

1905.} On Vegetable Assemilation and Respiration. . 419 
amount. 00110 would only be maximal at 24° C. . Light is thus falling 
off faster than the temperature, and has become in its turn the limiting 
factor ; the decreasing assimilation is henceforward a direct measure of the 
light intensity only. | , | 
This series of readings in the waning intensity of afternoon diffuse light 
is represented graphically in fig. 3, and it will be seen that assimilation is 
considerable right up to sunset. 
.The assimilation, of 00033, in the last reading but one could ave been 
carried out with a temperature of 7° C. Finally, there is no appreciable 
light or assimilation, and from 815 to 9:15 we get, instead, respiration 
with an output by the leaf of 0:0010 gramme COsz. 
Experiment III.—The leaf in this experiment was exposed to a natural 
sequence of irregular illumination throughout the day. The leaf-chamber 
was inside the water-bath and kept faced normal to the sun (or, when 
cloudy, to the approximate position where the sun would be) throughout the 
day. The only interference with the natural sequence of things was an 
alteration in the temperature of the bath about 4p.m. The temperature had 
been kept at about 18° C. by circulating water until 3.35 when heating 
up (by a primitive and very slow method) was begun. The heating up lasted 
for two hours, and then the bath slowly cooled down again. 
The first three readings—0-0090, 0:0081, 0:0093—are maxima for the 
temperatures of 21°-0, 19°1, and 21°3 C. respectively, and these probably 
were the average temperatures of the leaf in the respective readings, heated 
up by the diffuse light and feeble variable sun. Possibly the second reading 
is really just limited by the dull light. The fourth reading of 0:0067 is 
clearly so limited as it is well below the maximum even for the bath-tem- 
perature. In the fifth reading dull light prevailed at first, but later gave 
place to sunshine, and the assimilation is 0:0077, a slight increase. During 
this reading heating up of the bath was begun, but this is not the cause of 
the rise of assimilation, as 18° C. would have been adequate for what takes 
place. The sixth and seventh readings both show a large increase due to 
the improved light, and are a direct measure of this. 
In the eighth reading the sun sinks behind a ridge of the roof, and assimi- 
lation drops to 0°0043, while there is still appreciable assimilation up to 
sunset, after which the COz of respiration begins to escape from the leaf. 
_ The values obtained after the sun has gone below the roof are, of course, 
diffuse light values, and they agree closely with those of Experiment II 
under the same conditions, as fig. 3 shows. | 
272 
