54 AN INTRODUCTION TO MODERN GENETICS 



fore be impossible. Thus in the haploid phase we can observe the effects 

 of segregation uncomplicated by those of fertilization. From a cross 

 between two different gametophytes of, for instance, a moss, in which 

 the haploid and diploid phases are of more or less equal importance, 

 we may expect to, and do, obtain a hybrid sporophyte; but Fi gameto- 

 phytes which are formed from this hybrid are each of them "pure," 

 that is, of each pair of contrasting characters they show one only. This 

 is a demonstration that heredity is particulate and not blending, as the 

 pre-Mendelians thought; each gene retains its individuaUty without 

 contamination by any other character which may happen to be associ- 



Fig. 19. Germination of a 

 Spore Tetrad in the moss 

 Funaria hygrometrica. — The 

 haploid spores were derived 

 from a sporophyte hetero- 

 zygous for the factor Gg 

 which causes rapid growth. 

 Two spores are fast-growing, 

 and have already sent out a 

 long and a short thread, 

 while the other two slow- 

 growing ones have only 

 formed one medium-sized 

 thread each. 



(After Wettstein.) 



ated with it in a hybrid. New combinations of factors may of course 

 be formed in the gametophytes, and give assemblages of characters 

 which, however, are not blends but are mixtures of units from one 

 parent with units from another. 



The inheritance of haploid phase characters has been followed in 

 most of the orders of plants where it is to be expected.^ Particularly 

 interesting results have been obtained in mosses and some fungi, 

 where it has been possible to isolate the four spores formed by the 

 reduction of one diploid cell in a sporophyte and to show that, as 

 regards any one character, they germinate into two gametophytes of one 

 allelomorphic type and two of another. Similar tetrad-analysis of spores 

 from doubly heterozygous sporophytes is dealt with later. 



It should be noted that all the factors known in plants with a more 

 or less equal alternation of generations affect only one of the two 



^ General, Sansome and Philp 1932; Ferns, Anderson-Kotto 193 1; Bryo- 

 phytesy Allen 1935; Fungi, Dodge 1936, Kniep 1929. 



