98 



BACTERIA AND FERMENTATION 



gives more than a score of different members of this family of 

 Saccharomycetes* But before mentioning some of the chief of 

 these, it will be desirable to consider a number of properties common 

 to the genus. The yeast cell is a round or oval body of the nature 

 of a fungus, composed of granular protoplasm surrounded by a 

 definite envelope, or capsule. It reproduces itself, as a general 

 rule, by budding, or gemmation. At one end of the cell a slight 

 sv^elling or protuberance appears, which slowly enlarges. Ulti- 

 mately there is a constriction, and the bud becomes partly and 

 at last completely separated from the parent cell. In many cases 

 the capsules of the daughter cell and the parent cell adhere, thus 

 forming a chain of budding cells. The character of the_ cell and its 

 method of reproduction do not depend merely upon the particular 

 species alone, but are also dependent upon external circumstances. 

 There are differences in the behaviour of species towards different 

 media at various temperatures, towards the carbohydrates (especially 



Fig. 13. — Diagram of Ascospore Formation. 



Fig. 14.— Gypsum Block. 



maltose), and in the chemical changes which they bring about in 

 nutrient liquids. In connection with these variations Professor 

 Hansen has pointed out that, whilst some species can be made use 

 of in fermentation industries, others cannot, and some even produce 

 " diseases " in beer.f 



One of the most remarkable evidences of the adaptability of the 

 yeasts to their surroundings, and a specific characteristic, occurs in 

 what is termed ascospore formation. If a yeast cell finds itself 

 lacking nourishment or in an unfavourable medium, it reproduces 

 itself not by budding, but by forming spores out of its own intrinsic 

 substance, and within its own capsule. To obtain this kind of 

 spore formation Hansen used small gypsum blocks as the medium on 

 which to grow his yeast cells. WeU-baked plaster-of -Paris is mixed 

 with distilled water, and made into a liquid paste. The moulds are 

 made by pouring this paste into cardboard dishes, where it hardens 



* Micro-organisms and Fermentation, 



f E. C. Hansen, Studies in Fermentation (Copenhagen), p. 98. 



