282 KNUDSON: CAMBIUM DEVELOPMENT IN AMERICAN LARCH 
While in trees A and C the region of the first growth of xylem 
could not be definitely located, more fortunate results were ob- 
tained with tree B. No xylem was formed in tree B until May 25 
and then neither at the top nor at the base of the trunk, but in the 
middle region. By May 25 then the xylem in that part of the 
tree between the fourth and fifth whorls of branches was four cells 
in thickness,—while between the sixth and seventh whorls it was 
five cells and between the seventh and eighth it was two cells in 
diameter. The most marked development again occurred im- 
mediately after May 25. The detailed figures are given in TABLE 
Iv. 
Season and duration of diameter increase-—In order to bring 
out more clearly the period of greatest diameter increase, the 
combined values for increase of phloem, cambium, and xylem, 
in trees A and C are given in TABLE vy. 
- From the table it is at once evident that very little growth 
took place previous to May 25, and the growth which occurred 
was confined, as before indicated, entirely to the cambium and 
phloem. From May 25 to June 3a very marked increase occurred, 
and during that time in the middle regions of the trunk nearly one 
half of the total diameter increase was completed. In TABLE VI 
is given the daily average increase in rows of cells and also the 
daily diameter increase in trees A and C. 
TABLE VI 
Period May 25-June 3 June 3-June rs / June 15-June fe June 30-July 6* 
No. rows of cells......... | 4.2 Z; 1.4 I. 
Dia. in microns.......... 94.5 | 30. | 4o. 48. 
By July 6 the ring of xylem was almost complete, judging from 
the extent of the summer wood which was then in the process of 
formation, though not yet completely developed. Unfortunately 
no further series of cuttings were made after July 6 to determine 
the succession with regard to growth cessation. 
In TABLE VII is given the combined diameters of the phloem, 
cambium, and xylem tissue in trees B and D, as well as the total 
number of cells. 
* For cutting nos. 9-10 only. 
