SUPPLEMENT 23 



activity, it may, as we have seen, partially replace potassium. The non- 

 essential nature of a number of other elements has not been proved with so 

 much certainty as that of sodium. In the first place, we may mention chlorine, 

 an element which we have spoken of as non-essential to the majority of plants 

 that have been investigated, with the exception of buckwheat. According to 

 BEYER (1869) the same is true of peas and oats. It would be specially interest- 

 ing to know how marine and seashore plants behave in this respect, plants 

 which always absorb large amounts of this element. 



Just as halophytes absorb large quantities of chlorine, so other plants 

 take up certain other elements abundantly, elements which are only sparingly 

 absorbed by other plants and which may be regarded as unessential. It may 

 be questioned in this relation whether here also specific differences may not 

 exist whether, in fact, these elements, e.g. Si, Al, Mn, are not essential to 

 certain plants. Si occurs as SiO 2 in Diatoms, in grasses, and in Equiseta in 

 large quantities (comp. Analyses 10 and n, p. 80). In grasses it is localized 

 in the cell-walls of full-grown organs, but is completely wanting in embryonic 

 parts and in the seed. SALM HORSTMAR held that SiO 2 was an essential con- 

 stituent of the plant, but SACHS (1862) showed that maize could be grown, 

 without detriment, in a ' SiO. 2 -free ' water-culture. The evidence is, how- 

 ever, not quite conclusive, for the ash of maize plants grown in the ' SiO 2 -free ' 

 solution still contained 0-7 per cent, of SiO 2 (instead of 18-23 per cent.), which 

 they very likely had absorbed from the glass of the vessel in which the culture 

 was made. Similarly, JODIN (1883) cultivated four generations of maize one 

 after the other in ' SiO 2 -free ' solutions, but he was not successful in com- 

 pletely excluding Si, for in the second generation there was even more SiO 2 

 present than SO 3 . 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 silicic acid in 

 grasses are certainly unnecessary, but that it has not been proved that they 

 can get on equally well when silica is entirely absent. The question deserves 

 a critical investigation, all the more since O. RICHTER (1906) has proved that 

 Diatoms cannot really thrive without SiO 2 . 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. 



Just as SiO 2 is a plentiful constituent of the plants above mentioned, so 

 aluminium occurs abundantly in other forms. In Lycopodium complanatum, 

 L. clavatum, andL. chamaecyparissus, 23-39 P er cent- of the ash consists of Al, 

 and in species of Symplocos and in Orites excelsa it is present in quantity, 

 while in the majority of plants, as also in many species of Lycopodium, only 

 minute traces are demonstrable (comp. ROTHERT, 1905). It would not be 

 astonishing to find as a result of further research that Al was essential to 

 many species of Lycopodium and to Symplocos ; nor would it be surprising if 

 iodine, which occurs in such quantity in Marine Algae, were found to be essen- 

 tial to these plants, or that lithium was not superfluous in such types as always 

 accumulate it (TSCHERMAK, 1899). The same is true of manganese, found 

 especially abundantly in aquatic and swamp plants (GossL, 1905). 



Having now discussed the question as to essential and unessential minerals, 

 we must next study those which are injurious to plants. It need scarcely be 

 said that every salt, even the most essential one, may be sometimes injurious 

 in high concentration, owing to its osmotic activity ; but an injurious effect 

 may also be produced by it owing to its chemical nature, and this is what we 

 mean by a poisonous effect. The salts of the heavy metals, even in quite 

 minimal doses, operate injuriously or may bring about a fatal result. Even the 

 essential potassium salts are poisonous. According to STIEHR (1903) the root- 

 hairs of Phloeum are killed by a 0-5 per cent, solution of KC1. The salts of 



