386 THE FOOD OF PLANTS 



of grape or cane-sugar yields a very suitable nutrient fluid, or the inorganic salts 

 may perferably be supplied by dissolving i grin. NH, NO 3 , 0-5 grm. KH 2 PO 4 , 

 0-25 grm. MgSO 4 in 200 cc. of water. Under certain conditions it is better to 

 use this solution either half diluted or twice concentrated, while a trace of iron 

 should be added, and for comparative researches always precisely the same amount. 

 An addition of 0-2 grm. Cad., is to be recommended though not absolutely 

 necessary. To supply nitrogen in organic form i grm. of asparagin or i to 3 grm. 

 peptone must replace the ammonium nitrate. 



Of the different saline solutions which Nageli employed the following may be 

 mentioned: (i) 100 grm. H 2 O, i grm. (NH 4 ) 2 Q H 4 O c , o-i grm. K 2 HPO 4 , 0-02 grm. 

 MgSO,, o-oi grm. Ca C1 2 ; (2) 100 grm. H,O, i grm. peptone, 0-2 grm. K 2 HPO 4 , 

 0-04 grm. MgSO v 0-02 grm. Ca CL,. 1 In place of these mixtures about 0-4 grm. 

 of the ashes of peas or wheat grains may be used, after neutralizating with nitric 

 acid and adding an ammonium salt. 



The nutrient solution first employed by Pasteur 2 contained 100 grms. water, 

 10 grms. cane-sugar, o-i grm. ammonium tartrate, and the ashes of i grm. of yeast, 

 and frequently fresh ammonium tartrate was subsequently added. All these nutrient 

 solutions are also adapted for the culture of bacteria, and for those which require 

 peptone a suitable nutrient solution must contain 3 per cent, of peptone, 2 per cent, 

 of cane or grape-sugar, and i per cent, of meat extract. 



The optimal concentration of grape-sugar is reached when 5 to 15 per cent, 

 is present, but the growth of many fungi is suppressed only when the concentration 

 reaches as much as 50 to 60 per cent. 3 The same result is produced by an 

 isosmotic concentration of any other non-poisonous substance, and the optimal 

 concentration is isosmotic with 5 to 15 per cent, of grape-sugar. 



The works already quoted give a full account of the mode of cultivation upon 

 gelatine, agar, gelatinous silicic acid (Sect. 63) or other solid nutrient media, and the 

 use of solid media is of especial importance for the isolation and detection of 

 bacteria as well as for the preservation of pure cultures. Bacterial germs develop 

 in non-nutrient gelatine, &c., only where a drop of nutrient fluid is added and 

 as far as it diffuses, so that by means of this auxanographic method of Beyerinck's 4 , 

 the nutritive value of a given organic or inorganic substance can readily be 

 demonstrated, and not only can the action of different substances be noticed 

 upon the same plate, but also their conjoint influence can be observed where the 

 diffusion zones of the different drops meet. 



1 Other data are given by Nageli, I.e.; Hueppe, I.e., p. 238; Zopf, Die Pilze, 1890, p. 172; 

 Wehmer, Bot. Zeitung, 1891, p. 272 ; Pfeffer, I.e., &c. 



3 Pasteur, Ann. d. chim. et d. phys., 1860, iii. se"r., T. LViri, p. 383 ; 1862, T. LXIV, p. 106. 



8 Eschcnhagen,tiberden Einfluss von Losungen versch. Concentr. auf Schimmelpilze, 1889, p. 55 ; 

 Bruhne, in Zopf's Beitriigcn z. Physiol. u. Morph., 1894, Heft 4, p. 15; Klebs, Beding. d. Fort- 

 pflanznng, 1896, p. 465. 



4 Beyeiinck, Bot. Zeitung, 1890, p. 201. 



