1905] BRIEFER ARTICLES 151 
the cup, killing the living cells of the disks, thus preventing any loss of 
weight by respiration. The cup with its contents is then placed in the 
drying oven. In the evening, using the other cup, an equal number of 
disks is cut from corresponding positions in similar leaves, or, better, is 
cut from the second halves of leaves, from the first halves of which the morn- 
FIG. 4. 
ing disks were taken. These are treated as for the first 
series and the cup is placed beside the first in the drying 
oven. When thoroughly dry both cups are carefully 
weighed, the weights of the cups (stamped upon them, 
together with the letters M and N respectively to distinguish 
the morning and night cups) are subtracted, when the remainder gives the dry 
Weight, which is always greater for the evening than for the morning 
set. If steps be taken (by use of the methods described by SACHS) to 
eliminate transfer of material into the stem, the difference in the weights 
shows approximately the amount of substance formed through photosyn- 
thesis in the particular plant studied in so many hours per so many square 
centimeters of surface. In practice the performance of the instrument 
1S very satisfactory. 
In addition to the above-mentioned educational demonstration use, the 
instrument should also serve in those phases of ecological investigation 
where it is desirable to determine the relative photosynthetic powers of 
