4 i2 THE FOOD OF PLANTS 



If potassium can -actually be replaced by rubidium in fungi, this is 

 the only case known in which two elements are completely interchange- 

 able. The conclusion which Nageli 1 based upon Loew's experiments, 

 that in the case of fungi magnesium, calcium, strontium, and barium can 

 mutually replace one another, has been proved by Molisch and Benecke 

 to be incorrect, and it is now easy to show that fungi require magnesium, 

 but not calcium. It is possible that in other organisms various substitutions 

 of one element for another may be possible, and we may speculate that 

 under totally different conditions in other worlds organisms may exist in 

 which the place of carbon is taken by silicon. 



Great caution must be used in drawing any general conclusions from 

 results obtained with a very limited number of plants, for in different parts 

 of the vegetable kingdom all kinds of specific peculiarities have been 

 developed. Our knowledge of the vital processes of metabolism is insuffi- 

 cient to warrant our making the statement that potassium must always 

 be essential but not calcium, or that magnesium can never be replaced 

 by any other element. In any case calcium does not appear to take an 

 essential part in the normal metabolism of fungi or even of certain chloro- 

 phyllous plants. It remains to be determined whether in such cases 

 magnesium or potassium takes on the functions performed by calcium in 

 the higher plants, or whether the latter have developed new functions for the 

 performance of which calcium is essential, or have become so specialized 

 that the continuance of life is impossible in its absence. The fact that 

 iron is necessary to all plants shows that it is not merely of importance 

 for the formation of chlorophyll. 



Full development is only possible when a certain minimal amount 

 of each essential element is provided, although a plant usually assimilates 

 much more than the minimal amount when the supply is abundant. The 

 fact that such an extra consumption is possible shows that the elements 

 in question are used for purposes which can to a large extent continue in 

 their absence. It is easy to understand, for example, that when a neutraliza- 

 tion of organic acid is necessary, it may be immaterial whether potassium or 

 sodium, or even calcium or magnesium, is utilized for this purpose, whether 

 the object is simply to prevent any injurious action which the free acid 

 might exercise, or to produce osmotically active salts. For these and 

 similar purposes one element may be replaced by another or even by an 

 organic compound, although the element in question may be essential 

 for certain definite functions. 



The dissimilar composition of the ashes of plants grown in different soils 



1 Nageli, Bot. Mitlh., 1881, Bd. Ill, p. 458. Benecke (I.e.) has also shown that Mg cannot 

 be replaced by beryllium or the other metals of the zinc series. 



