454 Dr. F. F. Blackman and Miss G. L. C. Matthaei. [Apr. 11, 
Caustic alkaly still air. .:..../....... 00117 
if WING TN Ae be 00148 
Helianthus leaf, bright day ......... 0:00'79* 
i dull day me dys a. 0:0077 
Sachs arrived ata much higher number for Helianthus, viz., 0°150, but this 
was not directly observed; it was arrived at by employing a leaf attached to 
its plant and making a large allowance for translocation, based on another 
experiment. Sachs’ number probably is much too high. 
From the fact that the same number was arrived at by Brown and Morris 
on two days, one brightly hghted and the other dull, we may conclude that 
the intake of COz was really limited by the diffusion possibilities and not by 
the light or temperature. 
This view is supported by the experiments in the present paper, which 
show that this intake of CO2 can be much surpassed with moderate illumination 
when the COs-supply is not a limiting factor. Thus with an adequate 
pressure of CO2 we have recorded :— 
Experiment XVI. Helianthus, sunlight only......... = 0:0264f 
x XVI. is diffuse light only .... = 00151 
” XI. H PAU ASUE Pie oe caste oa = 007264 
7 xe r diffuse light only .... = 0°0192 
It is probable then that a leaf of Helianthus is in Nature limited to an 
absorption of 0:0077 gramme COz per hour per 50 sq. cm. The value may be 
higher even up to the 0:0150 of Sachs, but it does not seem likely that the 
leaf should be as efficient as an equal area of caustic alkali, even though this 
particular leaf has two absorbing surfaces. The evidence, on the whole, is 
not in favour of the view that leaves with stomata on both surfaces are much 
more efficient absorbers than those with stomata on one surface only. There 
are probably few leaves more active in assimilation than those of Helianthus, 
i.e. few offering less mechanical hindrance to the ingress of COs, and if the 
lower value of 0:0077 is too low for this plant, it may be the limit for a 
number of other plants.t 
We will endeavour then to picture the state of things that holds with a 
leaf limited to this intake by the low pressure of CO: around it. | 
In setting out to compare the energy available for photosynthesis with the 
* A leaf of Catalpa (conditions of experiment not stated) gave also 0°0078, ef 
Horace Brown, 1899, loc. cit. 
+ These numbers are the apparent assimilation, 7.¢., the real assimilation less the respira- 
tion. 
t See, however, Note C, p. 459. 
