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). 1 Recently, also, some calibrations of the 

 variations in volume of trunks correlated with temperatures have 

 been made in New York. 2 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. Variations 

 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. 3 



1 Hall, C. E. Notes on tree measurements made monthly at San Jorge, Uruguay, from Jan. 

 12, 1885, to Jan. 12, 1890. Trans. Bot. Soc. Eclinburg, 18:456. 1891. 



s Trowbridge and Weil. The coefficien t of expansion of living tree trunks. Science, 48 : 348-350. 

 1918. 



* 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 Deo. 1, 1917. 



