OUTLINES OF THE PHYSIOLOGY OF NUTRITION 201 



I. The Food of Plants. The first step necessary in 

 order to understand anything about the nutrition of 

 plants is to learn what chemical elements a plant 

 contains. Probably most of the elements which exist 

 have been detected at some time or other in plants ; 

 those, however, which are constantly present, are only 

 thirteen in number, namely, carbon, hydrogen, 

 oxygen, nitrogen, sulphur, phosphorus, potassium, 

 calcium, magnesium, iron, sodium, chlorine, and 

 silicon. But although these elements are very con- 

 stantly found, on chemically analysing green plants, it 

 does not follow that they are all necessary to the life 

 of the plant. Many of them we know to be so, 

 because they enter into the chemical composition of 

 the substances of which the tissues of the plant 

 consist. For example, the three elements, carbon, 

 hydrogen, and oxygen, constitute cellulose and starch ; 

 the proteids, of which the protoplasm is composed, 

 contain, in addition, nitrogen and sulphur; while 

 phosphorus is regularly present in the substance of 

 the nucleus. As regards the other seven elements, 

 analysis does not tell us whether they are necessary 

 to the life of the plant, or whether they may 

 not be taken up simply because they are present in 

 the soil. 



This question can only be answered by experimental 

 cultures, so arranged that we know exactly what 

 chemical bodies are supplied to the plant. 



The simplest method is by means of cultivation in 

 water. We must take care that the water used is pure 

 to begin with, and therefore distilled water must 



