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Dr. H. T. Brown and Mr. F. Escombe. [Apr. 28, 



diaphragm in a given time is also directly proportional to the density 

 of that gas in the moving stream of air which flows over the outer 

 surface of the diaphragm. 



But this latter case exactly defines the physical conditions under 

 which atmospheric carbon dioxide enters the tissue of a living leaf, 

 the multiperforate diaphragm being represented by the cuticle and 

 epidermis, pierced with numerous stomata, and the inner absorbing 

 chamber by the intercellular spaces of the parenchyma, bounded by 

 the chlorophyll-containing cells in which the process of photosynthesis 

 goes on (Joe. cit.). 



From these considerations one would be led to expect that under 

 conditions favourable for photosynthesis, such as are fulfilled by the 

 incidence of a sufficient amount of the right kind of radiant energy, a 

 living leaf would be able to absorb amounts of carbon dioxide from 

 the surrounding air which, within certain limits of concentration, are 

 directly proportional to the partial pressures of that gas. 



One essential condition would of course be that the actively assimi- 

 lating organs of the leaf in Avhich the photosynthesis takes place 

 should act as perfect absorbers, that is to say, they must be able 

 to deal with the carbon dioxide as fast as it is brought within their 

 sphere of influence by the physical processes of diffusion. "When this 

 condition is no longer fulfilled, either through lack of a sufficient 

 amount of radiant energy of the right wave-length reaching the active 

 centres of photosynthesis, or through the natural limit of the meta- 

 bolic activity of those living units having been reached in other direc- 

 tions, we should then expect the intake of the gas into the leaf to be 

 no longer proportional to its partial pressure in the outer air. 



We have been able to verify these deductions exjDerimentally, and 

 to show that a living leaf is really able, within certain limits, to 

 respond to increased amounts of carbon dioxide in the air surrounding 

 it, in such a manner as to indicate an approximate proportionality 

 between the photosynthetic work it can accomplish, and the partial 

 pressure the gas exercises in the air bathing the leaf surface. 



In these experiments the leaves, which were in some instances still 

 attached to the plant, had their laminae enclosed in air-tight glazed 

 cases, through which was aspirated a sufficiently rapid stream of air the 

 volume of which was determined by means of carefully standardised 

 meters. The carbon-dioxide content of the air was determined both 

 before and after its passage over the leaf, and the area of the leaf was 

 also accurately known. A full description of the apparatus employed 

 will be given in a subsequent paper ; it was so arranged that large 

 volumes of air could be employed, containing known amounts of 

 carbon dioxide either larger or smaller than the normal amount in the 

 atmosphere. 



Experiment I. — In this case comparative experiments were made on 



