19 is] brown—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 da!y, 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 i, 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 prevaiHng 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 vatriabUity in growth intensity may be 



