of the annual zone. 



Evidently a late and short spring and a prolonged summer are 

 favourable to an increase of weight in wood. It is also evident 

 that special localities and seasons will affect the differences 

 between the amount of spring- and summer-wood. Comparative 

 densities of stocking will also affect this question, for in crowded 

 woods vegetation begins later in the spring than in more open 

 woods. 



The quantities of woody substance in difterent woods vary 

 considerably according to species, and to the energy of growth 

 due to local conditions. Amongst the indigenous trees of 

 Central and Western Europe, the oak contains the largest 

 amount, and the silver-fir the smallest, of woody substance ; 

 Hartig states that broad-leaved species exceed conifers in this 

 respect by about 25 to 30 per cent. 



The energy of nutrition varies with the locality ; not only 

 must the soil be considered, with its widely-differing powers of 

 j)roductiveness, but also the powerful aids it receives from heat 

 and light. They are chiefly influential during summer, when 

 they meet with the fullest development of foliage and roots, and 

 most woody substance is therefore formed in summer. The 

 immense importance of these factors on the structure of the 

 annual zones of wood, in localities where the soil is equally 

 productive, is very noteworthy. From the harmonious or dis- 

 cordant working of all the factors of nutrition a number of 

 phenomena arise — for instance, the comparatively high specific 



