The Cytoplasm as Specific Substrate 229 



Weier and Stocking, 1952). A discussion from the point of view of 

 particulate cytoplasmic inheritance must start with an appraisal of 

 the nature of the plastids. Plastids as the seat of chlorophyll, xantho- 

 phyll, and so on are the seat of the autotrophic metabolism of plants 

 synthesizing starch by photosynthesis. The localization of this property 

 in definite bodies, included in the cytoplasm, is certainly a very specific 

 feature and not a general protoplasmic one. Thus it is understandable 

 that some students distinguished the totality of plastids as a plastidome 

 from genome and plasmon to which nowadays could be added also 

 the chondriome (Dangeard, Renner). If this distinction is correct, the 

 genetic behavior of the plastids would be a problem outside that of 

 cytoplasmic inheritance. This is not a purely academic or semantic 

 question. It is well known that a number of animal species in Protozoa, 

 Hydrozoa, Platyhelminthes, and Mollusca have returned to purely or 

 partial autotrophic metabolism by incorporating in their cells sym- 

 biontic unicellular algae. This suggests the possibility, greatly enhanced 

 by the recent work on symbiosis by the Buchner school, that the 

 plastids may be considered as symbionts of the plant cells, that is, 

 dependent organisms like parasites and viruses. Many biologists are 

 favorable to such a view which, however, can neither be proved nor 

 disproved at present. But the possibility that it may be true makes us 

 very cautious when it comes to treating plastid behavior as a phenom- 

 enon of cytoplasmic inheritance or even as an example of plasma- 

 genes. 



There is still no consensus regarding the cycle of the plastids. 

 There is no doubt that plastids, once seen, multiply by division, as do 

 centromeres and kinetosomes (see II). We may call this a property 

 of self-duplication, though it seems rather formalistic to identify it with 

 the self-duplication of genie material. Unfortunately, there is no final 

 information in regard to the autonomy of the plastids. Some cytol- 

 ogists believe that plastids can be formed de novo, but more fre- 

 quently it is assumed that they are products of transformation of 

 mitochondria ( Guillermond ) . There is a primary probability that this 

 is true in view of the fact that both structures have to do with the 

 respiratory function (in the widest sense) of the cell. Mitochondria, 

 like plastids, are self-duplicating. Some authors (see DuBuy, Woods, 

 and Lackey, 1950) are very outspoken in asserting that "the chloro- 

 plasts of higher plants derive from the mitochondrial elements of the 

 cell and possess extranuclear hereditary entities." They base this 

 definite statement on their work, showing that plant mitochondria 

 like those of animals contain all the enzymes of the oxidation system 



