212 



SCIENCE 



[N. S. Vol. XLI. No. 1049 



clearer understanding of the factors involved. 

 These are as follows: 



The Gaseous Food of Plants. — Knowledge 

 of these centers about the discovery of carbon 

 dioxide assimilation (photosynthesis) and 

 oxygen respiration, the main points of which 

 were cleared up by Ingen-House (1YY9-1Y96) 

 and Senebier (1782-1800). Saussure (1804) 

 first proved that plants combine water with 

 carbon dioxide in carbon assimilation. 



The Minerai Food of Plants. — Saussure 

 (1804) recognized clearly the necessity of the 

 ash constituents of plants and that these were 

 derived from the soil. The conception, how- 

 ever, was much older, dating back at least to 

 Palissy in 1563. These ideas, however, met 

 with little acceptance until after 1840, when 

 the writings of Liebig and the experiments 

 of Boussingault, Salm-Harstmar and others 

 cleared up all the important points before 

 1860. Liebig must be considered as the great 

 dynamic force that impressed the importance 

 of this knowledge on agriculture. While some 

 of Liebig's ideas were erroneous, his writings 

 profoundly afl^ected agriculture and his gen- 

 eral ideas of the importance of mineral fertil- 

 izers dominated scientific agriculture until the 

 beginning of the present century and still 

 exercise a potent influence. The fertilizer 

 experiments conducted by Lawes and Gilbert 

 at Eothamsted still remain the most extensive 

 of their kind, and their results have contrib- 

 uted much to support Liebig's theory. 



The Organic Food of Plants {Nitrogen'). — 

 Liebig believed that all ordinary plants ob- 

 tained their nitrogen directly from the am- 

 monia in the air, but Boussingault (1851-5) 

 proved that various plants would not thrive in 

 a soil containing all essential elements but 

 nitrogen, but grew normally if nitrates were 

 added. 



While the fact had been known long previ- 

 ously that ammonia became changed into 

 nitrates in soil, Schlosing and Muntz (187Y) 

 first proved that it was due to microorganisms, 

 which were finally isolated by Winogradsky in 

 1890. 



Hellriegel (1888) demonstrated that legumes 

 are able to utilize atmospheric nitrogen through 

 the agency of bacteria in the root nodules. It 



was previously known that these plants could 

 obtain more nitrogen than was present in the 

 soil. 



Plant Breeding. — ^Three other discoveries 

 have led to great improvement in our crop 

 plants themselves. These are: (1) The proof 

 of the sexuality of plants by Camerarius 

 1691^; (2) the hybridization of plants by 

 Kolreuter, ITeO-lYVO ; (3) the discovery of the 

 laws of hybridization, Mendel, 1865. 



Improvement in Mechanical Appliances. — 

 The development of improved machinery for 

 the tillage of the soil, the sowing of the seed, 

 and the harvesting of the crop has had a pro- 

 found influence both in increasing the amount 

 and decreasing the cost of production. The 

 invention and improvement of agricultural 

 machinery has been the work of a long list of 

 inventors. 



Control of Insects and Diseases. — The im- 

 portant methods for the direct control of in- 

 sects and plant diseases center about the dis- 

 covery of Bordeaux mixture by Millardet in 

 1885; of the use of Paris green for biting in- 

 sects beginning about 1868 ; the value of kero- 

 sene emulsion for sucking insects about 1877; 

 and the development of fumigation with hydro- 

 cyanic-acid gas, 1886-1888. 



Indirect methods of control have been greatly 

 advanced by the investigations of both ento- 

 mologists and plant pathologists. 



Of these six lines of advance three are due 

 almost wholly to chemists, one to mechanics, 

 one wholly to botanists, and one partly to 

 botanists and partly to entomologists. It may 

 be argued that the chemists' contributions are 

 really plant physiology, but this does not alter 

 the fact that the work was done by chemists 

 and that further research into the food of 

 plants, at least of crop plants, is still largely- 

 directed by chemists and not by plant physiol- 

 ogists. 



At the 1914 session of the Graduate School 

 of Agriculture held at the University of Mis- 

 souri an incidental discussion led to a general 

 expression of opinion regarding the training 

 of American agronomists. There was com- 

 plete agreement that the botanical side of their 

 training is wholly inadequate. Indeed with. 



