230 Cytoplasm as Seat of Genetic Properties 



and that also the chloroplasts contain oxidizing enzymes. They claim 

 further that so-called plastid mutation (cases of variegation) produces 

 a derangement of the enzyme systems which becomes visible in the 

 mitochondria as well as in the plastids. Assuming the correctness of 

 the findings, they might, however, be explained in more than one way. 

 Another opinion is that developing plastids go back to granules 

 which look exactly like mitochondria but are different from the begin- 

 ning. Others maintain that they have demonstrated such a difference 

 by staining reactions. Further claims are that the primordia of plastids 

 go down in size to the limits of visibility and beyond that into the 

 molecular level. If the latter is true, the plastids may be actually 

 autonomous but changing from a molecular unit to a visible granule 

 to a large disc of complicated chemical constitution (permitting all 

 the steps of photosynthesis). If this is so, we might compare the 

 plastid, for description's sake only, to an organism and its develop- 

 ment from primordia to the development of an organism from an egg, 

 the egg in this simile being the equivalent of the invisible molecular 

 plastid primordium. In favor of this view is the fact that plastids may 

 be affected by antibiotics and then cease multiplying! (see Ephrussi, 

 1953 ) . A parallel to the vegetative phase of phage could be established, 

 especially since it has been maintained that chloroplasts contain DNA 

 as well as RNA (Metzner, 1952). 



This brief survey already shows that the morphological facts do 

 not permit us to decide upon the nature of the plastids, though it looks 

 as if they were autonomous, seff-reproducing, organism-like structures 

 with a kind of life cycle, but certainly not cytoplasmic differentiations. 

 The electron microscope reveals a disc-shaped structure with different 

 layers, which at present does not help the understanding, though it 

 may have a special meaning for the process of photosynthesis. We 

 mentioned already that it has been maintained that plastids contain 

 DNA. But the experts on staining reactions are not convinced that 

 these claims are valid, and it is therefore indicated that no conclusions 

 should be based upon the statements. Conclusions concerning the 

 nature of the plastids and their possible relation to cytoplasmic in- 

 heritance will have to be reached mainly from a study of the genetical 

 facts, as there is clearly an interrelation between plastid behavior and 

 genetic constitution. 



Correns ( 1937 ) states that the decisive difference between plastids 

 and symbionts is that the behavior of the plastids is controlled in the 

 majority of cases by the genie material in the chromosomes. The 

 simplest case is, of course, given when differences in the quantity of 



