20 GROWTH IN TREES. 



The measurements in the first series were made by bringing a modified 

 auxograph lever in contact with the upright leader of a young pine which had 

 been surrounded by a platform and held rigidly in position. The net gain of 

 the daily fluctuations during the whole of August amounted to nearly a milli- 

 meter in thickness. Now ensued a period of high fogs which during 3 days 

 equalized temperatures and humidity, reducing the daily variation, but 

 resulting in no increase. Finally, on September 5, rising temperatures and 

 humidity had the effect of producing a net gain which amounted to about 0.5 

 mm. before a rain came disarranging the clock-work of the recording cylinder. 

 Two days after this rain the instrument was again put in order and during 

 the following 2 weeks a daily increase in diameter was recorded which, on 

 September 27, had produced a net thickening of about 2 mm. The regis- 

 tration was discontinued for 2 weeks, but was resumed on October 15. The 

 increase had evidently continued during the interval, but was now coming 

 down to a minimum, as the net thickening in the following 2 weeks, termi- 

 nating October 29, was about 0.5 mm. 1 



Further observations on the variations of young stems were made in 

 the autumn of 1920. A small tree, No. 8, 8 or 9 years old, about 

 3.5 meters in height, was bent over so that the terminal leader which had 

 been formed by the growth beginning in February was held rigidly 

 on a block of wood, in such manner that the vertical swinging aim of 

 an auxograph might rest directly upon it. Another tree, No. 9, 5 years 

 old, about 2.7 meters in height, was similarly arranged but with the 

 internode formed during the previous year under measurement. 



The record of No. 8 shows that enlargement in diameter had ceased 

 on September 17 and that progressive shrinkage was recorded during 

 the following 10 days. Irrigation was followed by some increase. 

 The daily variation was not so great in the older internode of No. 9, 

 and irrigation was followed by a more pronounced and long-continued 

 enlargement. 



The auxographs were again brought into bearing at the same places 

 on the internodes of these two young trees for 10 days beginning 

 February 24, 1921. The terminal internode of No. 8 was now nearly a 

 year old and the buds at its tip had made an elongation to about 5 to 

 10 cm. during the previous month. The internode of No. 9, upon 

 which measurements were made, was now nearly 2 years old. The 

 buds at the tip of the terminal internode above it had made an elonga- 

 tion of 3 to 5 cm. It was soon seen that the two-year-old internode of 

 No. 9 had not awakened, as the pen traced a straight line, the soil 

 water supply being sufficient to maintain the water balance in the wood. 

 The one-year-old leader of No. 8 was already in course of growth, and 

 during the 10 days of the record showed an increase of 0.8 mm. in 

 thickness. A slight depression of the rate of growth occurred in the 

 midday period, especially on days of highest temperature. 



Passing from these measurements to variations in pine trunks, more 

 complex conditions are encountered. In securing dendrographic 



1 The records upon which the above statements are based were interpreted erroneously, 

 as stated in my account of 1919. See Carnegie Inst. Wash. Year Book for 1919, p. 76. 



