136 THE COLOURLESS PLANT CELL. THE YEAST PLANT 



a splitting of the sugar molecule without oxidation, 

 free oxygen only taking part in the reaction towards 

 the end of the process, resulting in the formation of 

 carbon dioxide and water. Alcohol may actually be 

 formed in ordinary plant cells if free oxygen is ex- 

 cluded. Thus alcoholic fermentation by yeast repre- 

 sents the first stage of the ordinary respiratory process 

 continued for a long time and very energetically. 



Alcoholic fermentation of sugar by yeast is only 

 one kind of fermentation. Sugar can be split up into 

 other substances than ethyl alcohol and carbon dioxide, 

 and a great variety of other organic substances can be 

 split up in a similar way. These varied fermentations 

 are mainly carried out by various members of the great 

 class of unicellular plants, the Bacteria, with which we 

 shall deal in the next chapter. 



PRACTICAL WORK. 



A. Yeast. 



(i) The corked bottle contains yeast fermenting a sugar solu- 

 tion. Dip a glass rod into lime-water and test the air in the 

 bottle for carbon dioxide in the same way as with the respiring 

 barley (p. 89). Repeat the test at the end of the practical work. 



(2) Place a drop of brewers' yeast on a slide, cover, and examine 

 with, the high power. Measure the length and breadth of several 

 of the isolated cells and determine their average size. Draw two 

 or three on a large scale, marking cell wall, cytoplasm and central 

 (nuclear) vacuole. A refractive mass (chromatin) can often be 

 seen at one end of the vacuole. Note also the granules and 

 small vacuoles that can often be seen in the substance of the 

 cytoplasm. Stain with Schulze's solution. The cell wall stains 

 b^ue (cellulose). 



(3) Trace the method of budding by which the yeast cells 

 multiply and draw examples of different stages. 



(4) Smear a drop of yeast culture thinly over a perfectly 

 clean coverslip and dry the smear over the flame of a lighted 

 match. Place five drops of methylene blue in a clean watch- 

 glass and fill it up with water. Immerse the coverslip in the 



