206 STRUCTURAL BOTANY 



formation of new organic substance is checked Their 

 exact mode of action, however, is unknown. 



We learn, then, by this method of water culture 

 that the essential elements for the nutrition of a green 

 plant are only ten in number, namely, carbon, 

 hydrogen, oxygen, nitrogen, sulphur, phosphorus, 

 potassium, calcium, magnesium, and iron. 



Something must be said about the source of 

 nitrogen. Nitrates, such as potassium nitrate, are, 

 as we have seen, quite successful as a source of 

 nitrogen to the plant. If we use ammonium com- 

 pounds, such as ammonium phosphate ((NH 4 ) 3 P0 4 ), 

 instead of nitrates, the plant will also flourish, but it 

 is possible that in this case the ammonium compound 

 is converted into a nitrate before being made use of. 

 If, however, we leave out nitrogen compounds from 

 the food-solution altogether, most plants will starve. 

 Yet we know that the atmosphere consists of f of 

 nitrogen to ^ of oxygen, approximately. In most 

 cases, then, the free nitrogen of the air cannot be 

 assimilated by plants. It appears, however, that 

 there are important exceptions to this rule, and that 

 plants of the Pea and Bean kind are able, under 

 certain conditions, to supply their demand for nitrogen 

 entirely from the air, while no nitrogenous compounds 

 are offered to their roots. The conditions under 

 which this takes place, however, can only be under- 

 stood after we have made ourselves acquainted with 

 some of the simplest vegetable organisms, for only by the 

 help of certain fungus-like companions can the Bean 

 plant and its allies make use of atmospheric nitrogen. 1 

 1 See Part II. (Flowerless Plants), p. 275. 



