434 FURTHER EVOLUTION 



in green plants are no different in principle from those 

 found in bacteria and fungi. H. Gaffron and H. Michels^^^ 

 both showed that the unicellular green alga Chlorella forms 

 lactic acid from glucose under anaerobic conditions. H. 

 Gaffron and J. Rubin^^^ showed that, under anaerobic condi- 

 tions, pure cultures of Scenedesmus give off CO2 and accumu- 

 late non-volatile organic acids, in particular lactic acid. In 

 connection with the extensive studies of photosynthesis 

 carried out by M. Calvin and his colleagues^ ^^ on the one 

 hand and by Gaffron and his group^^° on the other, it has 

 been shown that there are present in the cells of the green 

 algae Scenedesmus and Chlorella such important products 

 of the anaerobic breakdown of glucose as phosphoglyceric 

 acid, phosphopyruvic acid and hexose and triose phosphates. 



In recent years an enormous amount of evidence has been 

 collected showing that the glycolytic system of Embden and 

 Meyerhof, which is found in higher plants, takes part in the 

 synthesis as well as in the degradation of carbohydrates. All 

 the enzymes concerned with alcoholic fermentation have 

 been found in higher plants and some have been isolated 

 in a purified state. ^^^ For example, coenzyme II (TPN) 

 (triphosphopyridine nucleotide) has been found in various 

 leaves and also in potato tubers. Hexokinase, an enzyme 

 mediating the use of high-energy bonds, has been found 

 in spinach leaves. Wheat grains and the seeds of other plants 

 have been shown to contain oxoisomerase, and so forth. 

 Intermediate products of glycolysis such as acetaldehyde, 

 ethyl alcohol and lactic acid were found long ago in the 

 tissues of higher plants when they are made to live under 

 anaerobic conditions. ^^^ 



The striking uniformity of the glycolytic mechanisms 

 which underlie energy metabolism is found by investigation 

 to prevail among animals from the simplest flagellates to the 

 higher mammals and man. On the basis of studies of various 

 zoological types, a number of scientists have expressed the 

 opinion that the process of respiration of oxygen, which plays 

 such an important part in the animal world, is of relatively 

 recent phylogenetic origin. It represents a specialised 

 mechanism which has arisen in the course of evolution on 

 the basis of the more ancient, universal mechanism for the 



