CHROMOSOMES AND MENDELIAN HEREDITY 307 



fuse to form a diploid zygote (oospore or zygospore). When the zygote 

 germinates, the chromosome number is at once reduced, the monoploid 

 products of meiosis (here represented as spores) giving rise to the indi- 

 viduals of the next generation. The cycle is the same in the bryophytes, 

 except for the important fact that the zygote germinates to form a diploid 

 sporophyte in which meiosis is delayed until sporogenesis (Fig. 217). 

 In the body of a haplont or of a moss gametophyte, therefore, only one 

 member of each chromosome pair is present in each nucleus rather than 

 both members as in the diploid bodies of animals and plant sporophytes. 



The expectations based on the chromosome theory of Mendelism are 

 as follows. Two contrasting " allelomorphic " characters of the monoploid 

 individuals introduced into a zygote by crossing should segregate at once 

 in the next generation in the ratio of 1 :1. In these individuals, therefore, 

 there should be no "dominance," since only one factor of a given pair is 

 present. Every factor should have an opportunity to produce its full 

 effect upon the characters. With respect to a given pair of characters, 

 diploid individuals show one genotype in Fi (after pure types are crossed) 

 and three in F2, whereas monoploid ones show two in every generation. 



In the moss genera Funaria and Physcomitrium, von Wettstein^"* 

 studied the progeny of crosses between types differing in various pairs of 

 contrasting gametophytic characters and found that such characters 

 segregate 1:1 in the next generation. For example, when two types 

 differing in leaf form were crossed, the spores from the hybrid capsule 

 produced gametophytes of the two parental types in equal numbers. 

 The sporophytic characters (form and color of capsule) were inherited 

 as would be expected in diploid individuals. Von Wettstein was able 

 to show that the segregation of the factors for the gametophytic characters 

 takes place in each sporocyte: two spores of a quartet produce one type 

 of gametophyte and two produce the other. This clearly indicates a 

 segregation in one of the meiotic divisions. 



In the liverwort, Sphcerocarpos Donnellii, C. E. Allen^^ has shown that 

 the gametophytic character "polycladous" segregates from its allelo- 

 morph "typical" nearly 1:1 in the first gametophyte generation after a 

 cross, and that two spores of a quartet produce one type and two the 

 other. The same holds for the character "tufted." Both characters are 

 independent of sex, which had formerly been shown to be inherited in the 

 same manner (p. 382). Hence the spores produced by a single hybrid 

 sporophyte give rise, for example, to tufted males, tufted females, typical 

 males, and typical females. Ordinarily the spores of a single quartet 

 are of only two of these types, but Allen found a few exceptional cases in 



1* Von Wettstein (19216, 1923, 1924a6). Literature on heredity in bryophytes is 

 reviewed in 1924c. Later papers (1926, 1927, 1928, 1930) deal largely with heredity 

 in polyploid mosses and the hereditary role of the cytoplasm. 



1^ C. E. Allen (1924a6, 1925a, 1926a6c, 1930a6). For a general account, see 1930a. 



