456 BOTANICAL GAZETTE [JUNE 
During the spring and summer of 1920 the writer had the privi- 
lege of operating one of MacDoucat’s dendrographs in cooperation 
with him. Because of the backward season in the mountains, the 
instrument was placed on a specimen of Acer Negundo in the city 
of Ogden, Utah, on April 1, 1920, for the purpose of determining 
when growth actually begins in this species. A period of unusually 
cold stormy weather followed the installation of the instrument, 
during which quiescence and alternate shrinkage and enlargement 
were recorded. The tree showed some enlargement April 10-14, 
which, however, did not continue during the following four days 
when high winds prevailed. Diameter growth did not properly 
begin until May 19. The buds were just beginning to swell on 
April 1. By April 27 practically all of the leaves had unfolded, and 
on May 5 they were about one-fourth full size. The leaves had 
reached about one-half their natural size before the main period 
of growth began on May 19. 
Fig. 2 shows the daily march of the diameter changes. In 
order to afford a closer correlation, the march of the daily mean 
temperature of the air and of the cortical layer adjacent to the 
cambium or the zone of living tissue where the diameter growth 
actually occurs is plotted in the lower part of the figure. The 
greatest and smallest diameters reached during the day are plotted 
in the upper graph to show the diurnal variations. These diurnal 
changes apparently vary to some extent with the range in tempera- 
ture. On cool cloudy days with relatively warm nights, which have 
small temperature ranges, the diurnal change in the diameter 
is slight, while on warmer clear days with cool nights, having a great 
diurnal temperature range, the diurnal shrinkage or expansion 
may amount to as much as 0.5 mm., indicating a direct tempera- 
ture relationship. Moisture conditions are not considered among 
the critical factors causing the commencement of growth in the 
spring, because the soil is abundantly supplied with moisture from 
the melting of the winter snows and from the spring rains, until 
considerable growth has been made and fairly high temperatures 
obtain. As might be expected, there is a somewhat close corre- 
spondence between the trend of the two graphs, although the 
cambial temperature does not usually attain the extremes reached 
by the air temperature. 
