INTRODUCTION 3 



quantitatively, that the increase in the amount of 

 carbon, hydrogen, and oxygen, when plants were ex- 

 posed to sunlight, was at the expense of the carbon 

 dioxide of the air, and of the water of the soil. He 

 also maintained that the mineral elements derived 

 from the soil were essential for plant growth, and gave 

 the results of the analyses of many plant ashes. He 

 believed that the nitrogen of the soil was the main 

 source of the nitrogen found in plants. These views 

 have since been verified by many investigators, and 

 are substantially those held at the present time re- 

 garding the fundamental principles of plant growth. 

 They were not, however, accepted as conclusive at 

 the time, and it was not until nearly a half century 

 later, when Boussingault, Liebig, and others repeated 

 the investigations of DeSaussure, that they were 

 finally accepted by chemists and botanists. 



From the time of DeSaussure to 1835, scientific 

 experiments relating to plant growth were not actively 

 prosecuted, but the scientific facts which had accumu- 

 lated were studied and attempts were made to apply 

 the results to actual practice. Among the first to see 

 the relation between chemistry and agriculture was 

 Sir Humphry Davy. In 181 3 he published his 

 " Essentials of Agricultural Chemistry", which treated 

 of the composition of air, soil, manures, plants, and 

 of the influence of light and heat upon plant growth. 

 About this same period, Thaer published an important 



