YEASTS AND MOULDS 331 



human body-heat, some at 30 to 33 C., and most at air 

 temperatures. Growth at temperatures higher than the 

 optimum, in certain media and anaerobically, results in the 

 production of pleomorphic forms. Most of them die when 

 deprived of air or oxygen ; a few are anaerobic. Moisture 

 is absolutely necessary. They grow readily on organic 

 matter of all kinds. The natural media commonly used 

 in their study are bread, sterilized milk, beer wort, potato, 

 carrot, decoctions of fruits, etc. As these media are vari- 

 able in their composition from time to time, and the 

 problem of pleomorphism has to be faced, artificial media 

 of definite composition and reaction are preferred in 

 scientific work for giving comparable results. Growth is 

 best on solid media, standardized to an acid reaction of 

 -f 2 per cent (+ 20 per litre). 



YEASTS. 



The yeasts are fungi characterized by the mode of 

 multiplication known as " budding " or "gemmation" or 

 asymmetrical fission, and are hence called blastomycetes. 

 From their action in fermenting sugars they have also been 

 called saccharomycetes. Their botanical position as a 

 separate group is not well established, as a large number of 

 intermediate forms relate them closely to the moulds. 

 The usual yeast cell is round or oval in shape, 10 to 20 micra 

 long by 5 to 15 micra across, and occurs singly or in short 

 chains. Each cell is bounded by a cell-membrane composed 

 of cellulose, and of such a thickness (0-5 micron) that it 

 shows a double contour. Within the membrane is con- 

 tained the protoplasm, in which is a large number of 

 granules, globules, and vacuoles, and in old cultures a 

 nucleus is sometimes seen. When budding, the mother cell 

 throws out a small globular process, which gradually 

 enlarges until it attains nearly the same size as the parent 

 cell. By a gradual narrowing of the isthmus between the 

 mother and daughter cells, the daughter cell finally becomes 

 free. In addition to this mode of reproduction, most yeasts 

 can form spores called " ascospores." This takes place 

 when there is a lack of nourishment or where the conditions 

 of life are otherwise unfavourable. These spores are 



