18 PHOTOSYNTHESIS 



of photosynthesis can be attributed. The great complexity of the subject 

 itself is largely responsible, coupled perhaps with the unfortunate results 

 arising from the academic division of the sciences. Photosynthesis is a 

 phenomenon of plant life and thus naturally falls in the domain of botany. 

 But the botanists (with a few exceptions) have either not exhibited any 

 interest or have not been in possession of sufficient experimental skill and 

 knowledge of other sciences to make any great advances. Photosynthesis 

 as a chemical reaction has been dealt with by the chemist in a purely 

 theoretical manner, in that he has endeavored to apply the knowledge 

 gained from his chemical experiments to the reactions taking place in the 

 green leaf with utter disregard of many very important biological facts. 



Historically the next impetus which was given to the subject were the 

 investigations and writings of Liebig in Germany and of Boussingault in 

 France. The publication of Liebig's "Die Organische Chemie in Ihrer 

 Anwendung auf Agricultur und Physiologie" in 1840 exerted profound 

 influence on the application of the young science of organic chemistry to 

 various phases of life activities. Through Liebig's clear exposition the 

 origin of carbon compounds in plants was again correctly explained. The 

 humus theory, according to which the brown amorphous substance of the 

 soil afforded the chief source of nutrition, was refuted. It was shown 

 that the humus of the soil comes from the decomposition of carbon com- 

 pounds of plants and the chief source of carbon in plants is the carbon 

 dioxide of the atmosphere which is absorbed by the leaves and through 

 the action of sunlight is converted into compounds of high complexity. 

 These latter substances in turn serve the plant as material for its respira- 

 tory activity and for its growth. Similarly the plant is capable of 

 svnthesizing complex organic nitrogen compounds from the simple com- 

 pounds such as ammonia and nitric acid. The animal, unlike the plant, 

 is incapable of effecting any of these syntheses, of the inorganic to the 

 organic. The animal is therefore entirely dependent for its existence upon 

 the synthetic activities of plants. Thus did Liebig clearly outline the 

 function of plants on the earth and their relation to animal life, particularly 

 as expressed in the importance of agriculture to human existence and 

 civilization. He further elucidated the importance of mineral fertilizers, 

 rotation of crops and many other fundamental principles of agriculture 

 and showed clearly that the new science of chemistry was destined to be 

 of enomious importance in the physiology of both plants and animals. 



Liebig's views rested principally on the results of his predecessors, 

 Priestley, Ingen-Housz, Senebier, de Saussure, on general considerations 

 and observations and on calculations. He followed rather a deductive 

 method than an experimental one. 



Li France at the same time Boussingault was patiently following the 

 course of pure induction. Sometime before the appearance of Liebig's 

 book Boussingault had begun experiments on the nutrition of plants. His 

 first results were not decisive, but he persisted and between 1851 and 1855 

 perfected his method of water cultures. He was thus able definitely to 



