388 INTRODUCTION TO CYTOLOGY 



individuals. Unfortunately such species have been said to be "hetero- 

 thallic" along with those showing true sexual dioecism, thus creating a 

 confusion which should be clarified. 



It is evident, therefore, that the problem of sex in fungi is compli- 

 cated, as it is in higher plants, by factors which render certain bisexual 

 individuals or strains intersterile. While the differentiation into plus 

 and minus strains is plainly of this nature in the instances cited above, 

 there is evidence^^ that in others it may be truly sexual. A full under- 

 standing of the situation can scarcely be reached until much more has 

 been learned about the physico-chemical nature of the sexual states 

 themselves and about the factors by which they are controlled. 



Sex-chromosomes in Angiosperms. — There has long been reason to 

 believe that the heterophytic (dioecious) condition in flowering plants 

 is related to a qualitative difference among the pollen grains. Correns 

 (1907) concluded from his work on Bryonia that the eggs all have a 

 female sex " tendency " ; that there are two kinds of microspores, and hence 

 male gametes, with male and female tendencies respectively; and that 

 the male tendency dominates the female, so that half of the sporophytes 

 are staminate and half of them carpellate. Strasburger (1910a), who 

 worked with Elodea, Melandrium, and Mercurialis, assumed that the 

 tendency of all the pollen grains, and hence male gametes, is toward 

 maleness: some have a "strong" male tendency which dominates the 

 female tendency of the egg, staminate plants being so produced, while 

 others have a "weak" male tendency which is dominated by the female 

 tendency of the egg, carpellate plants so resulting. 



The view that the pollen grains are of two kinds with a differential 

 effect upon the sex of the offspring received cytological confirmation in 

 1923, when several species of heterophytic angiosperms were shown to 

 have heteromorphic sex-chromosome pairs. ^^ Not all heteromorphic 

 pairs in higher plants, however, are sex-chromosomes. 



Sex-chromosomes in heterophytic angiosperms are most commonly of 

 the XY type. In Elodea canadensis (Fig. 222), the first known case in 

 vascular plants, the microsporocytes show 24 pairs, including an unequal 

 XY pair. All of the pairs disjoin in the first division, and the second 



28 Chemical researches on strains of phy corny cetes (Satina and Demerec, 1925; 

 Satina and Blakeslee, 1925 et seq.). Vandendries and Brodie (1933) report evidence 

 for the view that repulsion between certain strains of a basidiomycete, Lenzites betulina, 

 is due to a radiation under the influence of a Mendelian factor pair and not to chemical 

 interaction. 



2» For lists of known cases, see Kihara (19296) and Sinoto (19296). Correns 

 (1928a) reviews the subject of sex in higher plants. Yampolsky (1922) lists the types 

 of sexual manifestation in angiosperms. Lindsay (1930) gives a list of heterophytic 

 plants whose chromosomes have been studied. Lindsay (1929) and Cooper (1929) 

 describe a heteromorphic pair in the homophytic Bougainvillea. For a discussion of 

 the application of terms implying sexuality to sporophytes, see Sharp (1925). 



