SPORE-FORMATION. 303 



1. The cells must be placed on a moist surface and have a 

 plentiful supply of air. 



2. Young and vigorous cells can exercise this function most 

 easily and rapidly. Old cells which lack nutritive material 

 can only develop spores with free access of oxygen. 



3. The optimum temperature for most of the species yet 

 examined is about 25 C. This temperature favours spore- 

 formation in all known species. 



4. A few Saccharomycetes likewise form spores when they 

 are present in fermenting nutrient fluids. 



A lack of food cannot, as Klebs assumes, be regarded 

 as a direct condition for spore - formation, since young 

 and well-nourished cells can also be induced to form spores 

 immediately without previous budding when they are 

 placed under conditions which favour spore-formation, but 

 are unfavourable to budding e.g., in water saturated with 

 gypsum, but with access of air and at a favourable tem- 

 perature. 



A growth of yeast is developed in the way described on 

 p. 290. Older cultures, developed in saccharine solution or 

 in wort, must be cultivated several times in aerated wort before 

 showing a normal formation of spores. A small quantity is 

 transferred to a previously sterilised gypsum block ; this block 

 takes the shape of a truncated cone ; it is enclosed in a flat 

 glass dish covered by a larger inverted dish, and is kept moist 

 by half-filling the dish with water.* If it is desired merely 

 to bring about the formation of spores, the apparatus may be 

 allowed to remain at the ordinary room temperature. 



The transferred cells develop through a few generations 

 by means of budding, and then spore-formation begins in the 

 mother-cells. 



Hansen was the first to give an accurate description of the 

 structure of spores and a detailed account of their evolution 

 founded upon observations of individuals. He distinguished 

 three typically different groups of Saccharomycetes which are 



* Ascospores can also be obtained when yeast is spread upon sterilised solidified 

 gelatine, prepared with or without nutritive solution (or on filter paper), and 

 kept in a damp place ; likewise in yeast-water and in sterilised water. Spore- 

 forming cells may also occur in the films of the Saccharomyces. 



