120 Canadian Record of Science. 
quence of which they are usually designated as the annual 
rings. Advantage has been.taken of this fact to reach an 
approximate estimate of the age of trees such as the great 
redwoods and sequoias of California, and it becomes of prac- 
tical value to the surveyer in re-establishing old boundary 
lines. 
In order, however, to correctly determine their relation 
to climatic influences, a few important considerations may 
be passed in review. 
The formation of such rings or layers of growth, is refer- 
able to periods of physiological rest and activity, which 
alternate with one another, together with the secondary 
influence of internal tension established between the wood 
and bark. Whenever the change of seasons is sharply 
defined, and the conditions which obtain during summer 
are favorable to continuous growth, there will be but one 
period of activity and one of rest ; consequently, but one 
layer of wood for a given year. There are notable excep- 
tions to this, however. The red maple has been known to 
form several such rings in one season, and the same is true 
of other plants, but many such cases find at least a partial 
explanation in the attendant conditions, which induce 
repeated periodicity within the same season. In the tropics, 
where the conditions for continuous growth are more favor- 
able, trees generally exhibit no rings whatever, and when 
they are developed, an explanation is usually to be found in 
local conditions. We therefore learn from this, that in- 
creasing cold, through inducing a more perfectly defined 
periodicity in growth, causes the formation of layers of 
growth which, in number, correspond approximately to 
the age of the plant, and this correspondence will be closer, 
other conditions being equal, the farther north, or the more 
remote from the equater, the lovation is. 
Finally, we may turn our attention to a brief considera- 
tion of those influences which vegetation is supposed to 
exert upon climate. The history of Southern Europe and 
Asia Minor, as well as the more recent history of this con- 
tinent, shows that with the removal of the large forests once 
