400 



Dr. H. T. Brown and Mr. F. Escornbe. 



[Apr. 28, 



thin white canvas screen, the separate glazed cases containing the 

 leaves being placed side by side under exactly similar conditions. The 

 duration of the experiment was 4 hours. 



Through case A ordinary air was passed, and through case B air 

 enriched with C0 2 up to 13*1 parts per 10,000 : — 



Leaf A. 



Area of leaf. 336 '4 sq. cm. 



Air passed in 4 hours (N.T.P.) 378 '82 litres. 



1 hour 94-70 „ 



€0 2 content of air entering case 2 "80 parts per 10,000 



leaving „ 1-70 „ „ 



Mean C0 2 content of air in contact -with leaf 



during experiment 2 *25 



C0 2 absorbed by leaf per hour 10*39 c.c. 



„ per sq. metre per hour .... 309 c.c. 



Leaf B. 



Area of leaf 312 '7 sq. cm. 



Air passed in 4 hours (N.T.P.) 325 -59 litres. 



„ 1 hour „ 81*39 „ 



C!0 2 content of air entering case 13 '1 parts per 10,000 



» „ leaving „ 6*8 „ „ 



Mean C0 2 content of air in contact with leaf 



during experiment 9 *95 ,, 



C0 2 absorbed by leaf per hour 51 *25 c.c. 



per sq. metre per hour 1639 c.c. 



The ratio of partial pressures of C0 9 in A and B is 2 '25 : 9*95 = 

 1 : 4*4. The ratio of COo absorbed per sq. metre per hour in A 

 and B is 309 : 1639 = 1 : 5*3, so that there is again a fair correspond- 

 ence of the ratios. 



Many other similar experiments were made, of which the two 

 quoted are typical examples. In all cases where the illumination of 

 the leaf was good, although the amount of intake of C0 2 into the 

 leaf was approximately proportional to the increased partial pressure, 

 the photosynthetic work of the leaf was always somewhat in excess of 

 what might be expected from the increased amount of C0 2 . 



This is probably due to the fact that under the particular conditions 

 of the experiment the air which was more highly charged with the gas 

 also contained a little more moisture than the ordinary air. Under 

 these circumstances the stomata of the B series would tend to open a 

 little more than those of the A series, a fact which would in itself 

 account for the correspondence between partial pressures and intake 

 not being so perfect as it would otherwise have been, since, other 

 things being equal, the intake is proportional to the linear dimensions 

 of the openings. 



In the two following experiments, where the illumination of the 

 leaves was of low intensity, the differences are in the other direction, 



