14 BULLETIN 193 



The readings were taken at about the same time of day and the 

 areas through which the photometer was moved were as nearly the 

 same each time as could be estimated. 



The result of this work is displayed in table VII. The values there 

 given are directly comparable, as they all refer to the common stand- 

 ard, 1. In this instance the standard was made on April 27, a rather 

 early date; but it seemed at that time desirable to make comparisons 

 while the solio-paper used in making the readings was still fresh. 



TABLE VII LIGHT INTENSITIES AT CERTAIN STATIONS IN A PEAT BOG, COMPARED 



TO A COMMON STANDARD. MOST OF THE TREES WERE SMALL, AVERAGING ABOUT 

 30 FEET. FIRST SISTER LAKE, ANN ARBOR, MICH. 



A careful consideration of these figures shows a wide divergence 

 in the light intensities existing in the different zones when these are 

 compared with one another. The only zone with consistently low in- 

 tensities was the Larix, in which case variations ranged from .125 to 

 .1333. In an adjacent Larix station, in which the cover vegetation 

 was Cassandra in a dying condition, a series of readings gave a max- 

 imum light intensity of only .0333. (Plate 1). April 5 was a cloudy 

 day with a light intensity of .0666 at the control station. A compari- 

 son of the values at other stations shows that with the exception of the 

 Larix station there was very little variation. The same fact is ap- 

 parent, but not so striking, in the data secured on May 17. On 

 brighter days greater variations occurred, as may be seen by ex- 

 amining the data for April 27 and May 31. Possibly these wide 

 variations are due to the fact that it is impossible to expose the photo- 

 meter in exactly the same place when it is moved under the tree 

 crowns. All possible care was taken to avoid error of this kind, but, 

 as has been pointed out already, exactness in this respect cannot be 

 obtained. 



In some instances the light intensity "in the shade" was higher 

 than in the open. (April 19, Larix-Populus station; May 17, same 

 station and Salix station). Such occurrences tend to raise a doubt 

 as to the actual value, indeed of the accuracy, of other readings. They 

 are due, of course, to variations in the density of the clouds at the 



