K.— BOTANY 



221 



siderable quantities. In the succulents the output of carbon dioxide is 

 usually very small and acid, particularly malic acid, accumulates in the 

 leaves and stems during the night and disappears during the day. It is 

 generally held that the malic acid arises as a product of respiration, but 

 two main views have been put forward to explain the actual part played 

 by this acid in the sequence of actions following the breakdown of carbo- 

 hydrate. Ruhland and his collaborators hold that this breakdown follows 

 the same course as in normal respiration as far as the pyruvic acid stage. 

 Owing to the inhibiting action of high concentration of acetaldehyde the 

 normal action of carboxylase is inhibited and the pyruvic acid, instead of 

 breaking down under the action of this enzyme to acetaldehyde and 

 carbon dioxide, undergoes synthesis to diketoadipic acid, which then 

 breaks down to succinic acid and formic acid, from the former of which 

 malic acid arises. 



Disappearance of malic acid is ascribed to its oxidation to oxalacetic 

 acid, the conversion of this to pyruvic acid, and the breakdown of the 

 latter on removal of the inhibitor of carboxylase. 



Bennet-Clark has pointed out a number of objections to this theory, 

 among which perhaps the most important are that formic acid does not 

 accumulate in the tissues, while, so far from succulent plants containing 

 a high concentration of acetaldehyde, the concentration of this substance 

 in succulents is very low (o-oi to o-ooi %), and is, in fact, too low to 

 have any appreciable inhibiting effect on carboxylase. 



From his own researches and a critical consideration of the work of 

 others, Bennet-Clark has shown that for each molecule of sugar which 

 disappears from succulents not more than one molecule of malic acid is 

 formed, so that for every molecule of sugar which is lost by glycolysis at 

 least two atoms of carbon must be involved in the formation of some 

 other material. This is not carbon dioxide, and in fact, no carbon 

 compound with one, two, or even three carbon atoms in the molecule 

 accumulates in the tissues, and Bennet-Clark therefore concludes that 

 the carbon compound formed from glycolysis along with malic acid 

 must be built back to polysaccharide. The carbon dioxide evolved by 



