THE ESSENTIAL ELEMENTS 411 



by direct experiment whether a particular ash constituent is essential or not. 

 If the latter be the case the plant grows without any supply of the salt 

 in question, whereas no development is possible in the absence of any one 

 of the essential saline constituents. 



It has been determined by growing plants in various nutrient solutions 

 that potassium, magnesium, phosphorus, sulphur, and iron, are essential 

 elements in all cases. The fungi and bacteria whose properties were 

 investigated by Molisch, Benecke, and Loew can develop normally without 

 calcium, whereas higher plants cannot grow unless they are abundantly 

 supplied with this element. This is also the case with many algae (Spiro- 

 gyra, Vaucheria, &c.), while others (Microthamnion, Stichococcus, Palmella) 

 can do without calcium *. 



Different plants may have different requirements, and it is quite possible 

 that various peculiarities may exist among the lower organisms. Indeed 

 according to Loew 2 and Benecke in the case of fungi, potassium may be 

 replaced by rubidium and perhaps by caesium also, whereas according to 

 Molisch no such substitution is possible in algae 3 . All researches agree 

 in showing that potassium cannot be replaced by sodium or by lithium, 

 and it is very remarkable that sodium which is universally present has not 

 found any such use in metabolism as would make it an essential element. 

 As, however, both potassium and sodium are essential to the higher 

 animals 4 , it is possible that plants also may exist which cannot live without 

 a supply of the latter element, although this has not yet been found to be 

 the case even in plants frequenting saline habitats. Seaweeds, however, 

 have not up to the present time been cultivated in the absence of sodium 

 chloride. The same considerations apply to silicon also, though it is 

 possible that in its absence organisms such as diatoms may be unable to 

 exist, for the inability to perform the functions connected with the presence 

 of silicon may render growth and development impossible. Similarly 

 aluminium and zinc are unnecessary, though large quantities of both 

 elements are often accumulated, and of all the other elements chlorine is 

 the only one which may be even of doubtful use in certain cases. 



1 Molisch, Bot. Centralbl., 1894, Bd. LX, p. 167 ; Sitzungsb. d. Wien. Akad., 1894, Bd. cm, 

 Abth. i, p. 566; 1895, Bd. civ, Abth. i, p. 799; 1896, Bd. cv, p. 633; Benecke, Bot. Centralbl., 



1894, Bd. LX, p. 195; Jahrb. f. wiss. Bot., 1895, Bd. xxvm, p. 487; Bot. Zeitung, 1896, p. 97. 

 Cf. also Sect. 74; Loew, Flora, 1892, pp. 374, 390; Bot. Centralbl., 1898, Bd. LXXIV, p. 258, and 



1895, No. 52. 



* Loew, Landw. Versuchsst., 1879. See also Bot. Centralbl., 1898, Bd. LXXIV, No. 7. 



8 According to Winogradsky (Bot. Centralbl., 1884, Bd. xx, p. 167), K may be replaced by 

 Rb, but not by Cs, in Mycoderma vini. Nageli holds that K may always be replaced by Rb and Cs, 

 but the experiments which favour this conclusion are not decisive. According to Wehmer (Beitrage 

 zur Kenntniss einheimischer Pilze, 1895, II, p. 107), fungi can grow without K if Na is present, 

 though they do so more slowly, but these results have been proved to be incorrect by Molisch and 

 Benecke (1. c.). The chief literature is given in Sect. 74. 



4 Bunge, Physiol. Chem., 1894, 3. Aufl., p. 107. 



