Relation of Annual Rings to Age. 167 



views, as generally held, are well expressed by Sachs*, who also 

 regards the alternation of density of structure as largely resulting 

 from external pressure f exerted by the more resisting cortical 

 structure, holding that the bark being relatively loose or of min- 

 imum tension in spring, the wood-colls first developed will there- 

 fore be larger and looser in structure, and the form least modified ; 

 but that as the tension constantly increases with continued devel- 

 opment of new structure within, the cells must show a constant- 

 ly increasing density and modification of form until, in the last 

 of the season, the difference is very pronounced. Id support of 

 this view he also cites the experiments of H. de Vries J by which 

 the influence of external pressure in producing such modification 

 is directly demonstrated. It would appear very doubtful, however, 

 whether we are justified in assuming this tension of the cortical 

 tissues to be the chief determining factor. If it were, then as the 

 tension steadily increases towards the close of the growing season, 

 there should be a correspondingly gradual change from the less 

 dense to the more dense structure. But this does not always occur. 

 Frequently the duct tissue ends abruptly, or, as we have seen, the 

 ducts are uniformly distributed ; there is an absence of ducts with 

 uniform density in the distribution of the wood-cells, or, as in 

 Ahius, the transition from larger thin-walled cells to those which 

 are smaller and thick-walled may occur very abruptly at or near 

 the close of growth for the season. Thus in the case of the 

 tropical woods examined, sixteen out of the twenty- three appear 

 to show that pressure of the bark is an unimportant factor, espe- 

 cially as eleven out of the sixteen show a regularity in distribu- 

 tion of the ducts through an otherwise uniformly dense woody 

 tissue. In these woods, also, when rings are developed their dis 

 tinction often rests — in five cases out of twenty-three — upon mere 

 superposition of rings successively upon one another, without other 

 structural difference, the lines of union or contract between suc- 

 cessive layers thus becoming the means of distinction. 



The general evidence here brought forward, therefore, seems 

 to indicate, in many instances at least, a more or less clo^e 

 correspondence between the number of rings and the periods of 



* Text Book of Botany, p. 652. 



t Ibid., p. 813. 



t Flora, No. 16, 1872. 



