KNUDSON: CAMBIUM DEVELOPMENT IN AMERICAN LARCH 289 
was now approximately 15 years of age. Cuttings were removed 
from the south side of the trunk below the Ist, 5th, 7th, and 9th 
whorls of branches. The results from the measurement of the 
xylem are found in TABLE XII. 
While no conclusions can be drawn respecting the inception of 
growth the indications again. point to the middle region. The 
figures are of interest mainly because of the fact that growth of 
xylem has occurred before May 25, which was the date of xylem 
development in 1909. 
FACTORS INFLUENCING DATE OF XYLEM FORMATION 
Temperature, moisture, and insolation are important factors 
in all growth phenomena. In 1909 and 1911 the opening of buds 
of larch began April 19. In 1909 the xylem formation began on 
May 25, while in 1911 the xylem formation began about May 20.* 
The temperature, moisture, and sunshine figures for the intervals 
between the bud opening and wood formation are found in the 
following table. The mean temperature for the day is considered - 
as the number of heat units for that day, and the total heat unitsT 
is considered as the sum of these mean temperatures. 
Period | Total heat units | Hours of sunshine Precipitation 
April 19—May 25, 1909... 1,810 | 234.5 | 2.89 
April 19—May 20, IgII... 1,769 257- yA Oe 
The number of heat units then during the interval before wood 
formation in 1911 was almost as great as that during the longer 
period of 1909, while the amount of sunshine was greater during 
the 1911 period. The amount of precipitation at this time of the 
year was not important as sufficient water was available both - 
seasons. In 1911, however, the mean temperature for the three 
days preceding wood formation ranged from 76° F.-78° F. and for 
the three days preceding this period the range of the mean temper- 
atures was 62° F.69° F. During 1909 the nine days preceding 
wood formation ranged in mean temperature from 50° F.-54° F. 
except one day which had a mean temperature of 58. The three 
* The exact date of xylem formation was not determined. The observation on 
May 22 showed a diameter increase of 6 cells. 
+ No daily mean temperature was below 34° F. 
