56 ELEMENTARY BIOLOGY 



also contain sulfur, and some phosphorus. From careful studies 

 of plants it is supposed that proteins are manufactured by certain 

 cells when these are supplied with carbohydrates and salts con- 

 taining the necessary elements. For example, nitrates contain 

 nitrogen, which the plant can use, phosphates contain phos- 

 phorus, sulfates contain sulfur, and so on. A green plant is 

 therefore capable of manufacturing its own food if it receives, 

 in addition to the water and carbon dioxid, a suitable supply 

 of minerals from the soil. Many plants without chlorophyl, as 

 certain kinds of molds, have also been shown to be capable of 

 manufacturing proteins when supplied with carbohydrates and 

 suitable minerals. 



