456 THE president's address. 



of the work done by the yeasts, or Saccharomyces, characterised 

 more especially by the formation of carbonic cUoxide (carbonic acid 

 gas) and alcohol ; putrefaction, or decomposition of organic or 

 inorganic bodies by bacteria, with liberation of free nitrogen or 

 ammonia. . 



It is important to bear in mind that the substances enumerated 

 above as characteristic of the activity of yeasts and bacteria 

 respectively, are by no means the only bodies liberated during 

 fermentation or putrefaction. The very varied smells and tastes 

 produced during these 2:)rocesses testify otherwise. 



The yeasts are true fungi, originally included in the genus 

 SaccharomyceSy and were at one time considered as representing 

 one of the simplest phases of plant life, owing to their minute 

 size and apparent absence of differentiation. Kecently, however, 

 thanks to improved microscopes, reagents, and stains, the presence 

 of nuclei has been demonstrated, and a sexual mode of reproduc- 

 tion observed. All veasts are unicellular orefanisms, the average 

 size being about 10 /x long. The commonest mode of reproduc- 

 tion is unisexual, and known as budding. When yeast is j^laced 

 in a saccharine solution kept at a suitable temperature, the yeast 

 cells raj)idly increase in size, and when a given stage is reached, 

 one or more minute bulges appear on the surface of the parent 

 cell. These bulges increase in size, and eventually become free, 

 each in turn adding to the number of plants present. By this 

 rapid method of reproduction, a quantity of suitable liquid, con- 

 taining only a very few yeast cells, soon becomes thick and turbid, 

 due to the rapid increase in numbers of yeast cells. During this 

 active period of growth, a brisk effervescence, due to the liberation 

 of carbonic acid, is going on, and proper examination reveals the 

 presence of a certain amount of alcohol in the liquid ; both sub- 

 stances being by-products resulting from the dissociation of the 

 sugar by the yeast plant. 



A second mode of reproduction is by the formation of endo- 

 spores, or spores formed within a yeast cell. This form of 



