428 THE FOOD OF PLANTS 



still be absent from partially green chloroplastids, which under other conditions 

 are capable of photosynthesis. 



Many plants, as Lemna trisnlca, Trapa natans, &c., store up iron in large 

 quantities (cf. Wolff, Ash Analyses), while an incrustation of ferric oxide is formed 

 on the outer surface of others (Sect. 23). In these cases the accumulation of 

 iron seems to be largely accidental, but it appears that certain bacteria may profit 

 by it through obtaining energy by the oxidation of ferrous into ferric salts (Sect. 63). 

 Ordinary plants are fully supplied with iron when 0-2 per cent, is present in the ash, 

 so that an adult maize plant of 200 grm. dry weight and yielding 89 grm. of ash 

 would contain 0-016 grm. of iron, and the crop from a hectare (2$ acres = 200 kg. ash) 

 would remove 0-4 kilogrammes of iron from the soil. 



Phosphorus is in all cases essential, for it forms a constituent of many 

 proteids, and nuclein contains as much as 6 per cent. (Sect. n). It is not known 

 what other functions phosphorus may discharge, and it is uncertain whether the 

 widely distributed lecithin compounds ' are associated with proteids, or are of im- 

 portance in fat formation, or have quite different functions to perform. Phosphorus 

 is present in the form of an organic compound in lecithins, and in certain proteids, 

 as well as in the globoids 2 deposited in seeds. None of these produces any 

 precipitate with an ammoniacal solution of magnesium sulphate, or with nitro- 

 molybdic acid. Reacting phosphates are liberated again by proteid decomposition 

 and by metabolism in general, and they may accumulate to a marked extent in 

 certain plants. Phosphates are usually present in a living cell in a dissolved 

 form, but may after death be precipitated in combination with calcium, or 

 magnesium 3 . 



In all cases hitherto examined phosphoric acid affords the best source of 

 phosphorus, and since pyro- and metaphosphoric acids readily change into ortho- 

 phosphoric acid the former may also serve as a source of this element 4 . It is 

 however possible that certain plants require or prefer their phosphorus in the 

 form of organic compounds (Sect. 64), and fungi indeed are able to obtain all 

 that they require from proteid compounds, while oats can partially satisfy their 

 need for phosphorus by absorbing lecithin 8 . Sub-oxidized forms of phosphorus 



1 On Lecithins, see Schulze und Frankfurt, Versuchsst., 1894, Bd. XLIII, p. 308. On the 

 tendency to associate with proteids, see Neumeister, Physiol. Chemie, 1893, I, p. 41. Glycerine- 

 phosphoric acid may also be present in plants, and the frequently marked percentage of phosphorus 

 contained in fats may be due to the presence of either of these compounds. 



3 Pfeffer (Jahrb. f. wiss. Bot., 1872, Bd. VIII, p. 465) obtained microchemical proof of the 

 presence of H S PO 4 as an organic compound in the globoids of seeds. Cf. also Schulze und Winter- 

 stein, Zeitschr. f. physiol. Chemie, 1896, Bd. XXII, p. 90. On the microchemical detection of H S PO 4 

 see also Zimmermann, Mikrotechnik, 1892, p. 51 ; Raciborzki, Bot. Zeitnng, 1893, p. 245, and the 

 literature here given ; Polacci, Malpighia, 1894, Bd. vm. 



s See Zimmermann, Mikrotechnik, 1892; Hansen, Flora, 1889, p. 441; Kohl, Kalksalze u. 

 Kieselsaure, 1889, p. 116. On calcium phosphate in living cells : Zimmermann, Beitriige z. Morph. u. 

 Physiol., 1893, p. 310. On solution of calcium phosphate, see Vaudin, Ann. d. 1'Inst. Pasteur, 1895, 

 T. IX, p. 636. On accumulation of phosphates : Schimper, Flora, 1890, p. 222. 



* Shown by manuring experiments by Eggerty and Nilson, Centralbl. f. Agr.-Chem., 1893, 



P- 378- 



4 Stocklasa, Bot. Centralbl., 1896, Bd. LXVI, p. 64. 



