THE NON-ESSENTIAL ASH CONSTITUENTS 435 



minimum of a particular essential element suffices when the others are 

 present in abundance. Just as calcium is necessary to most plants, but 

 not to all, so also may silicon or similar elements be essential to a few 

 plants only. In a condition of nature, where the competition with other 

 organisms is severe, the trifling assistance afforded by a non-essential 

 substance may be of decisive importance, although in cultivated and 

 protected plants no perceptible favourable influence may be exercised 

 by the substance in question (Sects. 76, 92). 



Silicon. Sachs first showed that plants could be developed in a nutrient 

 solution containing no silica, and he harvested a maize plant which contained only 

 0-7 per cent, of this substance in its ash, instead of the normal 18 to 23 per cent. 

 Knop and others have proved that silica is not essential to other grain crops, and 

 Jodin successfully cultivated maize through four generations without it, so that 

 the percentage originally present was reduced to the lowest possible minimum 1 . It 

 has not yet been conclusively proved that silica is not essential for diatoms and 

 for shave-grass (Equisetum hyemale) in which it is especially abundant, while 

 siliceous cystoliths can hardly develop normally when silica 2 is deficient. 



Silica is usually deposited in the cell-wall, more rarely in opal-like masses 

 within the cells, or in intercellular spaces. The tabaschir, sometimes found in 

 the hollow internodes of the bamboo, resembles an opal in being a watery crystalline 

 form of silica 3 . It is possible that organic compounds of silicon may be formed 

 transitorily or in special cases only, but researches in this direction have yielded 

 indecisive results *. It is still uncertain whether the silica enters into the metabo- 

 lism of the plant, or whether the absorbed silicates are simply decomposed and 

 the silica deposited. In any case if silica is directly absorbed in solution it must 

 be deposited as soon as the solvent is removed B . The deposition of silica must be 

 partly due to the specific properties and metabolic activity of the plant, for an. 

 abundant deposit of it is formed in submerged Diatoms, while in special cells of 

 higher plants grains of silica may be deposited. In terrestrial plants transpiration 

 not only aids in the absorption of silica but also in its deposition in the tissues, 

 for the latter takes place most markedly in the epidermal walls, although the 

 amount deposited is by no means directly proportionate to the transpiratory activity 

 either in the same or in different plants. The silica gradually accumulates until in . > 

 the ash of grasses it may form 50 to 80 per cent, of the whole, whereas in young \ ^ 

 organs the merest traces may be present. 



In those plants which accumulate silica its presence may be of some use, 



1 Sachs, Flora, 1862, p. 52; Knop, Versuchsst., 1862, Bd. in, p. 176; also Rautenberg u. 

 Kiihn, ibid., 1864, Bd. VI, p. 359; Birner u. Lucanus, ibid., 1866, Bd. vill, p. 141 ; Jodin, Ann. 

 d. chim. et d. phys., 1883, v. se"r., T. XXX, p. 485. 



a Kohl, Kalksalze u. Kieselsaure, 1889; Zimmermann, Beitiage z. Morph. u. Physiol., 1893, 

 p. 306. On cystoliths of calcium carbonate, cf. Sect. 74. 



3 For the distribution of silica, see Kohl, 1. c., p. 228. On siliceous bodies, see also Strasburger, 

 Bau u. Verrichtung d. Leitungsbahnen, 1891, p. 367. 



4 Ladenburg, Ber. d. Chem. Ges., 1872, Bd. v, p. 568, and Lange, ibid., 1878, Bd. XI, p. 823. 

 6 Lange detected dissolved silica in the acid sap of Equiseturn. 



F f 2 



