THE FOOD OF PLANTS. 87 



obtained, and into each is poured 100 c.c. of the fluid. To a we 

 make no addition ; to b we add 0'2 parts of acid Potassium phos- 

 phate (KH 2 P0 4 ), 0-02 parts of Calcium phosphate (Ca s 2 P0 4 ) 

 and 0'02 parts of Magnesium sulphate (Mg S0 4 ) ; to c we add 

 0'02 parts of Calcium phosphate arid 0'02 parts of Magnesium 

 sulphate ; and, finally, to d are added 0*2 parts of Sodium 

 phosphate (Na 2 HP0 4 ), 0'02 parts of Calcium phosphate, and 

 0'02 parts of Magnesium sulphate. Regarding addition of Ferric 

 salt, see p. 58. We now close the flasks with stoppers of cotton 

 wool, heat the fluids to boiling, and allow to cool again. We next 

 add to each of the four nutrient solutions 1 c.c. of slightly milky 

 yeast water (see Appendix), momentarily raising for this purpose 

 the plugs of cotton wool with which the flasks are closed. It is 

 best now to put the four flasks into the thermostat at a temperature 

 of 25 to 30 C., frequently shaking them. Vigorous fermentation 

 and multiplication of the yeast only takes place in the fluid 6, as 

 we may quantitatively determine by collecting the yeast on a dried 

 and weighed filter. If a slight turbidity appears in the fluids 

 a, c, and d (due to a small development of yeast or bacteria), 

 that must be taken as an indication of the presence of small 

 quantities of mineral substances or potash, as the case may be, 

 which were perhaps present in the sugar, or in the yeast water 

 added to the fluids. At all events, the yeast cannot develop 

 normally when mineral substances are wanting in the food 

 solution. But our experiments teach further that no extensive 

 vegetation of the yeast and no fermentation takes place even when 

 only Potassium is absent, 1 this not being replaceable by Sodium. 

 In the experiments the greatest care must be taken as to the 

 purity of the water, of the reagents, arid of the yeast material. 

 The more favourable the conditions in this direction, the more 

 striking will be the result of the experiments, 



1 For further information as to the food requirements of yeast, see A.. Mayer, 

 Lehrluch der Gahrungschemie, 1874, p, X21, 



33. The Forms in which Certain Mineral Substances Occur 

 in Plants. 



Mineral substances are by no means restricted in the plant 

 to inorganic compounds; they also occur very frequently in 

 organic substances also, e.g. Sulphur in proteids, Phosphorus in 



