THE GROWING PLANT 37 



60. Elements in plant-food. The chemical elements 

 regarded as essential in the food of plants are carbon, 

 hydrogen, oxygen, nitrogen, potassium, calcium, mag- 

 nesium, phosphorus, iron, chlorin and sulfur. Some other 

 elements that do not appear essential are also used by 

 plants. All of these elements, so far as they serve as 

 food, are absorbed by the plant in the condition of chem- 

 ical compounds, as water, carbonic acid and various 

 nitrates, sulfates and the like. 



61. Function of the different elements. Carbon is the 

 chief constituent of vegetable substances, and forms 

 about half of their total dry weight. Plants obtain their 

 carbon almost wholly from the air, in the form of car- 

 bonic acid gas, which is a compound of carbon and oxygen. 

 The leaves absorb and decompose this gas, retaining the 

 carbon and giving off the oxygen (58). Hydrogen and 

 oxygen are obtained by the decomposition of water, 

 which is a compound of hydrogen and oxygen. These 

 enter into the construction of nearly all tissues. Nitro- 

 gen is one of the constituents of protoplasm (13). Most 

 plants depend upon soluble nitrates in the soil for their 

 nitrogen supply, but those of the natural order to which 

 the clover belongs, the leguminosae, or the legume family, 

 including the clover, alfalfa, peas, beans and vetch, are 

 able to appropriate nitrogen from the air (260). Phos- 

 phorus and sulfur assist in the formation of albuminous 

 substances ; potassium assists in assimilation (58) ; cal- 

 cium and magnesium, while uniformly present, seem to 

 be only incidentally useful. Iron is essential to the for- 

 mation of chlorophyll (57). Lime, which is a compound 

 of calcium, appears to be essential to the fruiting of some 

 plants, as the peanut, while detrimental to the fruiting 

 of others, as the cranberry and huckleberry. 



