NUTRITION 93 



they are present in soluble form in the soil. Though some 

 few of these may be useful, they are not necessarily es- 

 sential. 



Chemical analysis reveals whatever is present in the plant- 

 body, but neither indicates the compound in which an ele- 

 ment exists in the living body, nor enables one to distin- 

 guish between necessary substances and those present in the 

 plant simply because they are present in the media in which 

 it lives in soil, air, and water. Only by the culture of 

 plants in media of known composition can the essential 

 elements and compounds be distinguished from the non- 

 essential, the useful but not essential from the absolutely 

 useless and the absolutely necessary. Analysis shows that 

 ordinarily 1.5-5% of the dry weight of plants is furnished by 

 the ash constituents of all sorts, though in some cases 

 10-30% is ash. Analysis alone cannot account for this dis- 

 crepancy. Culture in media of known composition shows 

 that the greater amount of ash is due to peculiarities of 

 the soil or to peculiarities of certain species or even families 

 of plants. For example, 18-23% of the ash of Indian Corn 

 is silicic dioxide, useful to the plant in hardening its outer 

 surfaces and making projecting parts and the edges of 

 leaves harsh and cutting, but not essential to its stiffness, 

 growth, and perfect maturity. * Diatoms and the scouring- 

 rushes (Equisetum) are much richer in silica than the 

 grasses, but it is not yet proved that it is indispensable 

 even for them. 



Analysis reveals the presence of sodium and chlorine, as 

 common salt, in all plants. Because culture without these 

 elements is so difficult that it is doubtful whether it has 

 ever been accomplished, no one can say whether they are 

 absolutely essential or not. Experiment has already con- 

 clusively shown, however, that they are needed only in the 

 smallest possible quantities if at all, though in certain cases 

 larger quantities may act as favorable stimulants. This 

 last is especially evident in the bacteria, the growth of 

 which in the artificial culture media and under the unnatural 



* Quoted by Pfeffer (Pflanzenphysiologie, Bd. I., p. 429, Engl. tranel. 

 p. 435) from Sachs (Flora, p. 52, 1862). 



