I.] GENERAL CHARACTERS AND STRUCTURE. 19 



any two " annual rings " is due to the sudden apposi- 

 tion of non-compressed elements upon closely-packed 

 and apparently compressed elements : the latter were 

 formed in the late summer, the former in the spring. 

 Moreover, the spring wood usually contains more 

 numerous vessels, with larger lumina than the autumn 

 wood, and for the same reasons as before : in this 

 particular case, again, the fibres of the autumn wood 

 are darker in colour. It should be stated, however, 

 that many dicotyledonous trees show these peculiari- 

 ties more clearly than the beech : others, again, show 

 them less clearly. 



Now it is obvious that, other things being equal, 

 the spring wood, with its more numerous and larger 

 vessels, and its looser tissue generally, -will yield more 

 readily to lateral pressures and strains than the denser 

 autumn wood ; and the like is true of the pines and 

 firs the closely-packed, thick-walled tracheides of 

 the autumn wood furnish a firmer and more resistant 

 material than the larger, thinner-walled tracheides of 

 the spring wood. To this point we shall have to return 

 presently. 



C 2 



