168 UNIVERSITY OF MISSOURI BULLETIN 



that none of the atmospheric nitrogen is utilized by green 

 plants. The discovery of the nitrogen fixing bacteria of the 

 soil and their relation to other plants was not only a revela- 

 tion of the first magnitude for pure science, but was of far- 

 reaching and fundamental importance in agricultural theory 

 and practice. 



One of the more important present day problems in 

 plant nutrition concerns itself with the relation of plant roots 

 to the soil. It has long been known that the soil of a given 

 field becomes in time exhausted and unable to grow as large 

 a crop of a given plant as at first. For years no one doubted 

 that this was due to the removal of the nutrient materials 

 from the soil. Within two or three years, however, the 

 theory has been seriously advanced that crop after crop does 

 not appreciably impoverish the soil, but that its apparent in- 

 fertility is due to the excretions, by roots, of substances that 

 are poisonous to the species producing them. That such 

 poisonous substances are actually excreted by roots has been 

 experimentally demonstrated, but whether, or not, the inabil- 

 ity to grow an unlimited succession of crops of the same 

 plant on the field is due entirely or even largely to this cause 

 awaits further experimental evidence. 



The development of the cell-theory, in 1839-40, by the 

 botanist Schleiden and the zoologist Schwann worked a revo- 

 lution in biological sciences. This doctrine, no longer a the- 

 ory but a firmly established fact, I need only to refer to, for 

 it is now taught in every high school. Thus do the university 

 studies of yesterday become the entrance requirements of 

 today. 



The elaboration by Charles Darwin of the theory of nat- 

 ural selection and the establishment of the fact of organic 

 evolution were accomplished in part by the study of plants, 

 and should therefore be mentioned in the list of botanical 

 achievements. These triumphs of the scientific method 

 changed the entire face of modern thought, and have given 



