Apomixis 513 



With certain gene combinations other chromosomes might be- 

 come the sex differential and therefore the "sex chromosome," 

 and in some particular gene combinations the female might be- 

 come the heterogametic sex. Emerson has developed two dioe- 

 cious strains ; in one the male and in the other the female is the 

 heterogametic sex. Although there is no assurance that dioecious 

 species arose in this manner throughout the course of evolution, 

 it is quite conceivable that they may have. It would also indi- 

 cate why certain organisms happen to be of the XY type whereas 

 others are of the ZW type. Jones has founcf that all his families 

 of the dioecious strain produce some individuals with intersexual 

 tendencies; tassel seed plants may possess an occasional func- 

 tional anther and silkless individuals may have a rare functional 

 female flower. The various families differ with respect to the 

 number and character of these intergrades and respond to selec- 

 tion. It is very probable that these factors which are known 

 to be hereditary are of the nature of modifying genes. 



Apomixis 



As we pointed out in Chapter 4, all reproduction is not by 

 seeds, nor does it involve sexual reproduction. Asexual repro- 

 duction that does not involve nuclear or cellular fusion and has 

 arisen as a substitute for sexual reproduction has been called 

 apomixis. The two main types may be classified, following Fa- 

 gerlind and Stebbins, as vegetative apomixis and agamospermy, 

 or apomixis through seed production. The first type does not in- 

 clude all vegetative reproduction but only those types that arise 

 as substitutes for sexual reproduction. Various structures such 

 as vegetative buds, bulblets, or proliferations would be included 

 if they arose in place of flowers or flower clusters. Agamospermy 

 is of three types. In one form, meiosis is normal, and a haploid 

 egg is produced from which develops a haploid sporophyte. It 

 most nearly resembles the situation we have mentioned in ani- 

 mals. In another type the nucellus or inner integument of the 

 ovary proliferates and forms a new sporophyte. Since the nucel- 

 lus and integument are part of the parental sporophyte, this type 

 is a direct formation of a new sporophyte from the somatic tis- 

 sues of the old. This type differs from vegetative apomixis in 

 that it involves a structure connected with the seed and some- 

 times is dependent upon, although not utilizing directly, fertiliza- 



