1915] BROW N—PINUS STROBUS 213 
The third (temperature) is a variable one, and to this the rapidity 
of cell division is apparently directly proportional. Some idea 
of the rapidity with which the formation of tracheids may go on 
may be drawn from the following. Basal cuttings were taken 
from the south side of the ‘‘Wolf”’ tree already described on May 3 
and May 10, 1913. The first cutting showed no evidence of 
formation of tracheids, while the other, taken a week later, exhibited 
7 tracheids in each row, complete as to size, with several smaller 
ones in process of formation. The growth in places farther up the 
stem must have been going on still more rapidly. While the 
period mentioned above was warm and humid and therefore espe- 
cially conducive to rapid growth, it may be safely assumed that in 
all white pine trees in the vicinity of Ithaca formation of tracheids 
is very rapid at the start. A large number may be formed in a 
relatively short time. 
Intensity of growth in aerial parts 
In the discussion which follows, the distinction between inten- 
sity of growth and amount of growth must be kept clearly in mind. 
The latter may be easily ascertained for the whole growing season 
or for any part thereof by measuring at a given period the amount 
of new tissue. Growth intensity, on the other hand, is constantly 
changing. It may vary from week to week, day to day, and even 
within one and the same day, as FRIEDRICH has pointed out (7). 
The amount of growth during a given period is then the sum of the 
prevailing growth intensities multiplied by the time each was in 
force. Let us take a specific example. Suppose a white pine 
first begins the formation of new xylem on May 1, and, on May 30, 
60 new tracheids were in evidence in each tracheid row. It does 
not follow that 20 tracheids were formed the first 10 days, and 20 
during each succeeding 10 days, making a total of 60. While 
the average growth intensity was two tracheids per day, the actual 
growth intensity may have vacillated on either side of this amount. 
It is obvious that it is quite impossible through comparative studies 
to obtain the prevailing growth intensity at a certain definite time. 
In order to do this the growth process would have to be actually 
observed. Some idea of the variability in growth intensity may be 
