376 THE FOOD OF PLANTS 



other cases the needs of the organism exercise a marked influence upon 

 its secretory activity, for when sugar is present in sufficient quantity the 

 secretion of diastase may cease, and similarly the presence of peptone 

 may inhibit the formation of proteolytic enzymes (Sect. 91). Hence it is 

 not surprising that the same results are not always obtained in experi- 

 menting with a particular plant ; moreover the action of a ferment is 

 largely dependent upon the external conditions and especially upon the 

 acid or alkaline nature of the medium. 



Although enzymes are very widely distributed they are not essential 

 to those heterotrophic plants which grow only upon soluble food-materials, 

 and it appears that a few bacteria have no power of excreting enzymes. 

 Hence it can only be determined empirically whether solvent enzymes aid 

 in the nutrition of phanerogamic saprophytes ; similarly the penetration 

 of root-hairs, &c. into dead leaves, masses of humus, or even into living 

 parts of plants is not necessarily an indication of a power of excreting 

 solvent ferments, but may simply be due to the powerful mechanical 

 pressure which growing organs are able to exert 1 . 



Extracellular digestion is only possible by means of excreted enzymes, 

 but it is erroneous to suppose that every solvent action in a living cell is 

 due to the action of ferments derived from the same cell or from neighbour- 

 ing cells of the same tissue. Solvent actions may be attained by other 

 means, as occurs, for example, in the removal of endosperm food-material, 

 for the embryo may absorb the latter without any secretory activity of 

 the cotyledons being necessary (Sect. 109) 2 . 



Among sixty-two species of bacteria, Fermi 3 found in twenty-four a proteolytic, 

 in twenty a diastatic, in two an inverting enzyme, and in sixteen none. Three 

 enzymes were found in Bacillus megatherium, and two in two other species. Among 

 fungi, Aspergillus niger and Penidllium glaucum can produce diastase, invertase, 

 cytase, pepsin, and fat-splitting ferments 4 . Aspergillus oryzae is able to secrete 

 diastase in great abundance, whereas, according to de Bary, Peziza sderotiorum 

 has not this power, although it is able to form a cytase or cellulose-dissolving 

 ferment 5 . Hormodendron hordei produces no diastase, but secretes invertase, 



1 Examples in the case of root-hairs: Drude, Biol. v. Monotropa und Neottia, 1873, p. 34; 

 Schlicht, Landw. Jahrb., 1889, Bd. xvm, p. 499. Mosses: Haberlandt, Jahrb. f. wiss. Bot., 1886, 

 Bd. xvn, p. 476 ; Hoveler, ibid., 1892, Bd. xxiv, p. 293. 



2 For examples of the excretion of ferments by fungi and bacteria, see Zopf, Pilze, 1890, p. 177 ; 

 Flugge, Mikroorganismen, 1896, 3. Aufl., Bd. I, p. 197; Lafar, Technische Mykologie, 1897; also 

 Sect. 91, and the literature there given. 



3 Fermi, Centralbl. f. Bact., 1892, Bd. xii, p. 715. 



4 Bourquelot, Centralbl. f. Physiol., 1893, Bd. vn, p. 660 ; Hansen, Flora, 1889, p. 88 ; Miyoshi, 

 1. c. ; Schmidt, Flora, 1891, p. 300. 



5 Wehmer, Centralbl. f. Bact., Abth. ii, 1895, Bd. I, pp. 152, 218 (A. oryxae); de Bary, Bot. 

 Zcitung, 1886, p. 422 (Peziza}. 



