THE ESSENTIAL ELEMENTS 415 



doubtful whether certain organisms can only obtain their phosphorus, iron, 

 and sulphur from the organic compounds which may form part or the 

 whole of their food *. Similarly a specific difference exists between aerobes 

 and anaerobes as regards oxygen, for the latter must obtain this gas in 

 a combined form, whereas to the former life is only possible when a supply 

 of free oxygen is assured. 



Even when the best possible food is supplied normal vital activity is 

 possible only when all the other essential conditions are fulfilled. Since 

 particular organisms may have special requirements which are not at first 

 sight apparent, a particular substance may be necessary for their growth 

 under certain circumstances, though it is not absolutely an essential one, 

 and may be dispensed with under different cultural conditions. Thus the 

 presence of salt may be merely a physical condition for the development 

 of marine algae, and it may be found possible to grow them in isosmotic 

 solutions of other salts 2 . The maintenance of a suitable nutrient medium 

 is also one of the physical conditions for growth, and for those fungi and 

 bacteria which excrete free acid the presence of calcium carbonate or some 

 neutralizing base is essential for continued development. The production 

 of acid is dependent upon the cultural conditions, and these may be so 

 altered that the nutrient fluid becomes alkaline (Sect. 86), in which case 

 the addition of a neutralizing acid becomes necessary. These powers are 

 possessed only to a slight extent by higher plants, but in their case also it 

 depends partly upon the nature of the nutrient salts whether the desirable 

 slight acidity is produced, or replaced by an alkalinity, which usually 

 injuriously affects the roots, and hence may retard or inhibit the develop- 

 ment of the entire plant. The presence of sodium or potassium chloride 

 may cause the acid reaction to be maintained (Sect. 23), so that under such 

 circumstances chlorine appears to be an essential element. In other cases 

 also it rnay have a definite function to perform. There is therefore a 

 possibility that under special cultural conditions those plants may be grown 

 without calcium 3 to which at present it seems to be an essential element. 



It has already been mentioned (Sect. 64) that the seeds of Orobanche 

 and the spores of certain fungi will germinate only when stimulated by 

 the presence of special chemical substances, but it is very doubtful 

 whether any of the constituents of the ash act in a similar manner. As 

 a matter of fact a certain acceleration of growth may be caused by the 



1 Cf. Sect. 64. On the necessity of organic iron compounds for animals, cf. Neumeister, 

 Physiol. Chem, 1893, I, p. 311. 



a [Pennington (Cont. Bot. Lab. Univ. Pennsylvania, 1, 1897, p. 203) finds that Spirogyra contains 

 as much chlorine and sodium as marine algae do.] 



8 Dehe"rain and Breal (Bot. Jahresb., 1883, p. 40) state that seedlings do not require calcium at 

 a high temperature, but Molisch (Sitzungsb. d. Wien. Akad., 1895, Bd. CIV, Abth. i, p. 799) has shown 

 the inapplicability of these observations. The temperature also affects the formation of free oxalic 

 acid (cf. Sect. 86). 



