8 GROWTH IN TREES. 
17. An irrigation test similar to the above was made with a small 
California live oak (Quercus agrifolia). The results were even more 
startling than those described for the pine. Within two hours the 
dendrograph, which had its contacts with the tree at least 3 meters 
from the absorbing surfaces, showed some enlargement, an action 
which may be directly connected with the fact that the vessels in this 
oak are numerous and large. 
18. The irrigation experiments might be held to simulate the effects 
of stream overflow which, if due to melting snows, would not be accom- 
panied by any marked higher humidity. It is seen to result in the 
formation of a tapering shell of wood which was as thick as the seasonal 
formation at the base of the trunk, but which had but half this thick- 
ness 8 meters higher up on the trunk. The layer of normal formation 
was of practically identical thickness at the two places. 
HISTORICAL. 
The rate and course of growth in length of seedlings and plantlets 
of various trees have been the basis of many measurements, and the 
forester developed approximate methods for estimating the yearly or 
seasonal increase in the height or diameter of trunks. An interesting 
series of monthly measurements of a number of trees was made at 
San Jorge, Uruguay, in 1885-1890. Evergreen trees, including 
conifers, eucalyptus, and acacia, showed greatest increases in October, 
a spring month, and least in July (midwinter), while deciduous trees 
increase most in midsummer (December) and actually decrease in 
girth in autumn (May)! Recently, also, some calibrations of the 
variations in volume of trunks correlated with temperatures have 
been made in New York.? The available data, however, did not re- 
cord the daily and seasonal changes or include information by the aid 
of which the activities of growing tips and branches might be correlated 
with changes in volume of trunks and stems. 
The most notable attempt to follow accurately the variations in tree 
trunks previous to the instrumentation described in the present paper 
was that of Mallock in 1917. He used an apparatus consisting essen- 
tially of two plates of glass superposed, or an arrangement of prisms 
in connection with a tape of invar passed around the tree. Vaziations 
in girth caused displacement of interference bands of light. Obser- 
vations on such displacements, at frequent intervals, showed that 
growth was greatest in the early part of the day, that actual contraction 
occurred in the afternoon, and that increases were closely related to 
temperature and rainfall.’ 
1 Hall, C. E. Notes on tree measurements made monthly at San Jorge, U: 
, ge, Uruguay, from Jan. 
12, 1885, to Jan. 12, 1890. Trans. Bot. Soc. Edinburg, 18:456. 1891. 
Pre arias and Weil. The coefficient of expansion of living tree trunks. Science, 48:348-350. 
3 Mallock, A. Growth of trees, with a note on interference bands formed by rays at small 
angles. Proc. Roy. Soc., 90, B, 186-191. 1919. Submitted Dec. 1, 1917. 
