CYTOLOa/CAL ASPECTS OF IIYBRIDITY 225 



ill nionoploid algae and in bryophytc gametophytes which exhibit com- 

 binations of characters from two unlike parents. (7) A nucleus from 

 one species in cytoplasm of another species, the cytoplasm carrying a 

 nongenic element having a distinct and persistent effect upon characters: 

 cases of "cytoplasmic inheritance" described in the next chapter. (8) 

 There is no hybridity within the protoplast, yet two geneticall}^ vmlike 

 kinds of protoplast are so intimately associated in a t>ody that both share 

 in determining its characters. 



Examples of the last condition in the foregoing list may be given here. 

 In a chimera two genetically unlike tissues together constitute an indi- 

 vidual plant as a result of local somatic mutation or of a graft involving 

 two species. In periclinal chimeras, which have one type of tissue over- 

 lying the other like a glove over a hand, the plant may combine characters 

 of the two species involved. The form of the leaf, for example, may be 

 determined by the inner component, while the character of its surface 

 is that of the outer component. Sometimes the characters of the oviter 

 component are apparently affected by the genotype of the inner com- 

 ponent. The chimeral condition may be reproduced vegetatively but not 

 sexually, since the spores are developed solely by one component or the 

 other — normally the one constituting the subepidermal cell layer. 

 Another example of such cellular association is afforded by certain slime 

 molds in which the numerous ameboid cells do not lose their boundaries 

 when they unite to form a pseudoplasmodium from which fruiting bodies 

 (sorocarps) develop. When pseudoplasmodia of two species are thor- 

 oughly mixed and grown under certain cultural conditions, the mixture 

 produces not only sorocarps of the two specific types but also some com- 

 bining in various ways the characters of the two species. An extreme 

 example of the association of unlike protoplasts is seen in lichens, whose 

 l)odies are made up of a fungus and an alga living in symbiotic union. 



This section is included in the chapter not to confuse our conceptions 

 of hybridity, but rather in the hope of supplying them with a broader 

 basis. The physical basis of heredity shows a striking fundamental 

 similarity throughout practically the entire organic world, yet it has 

 several variants in different groups of organisms. Cytogenetical 

 researches are being extended into more groups as time goes on. Hj^brid- 

 ity, the presence of unlike genetical protoplasmic elements from different 

 sources, exists in some degree nearly everywhere, but if the investigator 

 expects the physical mechanism of inheritance to be of exactly the same 

 standard type in every organism he will meet many puzzles. It has 

 sometimes happened that an elaborate and ingenious hypothesis has been 

 formulated to account for aberrant genetical data when an awareness of a 

 peculiar cytological or histological condition in the organism would have 

 suggested a much simpler explanation. 



