86 METABOLISM 



sidered, viz. nitrogen. When nitrogen in the form of nitric acid is absent 

 no noticeable increase in dry weight takes place even though all the other 

 salts be present, hence it should be mentioned here that nitric acid, in a form 

 capable of being absorbed from the soil in water, is essential, although, 

 not being found in the ash of plants, it need not be discussed here. It may 

 be noted that one of the characteristics of the substances which we have hitherto 

 been discussing is their capacity for resisting heat, but this characteristic is of 

 no consequence so far as the plant is concerned ; nor is it of any importance 

 whether, in ordinary combustion, the nitrogen is given off as a free gas or as 

 ammonia, since in the plant the nitrogen is firmly combined and only very 

 rarely escapes in the gaseous form. We will content ourselves with noting 

 that nitrogen, in the same sense as sulphur, potassium, phosphorus, calcium, 

 magnesium, and iron, is an essential food-stuff in every plant ; any further dis- 

 cussion of its characters would be premature (compare Lecture XI). 



To sum up, we may say that we have clearly established a function for 

 nitrogen, sulphur, and phosphorus, viz. that they undoubtedly take part in 

 the formation of the living substance, and we have, further, good ground for the 

 belief that to these elements must be added potassium, magnesium, and iron ; 

 on the other hand, it may be said with certainty that this is, in general, not 

 true for calcium. Scattered through the very voluminous literature on the 

 subject, reaching from the time of Liebig to our own day, we find many state- 

 ments, suggestions, and hypotheses as to the function of the inorganic salts. 

 Thus, according to Liebig, the bases act as neutralizers of the acids — a fact 

 which cannot be doubted — but it is not so easy to say why special metals 

 should be required for this purpose. Again, it is stated that potassium is 

 required for the formation of osmotically active bodies, that other elements 

 render possible, or play a part in, the circulation of proteids, or in the construc- 

 tion of the cell membrane, of the nucleus, or the other organs of the cell. We 

 must rest content with this brief summary of the literature and leave over any 

 detailed criticism for the present, seeing that the various views above referred 

 to have not been sufficiently established. 



In addition to the essential constituents of the ash the plant also absorbs 

 non-essentials from the soil, in greater or less quantity. Generalizations on 

 this question are, however, scarcely valid, and although, also, very few plants 

 are to be found which can subsist on the six elements mentioned above, there 

 are others again which make specific claims on the soil. Buckwheat, which, 

 according to Nobbe (1862), cannot fruit properly if chlorine be absent, 

 may be cited as a striking example of the existence of such specific differ- 

 ences. It is necessary, first of all, to examine substances which occur only in 

 certain plants. Thus it would not be surprising if it were proved that iodine 

 was an important nutrient for marine Algae, or if it turned out that aluminium, 

 which forms 22 to 39 per cent, of the ash of Lycopodium chamaecyparissus, 

 L. complanatum, and L. clavatum, and yet appears in the minutest traces in 

 most plants, including several other species of Lycopodium, has a special 

 function to perform in these plants. [Large quantities of aluminium occur in 

 species of Syniplocos and Orites (Czapek II). Jamano (Bot. Centrbl. 99, 2) 

 found that aluminium was of service in the development of barley.] 



Similarly lithium, which is, generally speaking, not only redundant but 

 even poisonous, may be useful to those plants in which Tschermak (1899) 

 demonstrated its constant occurrence in plants taken from various situations. 



We do know, however, that certain substances which appear to be absorbed 

 in large quantity are yet actually superfluous, although they must not be looked 

 upon as entirely functionless. Sodium, for instance, appears in almost all analyses 

 quoted in the table on p. 80 in larger quantities than the indispensable iron. It 

 maybe assumed that this element has some duty to fulfil, though we cannot prove 



