40 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
in carrying out the preliminary work connected with this instrument, and 
especially for the labour he bestowed on the calibration of the radiometer in 
absolute thermal units. 
Tke radiometer and recorder were constructed by the Cambridge Scientific 
Instrument Company, and their cost was defrayed by the Government Grant 
Committee of the Royal Society who kindly put the instruments at our disposal 
for this research. 
When a bridge-wire No. } is used in the recorder of this particular instrument, 
100 scale-divisions = 1 ohm, and with the radiometer in sunshine in the 
horizontal position, 1 scale-division (= 4 mm.) is equivalent to a radiation of 
00070 calorie .per sq. cm. per minute. With the radiometer in bright 
sunshine and inclined at an angle of 45° to the horizon, 1 scale-division 
= 0:0074 calorie per sq. cm. per minute, the difference in this case being due 
to the influence of air-convection on the coils. 

Part I].—EXPERIMENTS ON ASSIMILATION AND RESPIRATION. 
CONTENTS. 
PAGE 
Section (1).—The Rate of Assimilation of Leaves as deduced from the Direct Estimation 
of the Carbon Dioxide absorbed in the Photosynthetic Process .................. 40 
Section (2).—A Comparison of the Direct Method of Determining the Rate of Assimilation 
in Ordinary Air with the Weighing Method of Sachs............... cece cece eee 49 
Section (3).—The Relation of the Distribution of Stomata to the Rate of Gaseous Exchange 
DME GMO AGCRT Bho lao sc cic epteictl selsies viet ween es ei oiaialeile sete alt sejties stoi alse alsielre at eisre cee eRe een R ETE 61 
Section (4).—(a@) Experiments on Leaves exposed to Artificial Illumination in a Current of 
Orga Ary AGE co cegohs oP ionic A) one's sh ves choke miepe sien ste naueels oop aa dees nie aen ey ae ee ECan 66 
(b) eixperiments on the: Respiration of TLeaves..u-c...c2-dcss-eeece ses eee eae 68 
Section (1).—The Rate of Assimilation of Leaves as deduced from the Direct 
Estimation of the Carbon Dioxide absorbed in the Photosynthetic Process. 
In a previous paper dealing with the influence of varying amounts of carbon 
dioxide on the photosynthetic process of leaves,* it was shown that, providing 
there 1s an excess of radiant energy of the right quality incident on the leaf, 
the rate at which photosynthesis takes place is approavmately proportional to the 
mean partial pressure of the carbon dioxide in the air supplied to the leaf 
during the experiment. 
Another way of expressing this fact is as follows; If we denote the partial 
pressure of the carbon dioxide of ordinary air by p, that of artificially-enriched 
* ‘ Roy. Soc. Proc.,’ vol. 70 (1902), p. 397. 
