The Cytoplasm as Specific Substrate 231 



chlorophyll per plastid, or differences in shape and chemistry of the 

 starch formed, show simple Mendelian behavior. For the present 

 discussion, the most important examples are found in the group of 

 variegations based upon chlorophyll-bearing plastids. Variegation may 

 often be the result of ordinary Mendelizing mutation, but the chief 

 interest is centered upon less simple facts. 



For the interpretation it is also of importance to know whether 

 the pollen transfers plastids ( or leucoplasts ) into the egg at the time of 

 fertilization. The overwhelming majority of cytological and genetic 

 evidence is not in favor of this. But we mentioned before some facts 

 (II 2 C a) which lend themselves to the interpretation that a little 

 sperm cytoplasm enters the egg. If leucoplasts may vary down to sub- 

 microscopic dimensions, as is generally accepted, the possibility cannot 

 be denied that the pollen nucleus carries with a cytoplasmic sheath 

 some leucoplasts into the egg. We shall report below that genetic facts 

 exist which require this. 



bb. Plastids and surrounding cytoplasm 



The first group of facts which led to very different interpretations 

 in the discussion carried on over almost fifty years since Correns' ( 1903, 

 1909; see 1937) basic work, relates to what Correns called the "status 

 albomaculatus." Plants in this condition show a mosaic of more or less 

 large green and white spots. In the cells of the white sectors the 

 plastids are colorless and also degenerated. Flowers of pure green 

 branches produce only green offspring with whatever pollen; those 

 from pure white branches, only white offspring. The inheritance is 

 purely maternal. Mixed branches produce white, green, and mixed 

 offspring. 



Baur (1909) had proposed the explanation that the white (or 

 "sick") plastids were derived from normal ones by mutation, that the 

 egg cells contained both types, and that during development they 

 were sorted out by chance. The mixed condition might be the result 

 of fertihzation involving transmission of plastids by the pollen. Correns 

 proved experimentally that this does not happen. Thus the egg cells 

 must already have contained both types. If this is so, Correns argued 

 correctly, mixed vegetative cells should be frequent, especially at the 

 borders of white and green sectors. Actually, they are absent or rare, 

 and may require a different explanation. Correns showed then that an 

 arithmetical study of the consequences of the chance sorting out of 

 two types at cell division leads to results which do not agree with the 

 actual structure of albomaculatus. Correns therefore proposed a dff- 



