226 COLLEGE BOTANY 



Tlie aotivity of the yeasts is practically inhibited when the 

 liquid contains 14 per cent, of alcohol. They are used in the 

 manufacture of alcoholic beverages and in the making of light 

 bread. The bubbling of the CO2 through the dough of wheat 

 flour is the most important factor in bread-making, (c) Acetic 

 fermentation, which is caused by bacteria acting on alcoholic 

 solutions. It may be illustrated as follows : 



C2H5OH + 02 = CH3COOH + H2O 

 (Ethyl alcohol)- (Acetic acid) (Water) 



This is the action in the transformation of cider into vinegar. 



It will b© readily seen that all of these fermentations are in 

 fact respiration processesi within the bacteria or fungi involved. 



Anaerobic Respiration. — ^Respiration processes are some- 

 times cai-ried on without free oxygen, and some of the micro- 

 organisms perform the functions of respiration to the best ad- 

 vantage in the absence of oxygen. This is known as anaerobic 

 respiration and is characteristic of certain bacteria and fungi. 

 It results in the formation of CO2 and sometimes of alcohol, 

 lactic acid and hydrogen. It is evidently due to a rearrange- 

 ment of the atomic groups in the organic molecules. Anaerobic 

 respiration probably precedes the aerobic or ordinary respiration 

 in all cases, but this is not well understood. 



LABORATORY EXERCISES. 



Exercise 1. Select two lots (about 24 each) of beans or peas of about 

 the same weight. Heat one lot and determine the dry weight. Germinate 

 the other lot in wet filter paper in a dark closet. When the seedlings have 

 attained their maximum growth under these conditions, dry and determine 

 the dry weight. Compare with the first lot. Have they lost or gained 

 in weight? 



Exercise 2. Put beans or peas that are just beginning to germinate into 

 two wide-mouthed bottles. After 12 to 24- hours test the air in one with 

 a lighted taper and the other with baryta water. Make similar tests with 

 the air in empty bottles. Explain. 



Exercise 3. Select three large glass evaporating dishes, A. B and C. 



