A Conjugating " Yeast." 



345 



« A Conjugating ' Yeast.' " By B. T. P. Barker, B.A., Gonville 

 and Caius College, Cambridge. Communicated by Professor 

 Marshall Ward, F.E.S. Eeceived May 4, — Read June 6, 

 1901. 



(Abstract.) 



At the outset, the idea of a true yeast (Saccharomyces) which conju- 

 gates may appear anomalous in the extreme, but it is not improbable 

 that such an event has been observed before in such organisms, though 

 the phenomena have been misinterpreted. 



The yeast which is the subject of this communication was obtained 

 from commercial ginger, pieces of this substance being placed in sterile 

 saccharose-Mayer solution and kept at 25° C. until the organisms 

 situated on the surface of the ginger had attained vigorous growth. 

 These were separated by means of fractional plate-cultures of beer-wort 

 gelatine. 



The colonies of the yeast-form, as seen on beer-wort gelatine plate- 

 cultures, appeared to the naked eye as small rounded white dots, about 

 the size of a pin's head. Under the low power of the microscope 

 colonies on the surface of the gelatine had regular edges, while sub- 

 merged colonies had a woolly appearance, due to numerous radiating 

 branches. 



A pure culture was obtained from a colony developed from a single 

 cell kept under observation in a hanging drop of beer-wort gelatine. 



Streak cultures on beer-wort gelatine and beer-wort agar are of a 

 milky-looking brownish-white colour, and have well-marked regular 

 crenate edges. Streak cultures on potato and bread are milky- white 

 when moist, and chalky-looking when dry ; on pieces of moist ginger 

 their colour is darker. ' 



A yeast-ring is formed in old cultures on many liquid media, but no 

 films are produced. In tubes of beer-wort, which have been actively 

 fermenting, the ring makes its appearance in 10 — 14 days at 25° C. 

 It is milky-white in colour, and looks like a layer of cream, deposited 

 around the edges of the liquid. Such rings are also formed on dextrose- 

 Mayer, lsevulose-Mayer, saccharose-Mayer, and maltose-Mayer solutions, 

 being particularly well developed on those liquids which have undergone 

 an active fermentation. 



The vegetation of the cultures described consists of typical ovoid and 

 round yeast cells, and in the older cultures a few sausage-shaped and 

 many irregular cells also, some of the latter containing spores. 



Reproduction by budding in a typical yeast-like manner is the usual 

 method of growth, taking place best at 25 — 30° C, the maximum and 

 minimum limits being 37 — 38° C. and 10 — 13° C. respectively. 



Reproduction by spores occurs under the usual conditions of spore- 



