VI 



PREFACE 



the air can only take place when they are exposed to 

 sunshine, and especially emphasised the importance of the 

 process as one of nutrition : ' It seems that plants absorb 

 the phlogistic from the atmosphere which constitutes the 

 principal part of their nutriment ; the absorbed air is 

 elaborated by the organs of the plant, with the result that 

 the phlogistic is retained as nutriment, whilst the remainder 

 is restored to the atmosphere.' Senebier confirmed the 

 importance of exposure to light", and made it clear that 

 the process is mainly carried on in those parts of the plant 

 that are green in colour. This was the stage reached by 

 the end of the eighteenth century. 



Important progress was marked by the publication, in 

 1804, of Th. de Saussure's Recherches Chimiques. The 

 advances in chemistry, both theoretical and practical, 

 enabled him to make the experiments which proved, 



(1) that a green plant exposed to light absorbs carbon 

 dioxide and evolves a rather smaller volume of oxygen ; 



(2) that the plant at the same time assimilates the elements 

 of water ; and (3) that the gaseous interchange is accom- 

 panied by an increase in the dry weight of the plant. The 

 bearing of the gaseous interchange upon the nutrition of 

 the plant was thus clearly defined, and a beginning was 

 made in the direction of the quantitative estimation of 



* 



the process. 



The main factors were now recognised : light ; presence 

 of the green colouring matter, chlorophyll ; supply of a 

 limited quantity of carbon dioxide ; but much yet remained 

 to be explained. What, for instance, is the part played 

 by light in the process ? What is the function of the 

 chlorophyll ? What is the nature of the organic substance 

 formed ? Answers to these questions were slowly forth- 

 coming during the nineteenth century. Light, it was 

 ascertained, is the source of the energy required for the 

 chemical work of reducing and assimilating carbon dioxide 

 and water. The study of the spectrum of chlorophyll 

 showed it to be the means by which the light is absorbed 



