10 THE NITTRTTTVE PROCESSES 



two heads : — (i) As to the nature and condition of the nutrient 

 material ; and (2) as to the manner in which such nutriment finds 

 its way within the organism. 



As regards the first, it has been repeatedly shown by well- 

 known investigators that various species of Saccharomycetes 

 will grow, and to some degree thrive, in media other than the 

 saccharine fluids in which they are properly found ; indeed, that 

 they can be cultivated in solutions containing mineral salts only. 

 This leads at once to the conclusion that whatever absorption of 

 other materials may be going on, the cell-wall must necessarily be 

 pervious to certain inorganic salts, which, it should be noted, are 

 of course crystalloids, and therefore capable of passing the 

 membrane of a dialiser. 



Following up the indications thus obtained and cultivating spe- 

 cimens of healthy S. cerevist'ce in infusions of malt containing 

 various amounts of ammonia or nitrogen salts, certain fairly 

 definite differences in the results may be observed. These cul- 

 tures can be conveniently made in test-tubes plugged with cotton 

 wool and containing nutrient liquids carefully sterilised, and the 

 deposit of cells should be examined under a power of not less 

 than one-tenth inch. The examination of a number of such 

 cultivations will show that the vigour of the cell and the process 

 of multiplication is visibly increased by the mineral additions, 

 though not always with regularity. 



On making similar cultures of pure cells in solutions of 

 cane sugar, with and without the addition of nitrogen and 

 ammonia salts and alkaline phosphates, one finds that although 

 the vitality of the cells is maintained in the solutions of sugar 

 only, yet that it is with difficulty that the cells do not increase and 

 that the vigour of those remaining visibly declines. If these cells 

 be now placed in a fresh sugar solution, they will exert but little 

 decomposing action, and their vitality will come to a standstill. 

 But if they be now removed to a weak malt infusion (containing, 

 besides saccharin matter, albuminous and other nitrogenous com- 

 pounds, and some mineral salts), or to a solution of sugar 

 containing nitrogen, salts of ammonia, and potash, with some 

 tartrate of ammonia, they will rejuvenesce, so to say. The cell- 

 contents will again fill up the cell. The outer wall will recover 



