396 INTRODUCTION TO CYTOLOGY 



gametophytes would be expected to have no effect upon the sex expres- 

 sion, since the ratio would not be altered. 



Data such as the foregoing are still rather few, and generalizations 

 should therefore be tentative. At the same time they show clearly that 

 the development of the sexual states is a resultant of the activities of 

 both sex-chromosomes and autosomes. They also suggest one reason 

 for the rarity of polyploidy among animals as compared with plants 

 (Muller, 1925). In animals, which are so commonly unisexual with well- 

 differentiated sex-chromosomes, the addition of a set of chromosomes 

 seriously disturbs the normal X: autosome ratio, and hence development 

 and fertility, in the heterozygous sex. This offers a serious obstacle to 

 the establishment of triploid and other races with odd numbers of chromo- 

 some sets. On the other hand, in bisexual plants there is no such critical 

 ratio to be disturbed, so that triploid and pentaploid races are not 

 uncommon. The origin of tetraploid races directly from diploids in 

 animals may be expected occasionally through a duplication of all the 

 somatic chromosomes, since such duplication does not alter the X : auto- 

 some ratio. Noteworthy in this connection is the fact that heterophytic 

 plants tend strongly to remain diploid, or, when polyploid, to show only 

 even numbers of chromosome sets; whereas homophytic plants exhibit 

 very freely all types of polyploidy, with odd as well as even numbers of 

 sets. There is also evidence for the view that polyploid races in the 

 angiosperms tend to maintain their distinctness because of disturbed 

 relations between embryo, endosperm, and surrounding tissues in back- 

 crosses to the original parents.*^ 



Modification of Sex. — Field taxonomists and experimental workers 

 are well acquainted with the frequent intergrades between the purely 

 staminate and purely carpellate conditions in normally heterophytic 

 flowering plants, and with the changes in sex expression which may be 

 induced in both heterophytic and homophytic forms by appropriate 

 alterations in the environmental conditions.^'' The effects of age, nutri- 

 tion, length of day, and other factors have been investigated and shown 

 to be very marked in a number of plants, including Cannabis sativa, 

 Morus alba, Salix amygdaloides, Humulus japonica, Thalictrum dasycar- 

 pum, and Ariscema Dracontium. 



In only a few such cases are the cytological conditions known. In 

 Cannabis, which is normally heterophytic and is reported to have XY 

 sex-chromosomes,^^ the sex ratio is ordinarily 19 :lcf. When pollen 

 from anthers appearing on a carpellate plant was transferred to the pistils 

 of a normal carpellate plant, all of the offspring were carpellate. This 



« See Miintzing (1930, 1933), W. P. Thompson (1930a?>), and Watkins (1932). 

 "•See Schaffner (1919 et seq.); also Correns (1908), Bartlett (1911), Davey and 

 Gibson (1917), Stout (1919), and Yampolsky (1919, 1920). 

 61 Hirata (1924, 19276), Sinoto (1928). 



