1902.] 



Influence of Carbon Dioxide on Leaves, etc. 



399 



two successive days in August, 1898, with, two similar leaves, A and 

 B of Helianthus annuus, whilst still attached to the plant. These were 

 exposed to the strong diffused light of a clear northern sky under as 

 nearly as possible identical conditions, with the exception of the com- 

 position of the air drawn through the cases. 



Over leaf A was drawn normal air containing 2*8 parts per 10,000 

 of CO2, whilst the air passing over leaf B contained 25*53 parts CO2 

 per 10,000. 



Leaf A. 



Area of leaf 743 *1 sq. cm. 



Volume of air passed per hour, reduced to normal 



temperature and pressure 159 '03 litres. 



COo content of air entering case 2 '80 parts per 10,000 



,, ,, leaving case 1 *64 „ ,, 



Mean C0. 2 content of air in contact with leaf 



during experiment 2 "22 ,, „ 



COo absorbed by leaf per hour. . 18 '44 c.c. 



,, per sq. metre per hour 248*2 c.c. 



Leaf B. 



Area of leaf 863 *75 sq. cm. 



Yolume of air passed per hour reduced to normal 



temperature and pressure 72 *7 litres. 



C0 2 content of air entering case 25 '30 parts per 10,000 



„ „ leaving „ 4*12 „ 



Mean C0 2 content of air in contact with leaf 



during experiment 14 '82 „ 



C0 2 absorbed by leaf per hour 155 *7.c.c. 



per sq. metre per hour 1802 *8 c.c. 



It is manifest that if we wish to determine the relation of the partial 

 pressures of carbon dioxide to the rate of intake of that gas into the 

 leaf, we must employ the values representing the mean carbon-dioxide 

 content of the air in contact with the leaf during the experiment, 

 which may be taken as the arithmetical mean of the composition of 

 the entering and emergent air. In the above experiment we obtain 

 the following relations : — ■ 



Batio of partial pressures of COo in A and B, 2*22 : 14*82 or 1 : 6*6, 

 Batio of C0 2 absorbed per sq. metre of Leaf A and B in 1 hour, 

 248*2 : 1802*8 = 1 : 7 2. 



Thus by increasing the amount of C0 2 in the air passing over the 

 leaf about sevenfold, we have, under similar conditions of illumination, 

 increased the photosynthetic power of the leaf by a little more than 

 the same amount. 



Experiment II. — In this instance two similar leaves, A and B, of 

 Helianthus annuus (cut from the plant, and with their petioles immersed 

 in water) were exposed on August 25, 1899, to sunlight under a 



