418 INTRODUCTION TO CYTOLOGY 



pronounced differential influence than is usually the case. It remains for 

 future studies to reveal the nature of these extra-nuclear elements or 

 conditions and to demonstrate their location with respect to the plastids 

 manifesting the characters for which they are responsible. 



Mosses. — In bryophytes and vascular plants most if not all of the 

 cytoplasm of a given individual is derived from the mother, since the 

 male gamete brings little if any cytoplasm into the egg at the time of 

 syngamy. Hence it should be possible to detect any differential effect 

 of the cytoplasm upon inherited characters by comparing the results of 

 reciprocal crosses between unlike individuals. When the cross is made 

 in one direction the cytoplasm is contributed by one parent, while in 

 the reciprocal cross it is derived from the other; the nuclear constitution 

 after both crosses is the same. In general, it is found that reciprocal 

 crosses between races or Mendelian forms within the same species give 

 similar progeny, which is taken to mean that the cytoplasm is alike in the 

 parents, or that any slight differences are neutralized or dominated by the 

 nucleus in the offspring and therefore exert no differential influence upon 

 characters.^ In interspecific and intergeneric crosses, on the other hand, 

 it is often found that cytoplasmic differences are not thus neutralized, so 

 that it becomes possible to relate these to certain differences in character 

 exhibited by the reciprocal offspring. 



These principles are well illustrated by extensive researches on mosses 

 carried out by F. von Wettstein (1926, 1928, 1930). When two races of 

 Funaria hygrometrica are crossed, the reciprocals^ are alike; hence the 

 cytoplasms of these races are similar so far as genetic effects are con- 

 cerned. When F. hygrometrica is crossed with F. mediterranea, and 

 especially when it is crossed with the more distantly related Physcomi- 

 trium piriforme, the reciprocals differ and tend to be almost wholly like 

 their respective mothers in certain characters. Furthermore, such 

 differences persist through repeated back-crossing to the paternal species, 

 indicating that they are related to some stable element in the cytoplasm 

 which does not become altered by the paternal nuclear elements. 



This genetic element always present in the cytoplasm is called by 

 von Wettstein the plasmon.^ Hence the entire genetic complex consists 

 of factors in the chromosome set (the genom) and the plasmon. In the 

 interspecific crosses it is found that the relative effects of these two ele- 

 ments are not the same for all character differences. Thus paraphysis 

 form appears to be controlled wholly by the genom, the shape of the leaf 



^ Chlorophyll characters treated in the preceding section are not to be included 

 here. 



^ I.e., the gametophytes produced by spores from the reciprocal capsules. Certain 

 sporophytic characters have also been studied. 



8 Correns (19286) has employed Strasburger's term "cyto-idioplasm." It does 

 not include the results of previous nuclear influence nor ergastic materials. 



