RESPIRATION— AERATION— FERMENTATION 225 



art important factor, for it is by diffusion tliat air in the inteir- 

 cellular spaces ia being constantly renewed. Air also enters 

 the stem through the lenticels (see page 53 and Fig. 105). 



Aeration of the soil is also an important factor in plant 

 growth. There is a very evident interchange of atmospheric 

 gases in the root system of the plant. If the free aeration of the 

 soil is prevented by a packing of the soil or by saturation of 

 the soil Avith water, many species of plant are unable to grow. 

 Furthermore, th^ micro-organisms must receive nitrogen and 

 otiier atmosp'heric gases from the air. Therefore, the proper 

 drainage and cultivation are important factors in agriculture. 



Fermentation. — This term was originally applied to the 

 souring and decomposition of organic substances. It is due to en- 

 zymes which are produced by bacteria or other organisms. 

 Among the most common of these is (a) the fermentation of milk 

 by lactic acid bacteria. It may be illustrated as follows : 



(Lactose) (Water) (Glucose) (Galactose) ( Lactic acid) 



The bacteria use a part of the sugar as food and produce 

 lactic acid, carbon dioxide and sometimes other organic acids, 

 hydrogen and nitrogen. The formation of lactic acid inter- 

 feres with the action of the vaxious bacteria found in milk and 

 their activities usually cease when the liquid contains about 

 .8 per cent, of acid. (&) Alcoholic fermentation of sugar 

 solutions, which is most commonly caused by yeast When Og 

 is abundant the yeasts grow rapidly, using the sugar for food 

 and causing very little fermentation, but when the amount of 

 O2 is low the yeasts grow slowly and fermentation is more rapid. 

 It may be expressed as follows : 



CeHi20e = 2(C2H80)+C02 

 15 



(Glucose) (Ethyl alcohol ) (Carbon 



dioxide) 



