INFLUENCE OF CERTAIN CARBOHYDRATES ON GREEN 



PLANTS! 



Lewis Knudson 



The classical investigations of Priestley, Ingenhouz, de Saussure, and 

 others, near the beginning of the nineteenth century, established the 

 fundamental facts relative to the utilization of atmospheric carbon dioxide 

 by green plants. Yet, despite the discoveries made by these men and 

 by others who followed, the belief prevailed in the botanical, chemical, 

 and agronomic sciences that the source of the plant's carbon was the 

 brown or blackish organic material of the soil, which was designated 

 humus. Liebig (1847)," by his arguments based on theoretical rather 

 than on experimental grounds, contributed largely to the relegation of 

 this view. He asserted that the plant derived its carbon entirely from 

 the carbon dioxide of the air. Boussingault (1860) demonstrated by 

 experimental methods the ability of the plant to develop when organic 

 matter was entirely wanting in the substratum. His experiments demon- 

 strated not only the ability of the plant to procure its carbon from the 

 air, but also the fact that humus is not necessarily a source of nitrogen, 

 the contrary view having been formerly very generally believed. The 

 work of Sachs, Knop, and many others subsequently, with water cultures, 

 proved conclusively the ability of the plant to develop independently 

 of the presence of humus. 



Nevertheless the humus theory was revived from time to time, notably 

 by Breal (1894). An explanation was sought for the generally recog- 

 nized superiority of stable manure as a fertilizer, but no adequate 

 experimental evidence was produced in favor of the humus theory. 

 The interest now, however, is not confined merely to the narrower 

 aspect of the plant's relation to humus, a substance of indefinite character, 

 but is concerned rather with the relation of the plant to dissolved organic 

 substances that may occur in soils. One of the phases of this subject — 

 namely, the injurious effect of soil organic substances — has received 



^Contribution from the Laboratory of Plant Physiology, Cornell University. 

 2Dates in parenthesis refer to bibliography, pages 71 to 75. 



