394 MOULDS AND YEASTS 



the cut surface of a slice of potato ; place the latter on damp 

 blotting-paper in a Petri dish and incubate at 20 C. 



Scrape off daily a little of the yeast and mount in water. 

 Examine with an J objective for ascospores. 



Ex. 177. Take some plaster of Paris (or gypsum) and mix it 

 with water in the proportion of eight volumes of the powder to 

 three of water. Mould it rapidly before it sets into the form of 

 short cylinders or truncated cones about an inch in diameter 

 and | of an inch high, pressing the upper surfaces against a 

 sheet of glass to render them smooth. The material soon sets 

 into hard blocks which should be sterilized by heating when dry 

 in a hot-air sterilizer to 115 C. for one hour. 



Smear some of the creamy yeast from Ex. 1 74 on the upper 

 surface of a block, and place the latter in a deep Petri dish. 

 Pour in boiled water until the block is half immersed : then 

 place the whole in an incubator at 20 C., and examine small 

 portions of the yeast in twenty-four to forty-eight hours for asco- 

 spores. 



Ex. 178. Repeat the above experiments, using ordinary 

 brewers' yeast instead of German yeast. 



Ex. 179. Inoculate gelatine plates with a drop of water in 

 which, yeast has been distributed : isolate the yeasts from any 

 accompanying bacteria and grow them upon gelatine, agar, and 

 potato, noting the form and character of the pure colonies. 



Ex. 180. Prepare cover-glass films of yeasts from cultures in 

 beer-wort or other liquid media, and stain with Loftier' s methy- 

 lene blue or weak gentian violet ; wash, dry, and mount. 



Ex. 181. Mix some of the gypsum-block cultures showing 

 ascospores with a little water : spread a small drop on a cover- 

 glass, and after air-drying, fix and stain with carbol-fuchsin for 

 two minutes. Then wash in water and in alcohol to remove 

 colour from all parts of the cells except the spores : counterstain 

 with LofHer's methylene blue for three minutes. 



2. Torula. The genus Torula embraces single-celled 



