274 THE STKrCTUEE OP TIMBER. 



wood, and is characterised by the cells having comparatively 

 large internal chambers and rather thin walls, whereas the 

 autumn-wood formed later is much denser, the cells having 

 much smaller internal spaces and thicker walls. That is the 

 reason why, on examining the cross-section of a tree, we have 

 usually no difficulty in saying where a ring begins and ends. 

 The dense autumn-wood of one year abuts on the porous spring- 

 wood of the succeeding year, and the variations in colour which 

 these different degrees of density produce enable us to determine, 

 even by the naked eye alone, the exact limits of an annual ring. 

 We come next to consider the causes which are accountable 

 for the variations of the spring and autumn-wood. The more 

 generally accepted theory is Sachs', according to which the cells 

 of the spring-wood are large and those of the autumn-wood 

 small, simply because in spring less resistance is offered to the 

 cambium than in autumn. The pressure on the cambium in 

 spring is less than in autumn owing to the mollifying influences 

 of rain and frost during winter, whereas in autumn the new 

 wood formed earlier in the season has filled up all empty spaces 

 and induced considerable pressure on the cambium mantle. It 

 can be shown experimentally that increase or decrease of pres- 

 sure on the cambium influences the production of wood and, to 

 some extent, the shape of the cells, and on the whole the theory 

 of Sachs explains the variations in the wood ring very satisfac- 

 torily. Hartig, on the other hand, holds that the porous charac- 

 ter of the spring-wood is due to deficient nourishment of the 

 cambium, and that the greater density of the autumn- wood is 

 the result of the cambium being better provided with the mate- 

 rials requisite for the maintenance of activity in its cells. No 

 doubt in spring and early summer the tree cannot have the 

 maximum amount of food to offer to the cambium, for leaves 

 being scarce and the days being cold and comparatively short 

 assimilation is somewhat slow, and moreover much of the pro- 

 ducts of assimilation are required at that time to build up new 

 shoots and leaves. Later on in the season the products of assi- 

 milation being more abundant more food is placed at the disposal 

 of the cambium, which responds by producing firmer and more 



