CHAPTER II 



THE DEVELOPMENT OF KNOWLEDGE OF THE 

 FIXATION AND SUBSEQUENT CHANGES OF 

 CARBON DIOXIDE BY THE PLANT 



THE period 1860-1900 was exceptionally rich in investiga- 

 tions into the problems presented by the great question 

 of the nutrition of the vegetable kingdom. They attracted 

 large numbers of workers, among whom may be included 

 many of the keenest and most acute observers of the 

 century ; and though we cannot claim that the end of the 

 period found us in possession of all the secrets which are 

 connected with the nutritive processes, a very substantial 

 advance was made in that direction. 



At the beginning of the period the outlook was but poor. 

 A few isolated facts were known, but apart from the work 

 of Liebig in the forties, vegetable physiology had made but 

 little progress since the days of Senebier and De Saussure. 

 The early work of Priestley, Ingenhousz, and De Saussure, 

 had shown that plants take in carbon dioxide from the air 

 and give out oxygen ; Senebier had determined the con- 

 nexion between these two processes, and had made out 

 that it is in some way concerned in the construction of 

 organic substance. The observation of De Saussure made 

 a little later, that when a green plant is exposed to light 

 in air containing carbon dioxide it gains in weight, had 

 found a place in the literature of the subject. He had 

 also determined that the greater part of the increase of 

 weight of such plants is due to the carbon dioxide, together 

 with water absorbed from the soil. He had not, however, 

 connected the existence of the chlorophyll with these facts ; 



