62 PLECTOMYCETES [ch. 



1909 Klocker, A. Endomyces Javanensis, n. sp. C. R. des trav. du lab. de Carlsberg, 

 vii, p. 267. 



1913 VAN DER WOLK, P. C. Protascus colorans a new genus and a new species of the 

 Protascineae Group; the source of "Yellow-Grains" in Rice. Myc. Centralbl. iii, 



P- 'S3- 



1914 Ramsbottom, J. The Generic Name Protascus. Trans. Brit. Myc. Soc. v, p. 143. 



Saccharomycetaceae 



The Saccharomycetaceae, or yeasts, are widely distributed on, or in, all 

 sorts of sugary media; they occur mainly as separate cells which are only 

 exceptionally united to form a short mycelium. 



The individual cells are round or elliptical, bounded by a delicate 

 membrane and containing, in the cases studied in detail, a large nuclear 

 vacuole with a chromatin network and a well-marked, laterally placed 

 nucleolus. Division is amitotic. In the cytoplasm are refractive granules 

 of volutin, glycogen and oil. 



Multiplication is by transverse division and separation of the daughter 

 cells (Schizosaccharomyces), or more usually by budding, that is to say by the 

 formation of successive lateral outgrowths which ultimately assume the 

 form and size of the parent cell (Zygosaccharomyces, Saccharomyces, Saccharo- 

 mycopsis). Each bud receives a nucleus and cytoplasm and is cut off 

 by a wall. Before its separation it may itself bud again, and in this way 

 considerable colonies may be produced. 



Under suitable conditions, and especially when growing on a moist, solid 

 substratum, the cell contents may round themselves up to form one to eight 

 (usually two or four) spores. These so-called endospores are the ascospores 

 of the yeast, the ordinary vegetative cell functioning as an ascus either 

 independently or after conjugation with another similar cell. 



One of the most striking features of the yeasts and one which gives 

 them a considerable economic importance is the power possessed by many 

 species of producing alcoholic fermentation in certain sugars. This pro- 

 perty is due to the presence of the enzyme zymase which is secreted by 

 the yeast cells during fermentation, but which is not present in the resting 

 cells, being soon decomposed when the reaction comes to an end. The 

 activity of zymase is dependent on the presence of two co-enzymes ; the 

 first is a soluble phosphate which enters into temporary combination with 

 part of the carbohydrate, but it is ineffective in the absence of a second 

 factor of unknown constitution. The unknown co-enzyme is dialysable and 

 not destroyed by boiling; it may be separated from the yeast juice by 

 filtration under pressure, both filtrate and residue being inactive alone. 



Even the yeasts which produce the largest proportion of alcohol utilize 

 five to six per cent, of the available sugar in the formation of glycerine, 



