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 1st, 5th, yth, and 9th 

 whorls of branches. The results from the measurement of the 

 xylem are found in TABLE xn. 



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 unitsf 

 is considered as the sum of these mean temperatures. 



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.-yS 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. 



t No daily mean temperature was below 34 F. 



