2 CHAP. I. INTRODUCTION 



ment (i) of the absorption or intake of carbon dioxide ; 

 (2) of the evolution of oxygen ; and (3) of the increase of 

 dry weight due to the formation of organic substance. The 

 sources of error in the estimation of C0 2 -assimilation from 

 either the evolution of oxygen or the increase in dry weight 

 have hitherto been so great that the method generally 

 adopted in photosynthetic measurement has been that of 

 the absorption of carbon dioxide. This is by no means a 

 simple method but requires very complicated chemical 

 analysis. In spite of many difficulties, the ingenuity and 

 persistence of numerous investigators have rendered it as 

 accurate as its various sources of error would permit A 

 leaf or a number of leaves are, according to this method, 

 enclosed within a receiver through which a definite volume 

 of air containing carbon dioxide is passed. Analysis of the 

 inflowing and outflowing air, and its volume, give the 

 quantity of C0 2 decomposed by the leaves when in light. 



The most accurate means for the determination was 

 devised by Kreussler (1885-90) ; Bonnier and Mangin 

 (1886-91) introduced numerous improvements in the 

 analysis of the gas. F. F. Blackman (1895) analysed the 

 intake or evolution of carbon dioxide by the leaf, the gas 

 being passed through a standard baryta solution subse- 

 quently titrated against standard solution of hydrochloric 

 acid. The various analyses and necessary corrections for 

 estimation of the C0 2 absorbed are so numerous, and the 

 labour entailed so great, that the duration of even a single 

 observation is excessively prolonged. This introduces 

 difficulties in the maintenance of constancy of external 

 conditions, such as light and temperature. Variation in 

 the intensity of light during prolonged experiments causes 

 change in the aperture of the stomata through which the 

 gaseous interchange of the leaves takes place. The tem- 

 perature also undergoes indefinite variation, for the green 

 leaves enclosed in the receiver are surrounded by non- 

 conducting air which is transparent to the thermal rays. 

 The temperature of the leaves is raised by the absorption 



