Interference Bands formed hy Rays at Small Angles. 191 



growth was different in each case, but always greater in the early part of the 

 day than later. In fact, actual contraction was noticed on several occasions 

 between noon and 3 p.m. This showed that there was a considerable daily 

 component involved, but its magnitude could not be determined with any 

 certainty by observations which began at 10 a.m. and ended at 5 P.M. 



From June 21 to the end of July, while staying in the country, I made 

 constant observations day and night on four trees, devoting a week or ten 

 days to each. The records so obtained are reproduced in Diagrams I-IV, 

 together with the temperature of the air. 



It will be noticed that the increase of girth and the temperature curve are 

 rather closely related, the growth being most rapid when the temperature is 

 lowest, or nearly so. Also that rain has a great effect, any shower being 

 followed by increase of girth. 



A probable explanation of these facts may be found in the variable rate of 

 evaporation from the leaves, combined with a nearly constant flow of sap 

 into the roots. Presumably, the mean line through the diagram of girth 

 indicates the rate of formation of new wood, the divergence from the mean 

 representing the degree of turgescence in the bark and layers immediately 

 underlying it. 



The effect of rain may be partly mechanical, that is, it may act by merely 

 wetting the bark and thus causing it to swell ; at any rate, this may happen 

 in the case of heavy rain, but it must also act by checking evaporation from 

 the leaves, and in the case of light showers this is probably the most efficient 

 factor. 



I regret that I had no means of measuring the humidity of the air. In 

 any future trials this should be observed. The daily component, as well as 

 the average growth, varies largely in the different trees, and far more 

 extended observations would be required before any generalisation should be 

 attempted. 



The method of measurement, however, is simple and satisfactory, and the 

 results, as far as they go, seem sufficiently interesting to warrant their 

 publication. 



