540 ANAEROBIC RESPIRATION 



bulky fruits such as bananas, citrus fruits, melons, etc. often suffer from a 

 deficiency of oxygen and that anaerobic respiration is of frequent occurrence in 

 such tissues. (Gustafson, 1930) has shown that tomato fruits respire anaerobic- 

 ally when enclosed in an atmosphere of nitrogen or hydrogen and considers 

 it a possibility that they may carry on some anaerobic respiration even when 

 exposed to a normal atmosphere. 



On the other hand analysis of the internal atmosphere of some of the 

 cucurbitaceous fruits has shown the oxygen concentration to be almost as 

 high as in the atmosphere. While it is impossible to draw a definite conclu- 

 sion regarding the prevalence of anaerobic respiration in fleshy fruits, it seems 

 probable that this process occurs in at least some fruits of this type. 



Cacti will respire anaerobically when enclosed in an atmosphere of pure 

 nitrogen (Gustafson, 1932) and it seems probable that the deep-seated tissues 

 of succulent species may respire anaerobically under natural conditions. 



External factors appear to influence the rate of anaerobic respiration in 

 much the same way that they affect respiration of the aerobic type. The influ- 

 ence of temperature follows a time factor pattern similar to that found for 

 aerobic respiration. For germinating pea seeds the optimum temperature 

 (temperature at which the rate is maintained for a long time) is about 30° C. 

 (Fernandes, 1923). Various toxic substances appear to influence both types 

 of respiration in a similar manner. 



The Similarity between Anaerobic Respiration and Alcoholic Fer- 

 mentation. — Considerable evidence exists that the process of anaerobic res- 

 piration as it occurs in the higher green plants is identical with, or at least 

 very similar to the process of alcoholic fermentation as carried on by yeasts 

 and some other micro-organisms. The same summary equation is therefore 

 supposed to represent both processes. There are three main reasons for believ- 

 ing that these two processes are essentially similar : 



1. Hexose sugars are usually the substrate which is oxidized in both 

 alcoholic fermentation and anaerobic respiration. 



2. The principal end products — carbon dioxide and ethyl alcohol — are also 

 the same in the two processes. We have already seen that plant tissues evolve 

 carbon dioxide when subjected to anaerobic conditions. Alcohol also accumu- 

 lates in the tissues of higher plants during anaerobic respiration. The quan- 

 tities of alcohol produced in most plant tissues during anaerobiosis are less 

 than would be theoretically expected in terms of the generally accepted equa- 

 tion. A probable explanation of this fact is that other compounds besides 

 alcohol are produced as end products in this process as it occurs in the higher 

 plants. This does not necessarily invalidate the fundamental concept of an 

 essential similarity between anaerobic respiration and alcoholic fermentation. 



