NUTRITION 93 



ire present in soluble form in the soil. Though some 

 these may be useful, they are not necessarily es- 



-al analysis reveals whatever is present in the plant- 



t neither indicates the compound in which an ele- 



"• in the living body, nor enables one to distin- 



m necessary substances and those present in the 



because they are present in the media in which 



soil, air, and water. Only by the culture of 



media of known composition can the essential 



jud compounds be distinguished from the non- 



the useful but not essential from the absolutely 



md the absolutely necessary. Analysis shows that 



ily 1.5-5^ of the dry weight of plants is furnished by 



^h constituents of all sorts, though in some cases 



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



dpancy. Culture in media of known composition shows 



chat 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 (Eqvisetuin) 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, transl. 

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



