308 INTRODUCTION TO CYTOLOGY 



which all four were present. Whether this should be attributed to an 

 occasional disjunction of two chromosome pairs in different meiotic 

 mitoses or to some other cause is not yet clear. 



In Spirogyra, a typical haplont, three of the four nuclei formed at 

 meiosis in the zygospore degenerate. Hybrids between species are of 

 the two parental types in equal numbers, so far as any one pair of char- 

 acters is concerned. When two pairs of characters are considered, the 

 hybrids are of four types; when three are considered, they are of eight 

 types. These results are in accord with expectations based on the 

 independent assortment of factors and the equal chance of survival of the 

 four meiotic nuclei (Transeau, 1919). 



The pollen grains of angiosperms, since they normally belong to the 

 reduced phase of the life cycle, may be expected to show a 1 :1 segrega- 

 tion of parental pollen characters in each generation if such characters 

 are due primarily to the constitution of the monoploid nuclei in the grains. 

 Such a segregation occurs in a number of known instances. ^^ Contrasted 

 with these are certain other cases ^^ in which pollen characters (chiefly 

 color and shape) show dominance and behave as sporophytic characters, 

 which is not surprising in view of the fact that the grains are developed 

 in the midst of diploid sporophytic cells (tapetum, etc.) and are known 

 to make use of materials derived from them. Color, for example, may 

 be due to substances produced in the sporophytic cells and deposited in 

 all the spore coats irrespective of the genetic constitution of the proto- 

 plasts they enclose. Pollen may therefore exhibit "sporophytic" as 

 well as "gametophytic" characters, owing to the intimate association of 

 the two phases of the life cycle. 



The above results of researches on monoploid plants afford definite 

 confirmation of the chromosome theory of Mendelian heredity as origi- 

 nally developed through studies on diploid organisms. Very striking 

 additional confirmation will be brought out in later chapters dealing with 

 unusual types of chromosome behavior. 



The Gene Theory. — It is only for the sake of simplicity that Mende- 

 lian characters have been treated as if they might be due to single genes, 

 for this is far from what is actually assumed on the gene theory. It is a 

 well-established fact that the development of a given character involves 

 the complemental action of two or more pairs of genes; indeed, it is 

 doubtless due to the interaction of a very large number of genes, known 

 and unknown. When a single pair of genes is singled out for study in a 

 cross, it is done with the understanding that other genes affecting the 

 results are similar in the organisms used; in other words, the one pair is 



^^(Enothera (Renner, 1919a6), Oryza (Parnell, 1921), Zea (Demerec, 1924; Brink 

 and MacGillivray, 1924; Longley, 1925). 



'^ Epilobium, Papaver, and Geranium (Correns, 1900/), 1902\ Lathyrus (Bateson, 

 Saunders, and Punnett, 1905), Nicotiana (Thomas, 1913). See Correns (19286). 



