APTITDDK FOU F1B8I0N, 17 



dividuals of spruce and of some others of our species the concentric 

 rings, however, separate easily. 



II. Flexibility and elasticity. — It is obvious that elasticity in- 

 creases aptitude for fission, since as the wedge is driven forward, 

 the split extends by the force of the mere leverage exercised on the 

 sides by the wedge, the cutting edge of which does no work at all 

 once it has helped to introduce the tool into the wood. On the 

 other hand, when mere flexibility exists, there can be no such lever- 

 age, and the entire work of parting the fibres has to be done 

 throughout by the edge of the wedge. Mere flexibility helps 

 fission only in so far that without it the wood on each side of the 

 wedge would break off short. 



III. Contained moisture. — As a rule, green wood can be split 

 more easily than dry wood. Hence sapwood is easier to split than 

 heartwood, and wood felled during great activity of the sap at the 

 beginning of the season of vegetation than wood felled at any other 

 season. This greater facility is due to the slighter degree of 

 coherence between the fibres in a green state and to the greater 

 flexibility, up to a certain limit, of the wood. We say up to a cer- 

 tain limit advisedly, for if this limit is exceeded, as in the case of 

 extremely flexible woods, the diificulty of fission is increased (this 

 clearly proves what has been said in the immediately preceding 

 paragraph). 



IV. Frost. — It is obvious that frost makes the fibres brittle. 



V. Resin and oils. — Resin, by diminishing elasticity, renders 

 fission difficult, while fixed oils generally facilitate it. 



VI. The circumstances under which the tree has grown up. — 

 Growth in the midst of a proper leaf-canopy and with a sufficient 

 supply of moisture increases aptitude for fission by producing a 

 uniform tissue composed of straight, long and parallel fibres not too 



'closely connected together owing to a high degree of lignification. 

 Since the same conditions favour diametral increment, wood with 

 wide concentric rings is usually more easily fissile than wood 

 with narrow rings. And generally speaking we may say that the 

 wood of all vigorous individuals is easier to split than that of weak 

 ones. Hence the well-known fact that young stool-shoots are 

 much more easily split than seedlings of the same size. The wood 

 of trees grown in hot, dry climates, as it is always so highly lig- 

 nified, is more difficult to split than that of trees from temperate 

 localities. 



The ease or difficulty with which a wood can be split is a cir- 

 cumstance possessing considerable importance, since a great many 

 industries depend on this quahty of wood, especially the trade in 



