ASH. I 87 



it. It might serve, for instance, in place of another metal to neutralize acids 

 and in the form of a salt act as osmotically in the cell-sap. Again we may note 

 that silicon very rarely occurs in young organs or in seeds, although it is abun- 

 dantly present as silica in the shells of diatoms and in horsetails and grasses (see 

 p. 80, Nos. 10 and 11), and in the majority of cases is localized in the cell-walls of 

 old stems and leaves. Its accumulation does not necessarily point to a use in 

 metabolism, since its appearance maybe due merely to withdrawal of the medium 

 of solution. Salm-Horstmar held that silica was an essential constituent of the 

 plant, but Sachs (1862) showed that maize could be grown satisfactorily in a 

 silica-free water-culture. The evidence is, however, not quite conclusive, since the 

 ash of maize plants grown in the 'silica-free' solutions still contained 07 percent, 

 of sihcic acid (instead of 18-23 per cent.), which it had absorbed from the glass 

 of the vessel in which the culture was made. Similarly, Jodin (1883) culti- 

 vated four generations of maize in silica-free solutions, one after the other, 

 but he was not successful in completely excluding silicon, for in the 

 second generation there was more silica present than sulphuric acid. On the 

 other hand, some observers, e. g. Swiecicki (1900) have endeavoured to show 

 that silicic acid has a favourable influence on the plant. At present, all we 

 can say is that the large quantities of silica present in grasses are certainly 

 unnecessary, but that it has not been proved that they can get on equally 

 well when silica is entirely absent. As to whether silica is of use or not in the 

 Equisetaceae and Diatomaceae we are quite ignorant. Again, it is worthy of 

 note that although silica may be quite superfluous from the chemical point 

 of view it may be of great service to the plant in the biological sense. Our 

 knowledge of these subjects, despite the amount of work which has been ex- 

 pended on them, is still very imperfect, and it is possible to defend the assertion 

 that all the ash constituents have definite functions to perform, although these 

 have not as yet been determined in all cases, and although these constituents 

 cannot be considered as taking part in metabolic changes. 



Under the circumstances it is unnecessary to present a complete enumera- 

 tion of the 'non-essential' constituents of the ash. The occurrence of manganese 

 may, however, be specially noted, as leading to the consideration of a new series 

 of phenomena. It is not widely distributed in the earth, and yet is found, though 

 only in traces, in very many plants. Raulin (1869) has shown it to be instru- 

 mental in the development of Mould-fungi. Nevertheless it is certain that these 

 organisms can exist for generations without manganese, and that it must not be 

 looked upon as a nutrient. [See Gossl (Beihft. z. bot. Centrbl. 1905, 18, i, 119) 

 for a discussion of the distribution and functions of manganese.] Still more notice- 

 able are Raulin' s discoveries with regard to zinc, which have been recently 

 completely confirmed by Richards (1897). (Certain corrections of Richards's 

 results have been made by A. Richter, 1901.) Raulin showed that the addition 

 of 0-0005 per cent, of zinc sulphate to a nutritive solution materially aided the 

 growth of Fungi, and that a 0-003 per cent, solution of the same salt brought 

 about a doubling of the plant's weight. The greatest effect was observed 

 with this concentration, a further increase not only inhibited growth but 

 acted injiu-iously. There are quite a number of substances which behave 

 in a similar way, acting favourably in dilute solutions, but injuriously in 

 stronger. Cobalt sulphate gives an optimum effect with a concentration 

 of 0-002 per cent. ; nickel sulphate acts best in a 0-033 per cent, solution. 

 Ono (1900) found that an acceleration of growth took place after the addi- 

 tion of minute quantities of ithium nitrate, potassium arsenite, and sodium 

 fluoride to Algae, and of mercuric chloride and copper sulphate to Fungi. 

 But a poisonous effect does not always take place immediately, certainly not 

 in the case of silicon, which, according to Raulin and Richards, acts bene- 

 ficially. On the other hand, there are poisons which are injurious in small 



