432 BOTANICAL GAZETTE [JUNE 
irregularities do appear in others are insignificant. The curves 
plotted from these data are at least as regular as can be obtained 
from actual measurement during the season. And while the 
causal connection of the two curves is not actually demonstrated, 
I do not doubt that both represent the same thing, the different 
vital activity at different seasons, as the plant gradually arouses 
from its winter rest in spring and sinks into it in late summer and 
fall: The tree keeps its own record of its annual period of 
activity in the length of its needles; and we do very well if the 
record we construct from continuous measurement of the 7 
ity of growth is equally legible. 
The cause of this regular annual variation is the same com- 
plex, constituting its environment, which in some years stimu- 
lates the growth of longer, in others, of shorter leaves all sum- 
mer; with perhaps a larger part played by the temperature, and 
less by the moisture, in the production of the rhythmic than of 
the irregular variations. How little the regular annual period 
depends upon present conditions, and how largely it is heredi- 
tary, in response to the climatic conditions of the plant’s ances- 
tors, cannot now be said, but it probably depends very much upon 
the species. No conclusions can be drawn from the Arauca- 
rias*° in a local greenhouse, which show occasional, zones of 
short leaves, corresponding possibly to periods of rest, for the 
greenhouse is only an attempt to Pe their natural 
climate. 
TABLES. 
Except where otherwise specified, the measurements are 
made from lateral branches of the first order. In each table 
the average needle-length in successive zones, of ten needles 
each, from below upward, illustrating the regular yearly rise and 
fall in needle-length, is given first. Then follow the average 
needle-length of each year, the area of the cross section of a large 
needle, the length of the year’s growth of stem (the same on 
DE Bary: tl Anat. 513. Annual rings are not always regularly present 
in the wood of Araucari 
