36 GROWTH IN TREES. 



equalizing variations, which decreased in amplitude with the advance 

 of the season, until the record was but little away from a level line 

 until January 1921, when the instrument was dismounted to be re- 

 placed by one of improved pattern. The total increase during 1920 

 amounted to 12 mm., or about one-half that of the previous year. 

 The reduction of the measurements given was in fair agreement with 

 measurements of cores taken from the tree in February 1921. The 

 wood formed at the beginning of the season is characterized by ex- 

 tremely large vessels, which serve to denote the part of the layer formed 

 earliest in the spring. 1 



Vines has described the annual rings of Fraxinus excelsior as being 

 2 to 3 mm. in thickness and as consisting of an internal spring zone 

 of wide vessels with wood-parenchyma and rather thin-walled woody 

 fibers, external to which, later in the season, thick-walled woody 

 fibers form with scattered, smaller vessels surrounded by wood paren- 

 chyma. The wood formed at the close of the season consists chiefly 

 of wood parenchyma and small, very thick-walled vessels. He notes 

 that young trees of this species in a damp soil may form a layer of 

 wood 15 mm. in thickness in a season, which is practically equivalent 

 to that of the Arizona ash under conditions of irrigation. 2 



The daily variation in 1919 was greater than that seen at any other 

 time or in any other tree. Thus the daily shrinkage in March might 

 be as much as 0.4 mm., with an over-compensating increase of 0.6 mm. 

 by the following morning; in April these measurements were 1.1 mm. 

 and 1.4 mm. In May the variations rose to 1.6 and 2.1 mm. Cor- 

 related variations are displayed by elongating seedlings of the ash 

 during the same months. 



The materials of which the floating frames were constructed were 

 tested to determine whether or not some of this apparent variation 

 might be due to shortening and lengthening of the metal bars and rods 

 under changing temperatures. 



Calibrations of a bar of bario, such as was used in this instrument, 

 showed that its variation was 0.0000065 per unit part with 1 C. The 

 length of the axis of the yoke was 14.5 inches or 363 mm., and its 

 variation would therefore be 0.000025 mm. for 1, and 0.0005 mm. 

 for 20, which would represent the possible total in any one day. This 

 amplified 10 times by the levers would amount to about 1/200 of one 

 space on the record sheet. This would be wholly compensated by the 

 expansion and contraction of the short contact screws and by the varia- 

 tion in the instrument, so that for the purposes of this research the 

 instrumental error may be neglected. 



1 The above calculations are to be used in correction of the statements in the Carnegie Inst. 

 Wash. Year Book for 1919, p. 74. 



1 Vines, S. H. Text Book of Botany, p. 198. 1895. 



