416 INTRODUCTION TO CYTOLOGY 



(Baur, 1909, 1919). This form, which is characterized by white-mar- 

 gined leaves, often has pure green and pure white branches, as does 

 Mirahilis. Crosses either way between flowers on these two kinds of 

 branches may result in variegated offspring; inheritance is here not 

 purely maternal as in Mirahilis.'^ 



Theories of Chlorophyll Inheritance. — It has been suggested that 

 there are two types of non-Mendelian chlorophyll inheritance, biparental 

 and maternal, because the male gamete in some cases introduces cyto- 

 plasm into the egg while in others it does not. Maternal inheritance 

 would also occur if the male cytoplasmic elements were functionless after 

 entering the egg. Such explanations do not seem improbable in view of 

 the available descriptions of syngamy in angiosperms (p. 236), but 

 nothing can be stated with assurance until much more has been learned 

 concerning the history of cytoplasm and cytoplasmic inclusions through- 

 out the life cycle. 



With regard to the " albomaculata type" of variegation (la), it is 

 the hypothesis of Correns that the condition is due to a cytoplasmic 

 disease which prevents the normal development of the chloroplasts. It 

 is therefore delivered directly to the next generation through the egg 

 cytoplasm and is not transmitted by the male parent because the male 

 gamete introduces no cytoplasm into the egg. If it were due solely to 

 nuclear factors, it would be transmitted equally by both parents, since 

 the nuclear contributions of the two are equivalent. The variegation 

 is the result of different reactions in the unstable affected cytoplasm 

 during the development of the tissues in various portions of the leaf. 

 Winge contends that in types giving greens, whites, and variegateds (la) 

 the defect is in the plastids themselves, whereas in those giving rise 

 to variegateds only (16) it is in the cytoplasm in which the plastids lie. 

 For the " albomarginata type" (2) Baur holds that the appearance of 

 green and white tissues is due to a sorting out of two distinct kinds of 

 plastids or their primordia, green and white, as the cells multiply by 

 division.^ 



That variegation may be due to nuclear genes is suggested in the 

 hypothesis of Demerec (1927), who assumes the presence of a gene which 

 is highly mutable when associated with the cytoplasm of another plant, 

 and also in the hypothesis of Eyster (1928), who postulates a gene 

 composed of a number of "genomeres" assorting at random as the gene 



^ Other cases: probably (Enothera (Stomps, 1920; see Renner, 19246, on hybrids); 

 probably Viola (J. Clausen, 1930a); Hypericum perforatum (Correns, 1931). A blotch- 

 ing of the leaves in Avena is inherited in a similar manner (Ferdinandsen and Winge, 

 1930). 



^ This hypothesis is adopted by Gregory (1915) for Primula, E. G. Anderson (1923) 

 for Zea, Clausen (19276) for Viola, and Yasui (1929) for Hosta. These are examples 

 of class la. 



