402 



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



Leaf B. 



Area of leaf 433 "6 sq. cm. 



Air passed in 4 hours (N.T.P.) 250 '1 litres. 



1 hour 62-51 „ 



C0 2 content of air entering case 16 *1 per 10,000 



leaving „ 9 '2 



Mean C0 2 content of air in contact with leaf 



during experiment , , 12 '65 



C0 2 absorbed by leaf per hour 43 -12 c.c. 



„ per sq. metre per hour . . . . ; 994 "4 c.c. 



Eatio of partial pressures of C0 2 in A and B, 2'05 : 12*65 = 1 : 6-1. 

 Eatio of C0 2 absorbed per square metre per hour in A and B, 273*2 : 

 994-4 = 1 : 3-6. 



These experiments, and many others of a similar nature which might 

 be quoted, indicate beyond a doubt that, at any rate for a short period, 

 the photosynthetic functions of the leaf lamina are capable of being 

 intensified by increasing the amount of carbon dioxide in the surround- 

 ing air, and that under favourable conditions the response of the leaf 

 in this direction is approximately directly proportional to the amount 

 of C0 2 present. 



Experiments of this nature are necessarily limited to comparatively 

 short periods, and give us no information as to how far the plant, as a 

 whole, will respond to such changes in its atmospheric environment. 

 When first drawing attention to these facts in 1899* it was pointed 

 out by one of us that we were not justified, without direct experiment, 

 in concluding that the plant would be able to avail itself indefinitely of 

 the increased amount of plastic carbohydrate material formed in its 

 leaves under these artificial conditions, and that translocation, meta- 

 bolism, and growth may have become so intimately correlated that the 

 perfect working of the entire plant may only be possible in an atmo- 

 sphere containing the normal amount of three parts of CO2 per 10,000. 



In approaching this question experimentally we were led to make 

 certain preliminary experiments with a view to seeing how far slightly 

 increased amounts of carbon dioxide in the air would affect the actual 

 dry weight of plants grown in such an atmosphere. Should there be 

 any influence on the plant throughout its growth in any way com- 

 parable in magnitude with that of the increased photosynthesis in 

 leaves it could not possibly escape detection, even when using atmo- 

 spheres containing only two or three times the normal amount of C0 2 . 



Experiment V. — Two seedling plants of Vicia Faba were chosen which 

 had been grown in pots, and which were of the same age and as nearly 

 similar in appearance as possible. Above each of the pots was placed 

 a circular metal tray with a large central aperture, and an annular rim 

 which formed a water-seal for a large inverted glass beaker. The stem 



* Presidential Address, British Association, Section B, Dover. 



