54 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
shown that in this moderated illumination the particular rays active in the 
photosynthetic process are still considerably in excess of the requirements of 
the chloroplasts for the particular partial pressure of the carbon dioxide 
employed, there will be room for doubting whether in this respect the leaf 
enclosed in its case is not at a disadvantage as compared with a leaf receiving 
the full solar radiation under free-air conditions. 
We have made numerous experiments in two different directions in order 
to throw some light on this point. In the first place we find that under this 
moderated illumination, whether the diminution of intensity is due to natural 
cloud or to the interposition of a thin canvas screen, the assimilatory process 
always readily responds to slight increases in the amount of the carbon 
dioxide in the air of the case, and that within certain limits fixation of carbon 
dioxide is proportional, or nearly so, to the partial pressure of that gas. This fact 
in itself indicates that the special rays active in photosynthesis are still present 
in the moderated illumination in excess of the requirements of the leaf when 
it is in air of normal CO, content. 
Another, and still more conclusive mode of experiment was to place two 
similar leaves in their cases under the thin canvas screen, cutting off a certain 
determinate amount of the light from one of them by means of the apparatus 
with revolving sectors, already described in Part I, and measuring the relative 
rate of assimilation by estimating the carbon dioxide absorbed from a stream 
of ordinary air passed through the cases. 
The results of such a series of experiments are recorded in the following 
Table VI. It will be seen that the sunlight passing through the light canvas 
screen, which absorbed about two-thirds of the solar radiation, had to be further 
reduced by the radial sectors to one-quarter of its intensity before there was 
any sensible effect in reducing the rate of assimilation ; thus showing that the 
light which passed through the screen still contained more of the particular 
grade of radiant energy active in photosynthesis than the chloroplasts of the 
leaf could utilise. This appears to us to be a strong argument in favour of 
the leaves enclosed in the cases being under no disadvantage as compared 
with those under free-air conditions. In fact, for reasons already given, it 
would appear that we might expect better results from the cut leaves when 
placed under these artificial conditions than are attainable with leaves still 
attached to the plant in the open. 
