CHAPTER IX. 



FUNGI AND LICHENS. 

 BREWERY YEAST (SACCHAROMYCES CEREVISIAE). 



409. Structure of Yeast Cell. Before starting ex- 

 periments with Yeast, examine its structure, in the resting 

 condition, as follows : 



(a) Place a little dry Yeast on a slide, add a drop of 

 water, and stir it up with a needle, cover, and examine. 

 With the low power, note the extremely small size of the 

 rounded or ovoid cells. With high power, note (1) the 

 thin cell-wall, (2) protoplasm, often showing bright clear 

 dots (oil-drops), and (3) the central vacuole. 



(b) Treat preparations with (1) iodine, (2) chlor-zinc- 

 iodine, (3) potash ; note that there is no starch, that the 

 cell- wall does not give the reactions of cellulose, that the 

 protoplasm is stained brown by iodine, and that the wall is 

 made clearer by the disorganising action of potash on the 

 protoplasm. 



(c) Stain some Yeast with haematoxylin ; then press on 

 the cover-glass, and look for cells which have been crushed 

 these will show the empty ruptured wall and the ex- 

 truded contents. 



410. Pasteur Solution. To prepare a stock of Pasteur 

 culture solution for Yeast and other Fungi, weigh out, 

 powder, and thoroughly mix the following salts : 



Ammonium tartrate (NHJ^H^Oj. ... 50 grams 



Potassium phosphate, KH 2 P0 4 10 



Calcium phosphate, Ca 3 (PO 4 ) 2 1 gram 



Magnesium sulphate, MgSO 4 1 



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