6 INTRODUCTORY [chap. 



The true theory of the nutrition of the plant begins 

 very soon after the discovery of the composition of the 

 air. "Thus, Priestley observed that plants possessed 

 the faculty of purifying air vitiated by combustion or 

 by the respiration of animals ; and he having discovered 

 oxygen, it was found that the bubbles which Bonnet 

 had shown to be emitted from the surface of leaves 

 immersed in water consisted chiefly of that gas. Ingen- 

 housz demonstrated that the action of light was essential 

 to the development of these phenomena, and Sennebier 

 proved that the oxygen evolved resulted from the 

 decomposition of the carbonic acid taken up." 



Following up these results, de Saussure demonstrated 

 with as much quantitative accuracy as was then possible 

 that the oxygen which was split off by the leaf was 

 contained in the carbonic acid, and that the gain in 

 weight of the plant was practically represented by 

 its carbon ; combined with the elements of water to 

 make up such carbo-hydrates as sugar and starch. 

 De Saussure further arrived at very clear ideas as to 

 the source and value of the ash constituents of 

 plants : the nitrogen, which he also pointed out 

 as an invariable constituent of plants, he considered 

 to be either derived from the ammonia in the 

 atmosphere or the organic matters in the soil. Sir 

 Humphrey Davy, in his lectures before the Board 

 of Agriculture from 1802 to 1813, practically adopted 

 de Saussure's views, and emphasised the importance 

 of the ash constituents, which could come neither 

 from the air nor water, as he yet thought it necessary 

 to demonstrate. 



Though Davy made no advances towards ascertain- 

 ing the relative importance of these substances, and 

 was by no means certain that the plant derived all 

 its carbon from the atmosphere, his lectures did 



