THE FUNCTIONS OF THE ESSENTIAL ELEMENTS 423 



as to the part it plays in metabolism, nor does the analysis of the ash 

 afford any evidence as to the form in which the different elements exist in 

 the organism. The distinction between volatile and non-volatile products 

 of combustion is of course a purely artificial one', though this, together with 

 the widely different manner in which a green plant obtains its mineral con- 

 stituents and carbon-compounds, has caused the two classes of substances 

 to be regarded and dealt with from widely different points of view. As 

 a matter of fact it is when they enter into combination with carbon-com- 

 pounds that the ash constituents and the nitrogen become of physiological 

 importance. Each mineral element may serve a variety of purposes, and the 

 organic substances with which it combines may undergo such decomposition 

 that these elements are released again in the form of inorganic com- 

 pounds, while if the latter are continually reassimilated, the most active 

 decomposition and reconstruction may not cause either an increase or a 

 diminution in the percentage amount of a given element. The nitrogenous 

 enzymes afford examples of substances which may induce marked changes 

 without themselves being destroyed, and it is not impossible that certain 

 of the ash constituents may have a similar functional importance. The 

 frequent accumulation of potassium nitrate shows that the whole amount 

 absorbed of an essential element does not necessarily enter into metabolism, 

 or undergo metabolic metamorphosis ; nevertheless such accumulation is 

 possible only when the absorbed substance undergoes some change or other, 

 the character of which is determined by the selective power which the plant 

 possesses. Hence both the super-optimal absorption of an essential element, 

 and the selective absorption of a non-essential one, are physiological pro- 

 cesses due to the specific nature and vital activities of the plant under 

 examination (Sect. 22). 



It is probable that the really essential elements take part in the com- 

 position of the protoplast. This certainly applies to sulphur, which is 

 a constitutional element of most proteids, while several of the latter are 

 rich in phosphorus (nuclein, &c.), and the group of phosphorized substances 

 known as lecithins may also be of great importance (Sect. n). Although 

 sulphates and phosphates are found in abundance in the ash, the sulphur 

 and phosphorus are probably present in the protoplasm in very different 

 forms, for no sulphates or phosphates can be detected in resting seeds, and 

 hardly any traces are present in the primary meristem, and even in the 

 other tissues, when the plant is supplied with a minimal amount of sulphur 

 and phosphorus \ When a seed germinates, readily translocating phosphates 

 and sulphates are formed, while the proteids probably undergo further 

 disintegration into amides or even ammonia. 



Similarly iron occurs in the form of organic compounds, and it is 



1 Schimper. Flora. 1890, p. 223 ; Pfeffer, Jahrb. f. wiss. Bot., 1872, Bd. vm, p. 475. 



