126 CHAP. XVI. PHOTOSYNTHESIS WITHOUT CARBON DIOXIDE 



amplitude, proving that the sensitiveness of the plant 

 had remained constant during the whole period of the 

 experiment. 



The experiments which have been described show that 

 the photosynthetic evolution of oxygen takes place in acid 

 plants even without absorption of carbon dioxide. Hence 

 the evolution of oxygen is a surer index of photosynthesis than 

 the intake of carbon dioxide. Since organic acid serves as a 

 partial substitute for C0 2 , the quantity of this gas absorbed 

 by acid plants under light will be less than in non-acid 

 plants. 



Assimilatory quotient. — This introduces certain complica- 

 tions in the ratio of the oxygen evolved to the carbon dioxide 

 absorbed. Taking the following equation for the normal 

 photosynthetic production of carbohydrate : 



6 C0 2 + 6 H 2 = C 6 H 12 6 + 6 2 



the normal assimilatory quotient ~^- = I. But in acid 



plants the denominator of the fraction (C0 2 ) is less than 



normal ; hence ~Jr has a value greater than i. 

 LU 2 



Respiratory quotient.— In normal cases the respiratory 



CO 

 quotient — = I ; but in acid plants the quotient is less 



2 

 than i, and in extreme cases it is zero. 



The above considerations explain the variation from 

 unity of the assimilatory and respiratory quotients observed 

 in succulent plants. The biological feature of these plants 

 is that they are xerophytes, i.e. are adapted in their form and 

 structure to conditions (heat, dry soil, etc.) which involve 

 protection of the contained water against excessive trans- 

 piration. Their structural adaptations are a thick epi- 

 dermis with few stomata, and compact internal tissue with 

 relatively small intercellular spaces. These peculiarities 

 of structure affect, however, not only the exhalation of 

 water-vapour in transpiration, but also the gaseous exchange 



