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 II. 
TABLE II 
NUMBER OF ROWS OF NEW PHLOEM CELLS AND DIAMETER OF PHLOEM AND CAMBIUM 
TISSUES IN TREES B anp D 
Nov. 13 May 5 | May 11 May 25 | June 8 June 18 
Region of cutting coe ee ; 2 ce Dia. | Dia. 
No. _ No. oa No. ee No, ot No. i No.) pr 
Below 1st whorl..... o| 34 |— — | 75 |2 ' 68 | 8 | 170 11 | 228 
Below 2d whorl ..... ON SRS Sag S58. gop tae rr aes (ae 98 
Below 3d whorl ..... 34°13 68 4 | 82 |4 6|— | — |11 | 286 
Below 4th whorl..... o | 34 |4 | ro2 |4 | 102 |6 92 |12 | 221 |14| 262 
Below 5th whorl..... 0 | 34 | § 85 | 6 | tis eee | 262 
Below 6th whorl..... 0 | 34 |4 68 }8 | 107 |7 92 |I0 | 190 13 | 238 
Below 7th whorl... .. | OSF ars) ts ee ' So A oe mca poppe 279 
Below 8th whorl. .... o| 34 14 | 85 oe — |6 | 92} 8 | 170 |—| 
Below oth whorl... .. Ot Gare aa ot es 6 705) 8: | 293.412 | 245 
Below toth whorl....| 0 34°13 | Wer. 6 awe mags Bie ee as 238 
| 34 |3.1| 72 | 4.8 89 Is.3| 90 9.5 186 12 | 250 
Average. .....-. | 9 
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 III 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 
