174 Relation of Annual Rings to Age- 



reality in the next division. The third case, in which the rings 

 first increase continually in thickness, and, later, as regularly de- 

 crease, is illustrated in Betula alba, Ostrya virginica, and Fraxv- 

 nus americana. The fourth and last case, in which the rings are 

 all of more uniform thickness, is illustrated in Populus tremuloi- 

 des and grandidentata, Ahius serrulata, Betula lutea, Carpinus 

 caroliniana, Juglans clnerea, Ulnius americana, Rraxinusviridts, 

 Pruitus serotina, Acer rubrum and saccharinum. 



CONCLUSION. 



I hardly feel justified in drawing decisive conclusions from 

 evidence of so limited a character, but the facts here stated 

 furnish certain indications which it may be well to state as a 

 guide to future and confirmatory observations. They are as 

 follow : — 



(1) The formation of rings of growth is chiefly determined by 

 whatever operates to produce alternating periods of physiological 

 rest and activity. In temperate climates, where the seasons are 

 sharply defined, these periods are determined by the seasons them- 

 selves, but in tropical and sub-tropical latitudes other influences, 

 recurring at less regular periods, operate to determine them, there- 

 fore — 



(2) In cold climates, rings of growth are an approximately 

 correct index of age, but in warm climates they are of little or no 

 value in this respect. 



(3) Even in cold climates there is not an absolute correspond- 

 ence between number of rings formed and years of growth. 



(4) In warm climates the tendency is to obliteration of rings 

 and homogeneity of structure. 



(5) The distinction of rings is essentially due to structural 

 modifications, sometimes aided by local deposit of pigment or 

 resin, and this modification of structure is due in part to pressure 

 of the external structure upon the forming tissues, and, in part, to 

 physiological peculiarities of the plant iteelf as independent of 

 such presence. 



These indications are thus seen to be essentially in accord with 

 the views generally held at the present time, as already stated. 



(6) The influence of meteorological conditions in determining 

 the growth of each season is most important, particularly with 

 reference to rainfall. 





