I3 6 BOTANY OF THE LIVING PLANT 



In photosynthetic cellfl such as those of the leaf, the breakdown 

 ( ,t bu| on side by Bide with their synthesis, though only a 



small proportion of the sugars manufactured in photosynthesis is 



msumed in the respiration of the leaf. One result of this predomin- 

 ant oi photosynthesis is thai in green organs such as leaves, when 



*>sed to light, the gaseous interchange of photosynthesis, which 

 we have seen consists in t he absorption of carbon dioxide and 



ilution of oxygen, masks that of respiration and is alone in evidence. 

 In order to demonstrate the latter process in such organs light must 

 be excluded, photosynthesis being thereby prevented while respiration 

 continues, since it is independent of light. 



If we refer back to the equation for respiration we see that the 

 volume of oxygen absorbed should be equal to that of carbon dioxide 

 produced; so that the so-called Respiratory Quotient, that is the 

 r.itio of carbon dioxide given off to oxygen absorbed, should equal 

 unitv. Experiments show that this is usually approximately true. 

 There are exceptions, one being provided during the germination of 

 seeds with fatty food reserves, the fats providing the initial fuel for 



miration. Here a greater volume of oxygen is absorbed than of 

 carbon dioxide produced, giving a quotient of less than unity (fre- 

 quently in the region of 0-6). The probable explanation is that a 

 preliminary to respiration proper is here the conversion of the fats 

 to sugars. Oxygen is needed for this, over and above that subse- 

 quently required for the oxidation of the sugars. Conversely the 

 volume of carbon dioxide evolved may exceed that of oxygen ab- 

 sorbed ; carbon dioxide may even be produced without any absorp- 

 tion of oxygen. This type of respiration is displayed by plants when 

 they are deprived of oxygen. It is known as Anaerobic Respiration, 

 as distinct from the normal or Aerobic Respiration carried on in air, 

 such as we have so far been considering. 



Anaerobic respiration can be demonstrated by passing a few 

 germinating peas up into an inverted test-tube completely filled 

 with mercury and supported in a dish also containing mercury. After 

 some hours gas will be found to have accumulated over the mercury, 

 and will continue to increase in amount for several days. On testing 

 the gas it will be recognised as carbon dioxide. In this type of respira- 

 tion sugar is again consumed, though here the products are carbon 

 dioxide and ethyl alcohol. There is in fact evidence that in this 

 anaerobic respiration of higher plants we have a process very similar 

 to the well-known alcoholic fermentation promoted by the fungus Yeast. 

 Higher plants do not, however, live very long if they are deprived 



