74 CARNEGIE INSTITUTION OF WASHINGTON. 



ter. The only adjustments during the season are those which may 

 become necessary by the enlargement of the tree. 



A further study is being made of encircUng flexible bands which might 

 be used to replace the yokes in certain cases. 



Growth of the Arizona Ash, hy D, T. MacDougal. 



An Arizona ash {Fraxinus arizonica), 14 years old, in the grounds of 

 Dr. H. W. Fenner at Tucson, was selected for measurement. This 

 tree had been transplanted to this place when 2 years old and was 

 subject to irrigation, a condition not unusual to the tree in its habitat 

 along streamways. The trunk was slightly compressed in one diameter 

 and had a circumference of about 1 meter. An instrument with a 

 yoke of bario was adjusted to this tree on March 8, 1919, at which 

 time the flowers (staminate) were beginning to open, but no leaf- 

 surface had yet been displayed. 



Enlargement of the trunk began about March 10, although the enor- 

 mous daily variation in this tree made it difficult to fix upon the day 

 in which the increase was greater than the shrinkage. Enlargement 

 was interrupted for 6 days early in April, probably due to low tempera- 

 tures follo^ving a rainstorm, and in the latter part of July the increase 

 had been such as to jam the lever, giving a record simulating shrinkage. 

 Readjustment of the yoke and its bearing-points was followed by 

 records showing growth. Enlargement had ceased by the end of 

 October, at which time a total net increase of about 43 mm. had taken 

 place, showing that this young tree, which had made an average 

 increase of an inch in diameter yearly, was now approaching its 

 maximum stage of activity. 



The bearing-points of the yoke were seated on prepared places in 

 the bark which had been thinned down to 1 mm. at the beginning. No 

 examination has yet been made to ascertain what bark formation may 

 have taken place at these places, but it may be safely estimated to 

 amount to not more than 1 or 2 mm. 



The daily variation in thickness of the trunk, the difference between 

 the diameter at sunrise and at 4 p. m., was such as to indicate a shrink- 

 age of as much as 0.4 mm. daily in March, and the increase by the 

 following morning was 0.6 mm. In mid-April the shrinkage amounted 

 to 1.1 mm. and the overnight increase 1.4 mm. This type of variation 

 reached its maximum about May 1, when the daily shrinkage was as 

 much as 1.6 mm. and the following swelling as much as 2.1 mm. This 

 wide variation, which can be ascribed only to direct water-loss, was 

 probably controlled or lessened by the formation of new corky tissues, 

 although this matter needs morphological confirmation. The daily 

 shrinkage during the summer rainy season was reduced to a minimum 

 of 0.1 mm. or even less, and was not more than 0.2 or 0.3 mm. in any 

 day during the remainder of the season. 



