28 GROWTH OF THE PLANT 



matter are distributed in plants, and of what the dry matter 

 is ultimately composed. The percentages are general 

 averages of green plants. 



15. Form in which the Elements are Absorbed. — Carbon 

 enters the plant in the form of carbon dioxide through the 

 leaves. Oxygen, as an element, enters the plant for the most 

 part through the leaves, but also through the roots and 

 stems. This is the only one of the essential elements which 

 is taken up by the plant in the elemental form. As such, 

 however, it is not used in making compounds, but in 

 destroying compounds by the production of energy through 

 the oxidation of plant substance. The same thing has 

 been noted in the case of the germinating seed. For manu- 

 facturing plant substance oxygen is absorbed in combina- 

 tion with carbon as carbon dioxide, or in combination with 

 hydrogen as water, the latter being taken up through the 

 roots from the soil. Water also supplies the hydrogen 

 necessary for plant life. 



All the other elements enter the plant from the soil through 

 the roots. Phosphorus, nitrogen, and sulphur are absorbed 

 as phosphates, nitrates, and sulphates respectively of one of 

 the basic elements, usually calcium. Potassium commonly 

 enters the plant as carbonate or bicarbonate, but also as a 

 salt of one of the acid elements — that is, as a phosphate, 

 nitrate, or sulphate. Calcium and magnesium are absorbed 

 as bicarbonate, phosphate, nitrate, and sulphate; and iron 

 as hydrated ferric oxide. 



It is to be noted that these compounds are all in a highly 

 oxidized form. Lower oxidized compounds such as phos- 

 phites, nitrites, and sulphites are not plant foods. They 

 are rather plant poisons. The same thing is true of other 

 lower oxidized compounds. Ammonia may serve as a plant 

 food in some cases, but in this form it is a base and not an 

 acid. It is generally true, however, that nitrates are the 

 best form of nitrogen for crop plants, and usually are the 

 only form which can be utilized. Gaseous nitrogen of the 

 air for legumes is a special case, and will be discussed later 

 (Section 130). 



All of the plant foods must be inorganic in nature and 



