458 THE president's address. 



the wort. Saccharomyces cerevisiae is the species used in brewing 

 beer, and Professor Hansen, a Danish botanist, has shown that 

 this species has given origin, during its utilisation by roan, to 

 many distinct races, each of which behaves differently in the 

 fermenting vat. Some races produce sparkling beer, others pro- 

 duce a strong head, property of brightening quickly, and resisting 

 turbidity, and thus suitable for export beer, etc. The practical 

 outcome of this discovery is that at the present day, in many of 

 the large breweries, especially on the Continent, perfectly pure 

 cultures of the particular yeast giving the desired result are used 



for brewing. 



In bygone times, and even in the present day in country 



districts, a ginger-deer plant constitutes one of the heirlooms 

 transmitted from generation to generation, and used for the pur- 

 pose of manufacturing home-made ginger-beer. In appearance 

 the ginger-beer plant is not inviting ; somewhat resembling thin 

 sago pudding, with a slimy feel, and slipping between the fingers 

 w^hen crushed in the hand. This remarkable " plant" consists of 

 a mixture of two distinct organisms, the one being a yeast 

 — Sacchciromyces 2)yrifor'r)iis, the other a bacterium — Bacillus 

 vermiforinis. The occurrence of these two organisms, neither of 

 which alone can produce ginger-beer, is considered by Professor 

 Marshall Ward as affording an instance of true symbiosis. 

 When the ginger-beer plant is placed in water containing 

 crushed ginger, sugar, and a little tartaric acid, and kept at 

 the proper temperature, the yeast first commences work on 

 the sugar, setting up fermentation, the bacteria remaining, 

 for the time being, encased in a glairy sheath. After the 

 yeast has been active for some time, certain by-products, the 

 result of its i activity, check its progress, and would soon put 

 a stop to further fermentation. At this stage, however, the 

 bacteria emerge from their respective sheaths, and rapidly con- 

 sume the by-products which retard the activity of the yeast, and 

 by the mutual aid exercised between these two organisms, the 



