THE FUNCTIONS OF THE ESSENTIAL ELEMENTS 429 



are not assimilated by higher plants, but do not exercise any poisonous effect. 

 Phosphorus cannot be replaced by arsenic or boron 1 . 



A plant can absorb only those phosphates which may be rendered soluble ; 

 calcium phosphate may supply the large amount of phosphorus absorbed from 

 a water-culture, as it is soluble in water containing carbon dioxide. If only phos- 

 phate of iron is present chlorosis is apt to occur unless the water is frequently 

 agitated (Sect. 73). As the percentage of proteid increases so also does that of 

 phosphorus, but the relation between nitrogen and phosphorus may vary within very 

 wide limits ' 2 . 



Sulphur forms a constituent of most proteids, and the quantity present is 

 generally from 0-3 to 0-4 per cent., i. e. less than of phosphorus. Sulphur is also 

 present in oil of mustard and in other similar compounds, while in seeds the 

 greater part of the sulphur is frequently present in the form of organic compounds 

 which give no precipitate with an acid solution of barium chloride, although during 

 translocation they may be decomposed and form reacting sulphates. Certain 

 bacteria produce sulphur and sulphuretted hydrogen (Sect. 102), from both of which 

 Beggiatoa is able to obtain a supply of energy by oxidation 3 . Higher plants seem 

 only able to assimilate sulphur in the form of sulphates, but fungi can make use 

 of sulphurous and hydrosulphurous acids (H 2 SO 3 , H 2 SO 2 ), if they are present in 

 sufficient dilution to be non-poisonous 4 . Both higher and lower plants seem able, 

 however, to assimilate sulphur in the form of organic compounds such as taurin, 

 or isothionic acid, or even certain sulpho-acids. Nageli was unable, however, to 

 nourish fungi with urea sulphonate and rhodanammonium. 



The alkali metals. All research shows that in Phanerogams potassium can be 

 replaced by no other element 5 , and it is still doubtful whether rubidium or caesium 

 can be totally substituted for it in the case of fungi. Nageli, Molisch, and Benecke 

 all found that potassium could not be replaced by sodium, lithium or ammonium, 

 indeed even comparatively dilute solutions of lithium salts exercise a distinctly 

 poisonous action upon both higher and lower plants 6 . Sodium salts are, however, 



1 Knop, in Blomeyer's Ber. v. Landw. List, zu Leipzig, 1881, pp. 31, 51 ; Ville, Compt. rend., 

 1861, T. LIII, p. 822 ; Molisch, Sitzungsb. d. Wien. Akad., 1896, Bd. cv, Abth. i, p. 642. Bouilhac 

 (Compt. rend., 1894, T. cxix, p. 929) erroneously concluded that arsenic acid might replace 

 phosphoric. Schleiden incorrectly supposed (Grundz. d. vviss. Bot, 1845, 2. Aufl., Bd. II, p. 469) 

 that PH 3 could be directly assimilated. 



2 Literature by A. Mayer, Lehrb. d. Agr.-Chem., 1895, 4. Aufl., p. 264. 



3 \Yinogradsky, Bot. Zeitung, 1887, p. 489. Cf. Sect. 63. Sulphates, &c. : Schimper, Flora, 

 1890, p. 222; Berthelot et Andre, Compt. rend., 1891, T. cxil, p. 122; Tamman, Zeitschr. f. 

 physiol. Chemie, 1885, Bd. ix, p. 41 7 ; E. Schulze, Landw. Jahrb., 1892, Bd. xxi, p. 1 1 8. Cf. Sect. 80. 



* On the poisonous action of calcium sulphide on higher plants, cf. Fittbogen, Landw. Jahrb., 

 1884, Bd. xill, p. 755. Loew (Biol. Centralbl., 1891, Bd. XI, p. 377) states that Spirogyra can 

 assimilate methylsulphide. Nageli, Bot. Mitth., iSSi, Bd. m, p. 459. Phanerogams cannot 

 obtain their supplies of sulphur from sulphites (Birner und Lucanus, Versuchsst., 1866, Bd. vni, 

 p. 152), and similarly Knop (1. c., iSSi, pp. 31, 51) finds that Lisulphites do not afford an adequate 

 supply of sulphur. 



5 Lucanus, Versuchsst., 1866, Bd. vni, p. 146; Nobbe, ibid., 1871, Bd. xill, p. 399; Loew, 

 ibid., 1878, Bd. xxi, p. 389. 



6 Nobbe, Versuchsst., 1870, Bd. xill, p. 399; Gaunersdorfer, ibid., 1887, Bd. xxxiv, p. 175. 

 For fungi, Benecke, Jahrb. f. wiss. Bot., 1895, Bd. xxviil, p. 507. On the occurrence of Li in 



