2 SOILS AND FERTILIZERS 



in some mysterious way directly into plant tissue. Van 

 Helmont, in the seventeenth century, attempted to 

 prove this. " He took a large earthen vessel and filled 

 it with 200 pounds of dried earth. In it he planted a 

 willow weighing five pounds, which he duly watered 

 with rain and distilled water. After five years he 

 pulled up the willow and it now weighed one hundred 

 and sixty-nine pounds and three ounces." 1 He con- 

 cluded that 164 pounds of roots, bark, leaves, and 

 branches had been produced by the direct transmuta- 

 tion of the water. 



It is evident from the preceding example that any- 

 thing like an adequate idea of the growth and compo- 

 sition of plant bodies could not be gained until the 

 composition of air and water were established. 



The discovery of oxygen by Priestley in 1774, 

 of the composition of water by Cavendish in 

 1 781, and of the role which carbon dioxide plays in 

 plant and animal life by DeSaussure and others in 

 1800, form the nucleus of our present knowledge re- 

 garding the sources of matter stored up in plants. It 

 was from 1760 to 1800 that alchemy lost its grip and 

 the way was prepared for the development of modern 

 chemistry. 



The work of DeSaussure, entitled u Recherches 

 sur la Vegetation," published in 1804, was the first 

 systematic work showing the sources of the com- 

 pounds stored up in plant bodies. He demonstrated, 



