presidejst's address. 29 



Some of these may possess a greater capacity for fermenting 

 glucose in a dough than others. The conditions may favour a 

 more speedy multiplication of these desirable cells. In this %vay 

 the resulting yeast would be made up of a larger number of cells 

 capable of inducing fermentation in the dough. On the other 

 hand, we can suppose that the yeast-cells all become so altered by 

 their environment that they can ferment the glucose in the dough 

 more rapidly. It is not possible to explain with certainty just 

 what happens in this process. By cultivation from a single cell, 

 yeast suitable for a baker can be obtained. It is therefore clear 

 that the desirable yeast may be selected from the progeny of a 

 yeast not endowed with the required qualities. A similar 

 selection is made in the dough. This explains the rapid ripening 

 of doughs made with "sponges." The elucidation of what 

 happens is made clearer by study upon other organisms. 



When the Bacillus coli communis is grown in a nutrient liquid 

 containing glucose, both acid and gas are produced by the fer- 

 mentation of the glucose. The gas consists of carbon dioxide 

 and hydrogen, and the acids which are liberated, comprise lactic, 

 acetic, succinic, and formic acids. It has been found by W. J. 

 Penfold that strains af Bacillus coli coinmtvuis which do not pro- 

 duce gas from glucose, can be isolated by cultivation in the 

 presence of sodium mono-chloracetate. These races differ from 

 the typical parent-stock in their fermentative character on 

 glucose, and no individual member of the selected race appears 

 to retain the action of the typical parent micro-organism. The 

 presence or absence of the quality of liberating gas enables us to 

 recognise that, in the production of this character, every indi- 

 vidual of the selected race exhibits the altered function. If a 

 number of selected races are examined, it is, however, noted that 

 some show a complete absence of the gas-forming quality, while 

 others still retain more or less activity in this direction. The 

 careful studies of Egerton Gray have enabled us to learn some- 

 thing of the enzymatic changes which accompany this selection. 

 The estimation of the amount of the different products of the 

 decomposition of glucose supplies evidence to demonstrate that 

 lactic acid is formed in greater amount, and acetic acid, formic 

 acid, and alcohol in less amount, bv the selected strains than bv 



