KNUDSON : CAMBIUM DEVELOPMENT IN AMERICAN LARCH 279 



By July 6 the phloem was nearly 'complete with respect to cell 

 numbers; for, the average annual number of rows of phloem cells 

 produced during a period of three years was found to be seventeen. 

 At this time the cells were all very regular in form and no visible 

 differentiation had occurred. In DIAGRAM I are represented the 

 diameter measurements of the phloem at several different dates. 



In trees B and D the growth is similar to that in trees A and C, 

 though more vigorous growth resulted in tree B than in tree A. 

 In this tree also the development of cambium consists first in a 

 transformation of the peripheral cells. In this case, however, the 

 number of rows of phloem cells produced by May 25 was 5.6 with 

 nearly a 200 per cent increase in diameter. The detailed figures 

 are given in TABLE n. 



TABLE II 



NUMBER OF ROWS OF NEW PHLOEM CELLS AND DIAMETER OF PHLOEM AND CAMBIUM 

 TISSUES IN TREES B AND D 



Development of xylem. While in tree A there was a gradual 

 increase in phloem from April 19 to May 25, yet no xylem cells had 

 been formed. During the week of May 25 to June 3, coincident 

 with the marked increase of phloem, a very marked increase of 

 xylem occurred, over one third of the xylem being completed 

 during these seven days. TABLE in gives the figures obtained 

 for the number of rows of xylem cells formed, and the diameter of 

 the xylem tissue in the various cuttings at the different dates. 

 There is included also the diameter of the xylem tissue formed in 

 tree B during the years 1909 and 1910. In the first column the 



