82 



Agricultural Chemistry. 



the function of assimilation performed by the roots becomes 

 evident. 



While some plants, as for example, tobacco and the potato, re- 

 quire liberal supplies of plant food in readily available form, 

 others, especially the cruciferae (turnip, rutabaga and related 

 plants) and some of the gramineae (cereal grains and grasses), 

 display marked ability to attack resistant compounds in the soil 

 and obtain food from them. This difference is well illustrated 

 by the following data obtained by Merrill at the Maine Experi- 

 ment Station in studying the availability of phosphorus, when 

 supplied in various forms to different crops. Other requirements 

 of the plants than that for phosphorus were amply supplied. 

 The figures express the percentage yield of dry matter, the yield 

 with no phosphorus being taken as 100 per cent: 



These data show a widely variant power on the part of plants 

 to assimilate comparatively insoluble and unavailable compounds 

 of phosphorus. The great superiority of corn over barley and 

 of the tomato over the potato in utilizing the insoluble phosphates 

 is interesting as a demonstration that assimilating power is not 

 uniform for members of a plant family, but is a characteristic 

 of the individual species. The cruciferae, however, as a family. 



