The Cytoplasm as Specific Substrate 215 



interpretation. In this connection also the httle-known work of 

 Haberlandt (1935) on plant chimeras (Crataegomespilus) should be 

 mentioned. In periclinal chimeras containing the epidermis of only 

 one of the species, he found epidermal structures that were inter- 

 mediate between the species. In this case cytoplasm can hardly be 

 involved; instead, diffusible cytoplasmic products like auxins would 

 seem to account for the effect. Though not strictly concerned with 

 cytoplasmic inheritance, the facts suggest caution. 



The most important experiments in merogony, as far as our 

 present discussion is concerned, are those of Harder (1927) on 

 basidiomycetes. By a very intricate technique, made possible by 

 unique features of the apical cells of mycelia, hybrids between two 

 strains of Pholiota mutabilis could be produced containing a mixture 

 of the cytoplasms of the two strains with the haploid nucleus of 

 one. The mycelia obtained from these cells showed intermediate 

 growth habits similar to those of a real (nuclear) hybrid. Again the 

 same interpretation should be given as in the other cases: quanti- 

 tative shifts in genically controlled reactions owing to a foreign sub- 

 strate, combined with threshold actions. 



It is rather surprising that the cytoplasmic substrate differences 

 studied are in a number of cases typical for the lowest taxonomic 

 categories. In the pigmentation of Lymantria caterpillars, the cyto- 

 plasmic differences were found in crosses of all geographic races 

 which were sufficiently different for detection of the feature. In 

 Michaelis' Epilobium work this goes still farther. Here the cyto- 

 plasmic difference goes down even to otherwise indistinguishable local 

 communities. This means that a cytoplasmic evolutionary divergence 

 exists, which is even more sensitive than genie differences. If it 

 should turn out eventually that the well-founded suspicion that the 

 mitochondria are involved in all this is an actual fact, interesting new 

 problems of evolutionary divergence would arise. Formerly I doubted 

 ( Goldschmidt, 1940, p. 250) that the cytoplasm is a major factor in 

 evolution. The facts alluded to suggest caution for the time being. 



D. GENUINE PLASMON 



In the introduction to the last chapter it was pointed out that a 

 distinction should be made between a cytoplasmic influence upon 

 genically controlled processes which shifts the latter in a quantitative 

 way, and an actual cytoplasmic heredity, a genuine plasmon, which 

 by itself determines definite hereditary traits in the same sense as 

 genes are supposed to control characters. Such a distinction is a 



