76 CARNEGIE INSTITUTION OF WASHINGTON. 



either in seedlings, tips of branches, leaders, or trunks, although so far 

 no member has been measured which shows continuous growth. 



The auxograph records of the leader and of laterals of young trees 

 showed elongation during August, September, and October 1918, with 

 a decrease, retardation, or shrinkage during the midday periods, when 

 water-loss was excessive, the general course of variation being parallel 

 to that of the base of the trunk, as described below. 



The action of the main stems 1 year old of trees 3 m. in height pre- 

 sents a special case. Such a stem, about 2.4 cm. in diameter, still bore 

 leaves, had an unbroken epidermis, a chlorophyllose cortex layer over 

 1 mm. in thickness, a heavy cambial layer, the rays still composed of liv- 

 ing cells, and a medulla or pith 3 to 4 mm. in diameter. The mechanical 

 features presented may be roughly compared to those of a sunflower 

 stem which had attained its growth in length but not in thickness. 



The record of this stem was begun August 5, 1919, and nothing is 

 known concerning its behavior earlier in the season. In August, Sep- 

 tember, and October, in which period some increase in thickness took 

 place, the daily variations were the reverse of those displayed by the 

 trunk, as an abrupt expansion began at 9 a. m. and continued for 3 or 

 4 hours, after which a retardation or shrinkage ensued. 



The instrument was adjusted to the same position in May 1919, 

 and this young stem, now 2 years old, was seen to enlarge at an irregu- 

 lar rate throughout the season ; the daily variation was of the type in 

 which the greatest enlargement occurs during the midday period. 



The measurements of the base of the trunk of this pine were made 

 on a tree about 42 cm. in diameter, and a yoke was adjusted to it at a 

 height of 1.3 m. on March 24, 1918. Reproductive organs had been 

 formed earlier, the crop of pollen had been shed a month earlier, and 

 the tips of branches had elongated 3 or 4 cm., while the leaders of 

 young trees had grown ten times as much. It appeared, however, 

 that the base of the trunk had not begun to enlarge, as no increase was 

 recorded until March 30. 



The first interruption of growth occurred during a period of high 

 temperatures and low humidity at the end of May, after which time 

 the rate ^vas lower and subject to long interruptions, although increases 

 occurred as late as September, during which time the trunk, including 

 the thin layer of inner bark under the contact points, underwent a total 

 increase in thickness of 9 mm. or over a third of an inch. The actual 

 thickness of the shell of wood or "annual ring" may be safely esti- 

 mated at about 3 mm. The daily variation in thickness was such 

 that in the earlier part of the season the actual diameter of the trunk 

 at 2 p. m. might be 0.5 mm. less than at 8 a. m., and this increased to 

 0.7 or 0.8 mm. in June, but lessened to 0.4 nrni. in August. During the 

 period of active growth and greatest daily variation a trunk might show 

 a diameter at sunrise as much as 0.9 mm. greater than at 3 p. m. the 

 previous day. 



