408 FURTHER EVOLUTION 



adapted to the autotrophic way of life). Only those cells in 

 them which contain chlorophyll possess the chemical mechan- 

 ism for photosynthesis. It is in them alone that there occurs 

 that primary synthesis of organic substances which are used 

 as nutrients by all the rest of the colourless tissues of the 

 plant. These are nourished in a purely heterotrophic way 

 just as fungi are nourished by the addition of sugar to the 

 culture medium. The leaves, too, are nourished in this same 

 way in the absence of light. 



Thus, the metabolism of the plant as a whole is based 

 on a heterotrophic mechanism using organic substances as 

 nutrients although, in its green tissues, this mechanism is 

 combined with an additional specific apparatus whose func- 

 tion is to supply the whole organism with ready-made organic 

 substances. If the plant is supplied in some way with such 

 substances from without, it can exist even without its photo- 

 synthetic apparatus. This takes place under normal natural 

 conditions, in particular during the germination of seeds. It 

 can be demonstrated experimentally by, for example, raising 

 a whole adult plant of the sugar beet in the dark from a 

 one-year-old root. Finally, it may also be observed in cases 

 where higher plants have lost their ability to synthesise owing 

 to having become parasitic, e.g. in broomrapes.^* 



In all these cases the plant lives and nourishes itself on 

 exogenous organic substances while its photosynthetic auto- 

 trophic apparatus is completely inactive. But if even one link 

 of the enzymic chain of heterotrophic metabolism is dis- 

 rupted, all the vital activities of the plant cease and it is 

 destroyed. This may be observed, in particular, during the 

 specific poisoning of phosphoglyceraldehyde dehydrogenase 

 \s ith monoiodoacetic acid or of enolase with sodium fluoride. ^^ 



Hence it is quite clear that the vital processes of photo- 

 autotrophs, including the higher plants, are based on the 

 primary and ancient heterotrophic form of metabolism while 

 the ability to synthesise organic substances by using the 

 energy of light only arose in them as an accessory apparatus 

 on this basis. 



The situation is less clear in regard to the chemoauto- 

 trophs, mainly because their metabolism is, as yet, very little 

 studied in comparison with that of other organisms. Even 



