APOMIXIS 323 



undergo a further division with or without the formation of a cell 

 wall. The embryo arises from the unfertilized but diploid egg cell. 



An essentially similar condition occurs in the guayule, Parthenium 

 argentatum (Esau, 1946). This species comprises two races, one 

 with 36 chromosomes and the other with 72. The former is mainly 

 sexual and the latter mainly apomictic. Since the condition is not 

 absolutely fixed, the apomixis may be regarded as facultative. In 

 the 36-chromosome race there is usually an orderly sequence of 

 events in megasporogenesis (Fig. 184A-C) and gametophyte de- 

 velopment. Megaspore mother cells are seen in various phases of 

 meiosis, megaspore tetrads are abundant, and uninucleate embryo 

 sacs are associated with the crushed remnants of the nonfunctioning 

 megaspores. Views of pollen tubes in embryo sacs, followed by 

 syngamy and triple fusion, are common in artificially pollinated 

 flowers, and the development of the embryo and endosperm is 

 closely correlated. In the 72-chromosome plants, on the other 

 hand, there is no orderly sequence of stages in the development of 

 the embryo sac (Fig. 184D-F). Young stages tend to persist in 

 fairly large ovules; megaspore mother cells enlarge, become vacuo- 

 late, and directly assume the characteristics of uninucleate embryo 

 sacs without the intervention of the dyad and tetrad stages; em- 

 bryo and endosperm show little correlation in their development; 

 and frequently even impollinated flowers give rise to embryos. 

 Nevertheless pollination is highly beneficial or even necessary for 

 the continued development of the endosperm without which the 

 embryo does not grow to maturity. 



A good example of a semiheterotypic division, followed by the 

 formation of a restitution nucleus, is seen in Ixeris (Okabe, 1932), 

 also belonging to the family Compositae. The basic chromosome 

 number in this genus is 7. Species with the diploid number (2n = 

 14) reproduce normally by the sexual method, but /. dentata, which 



Fig. 184. Some stages in development of embryo sac in 36-chromosome and 

 72-chromosome races of Parthenium argentatum. A, 36-chromosome race; mega- 

 spore mother cell. B, dyad formation. C, tetrad of megaspores. D, 72-chromo- 

 some race; megaspore mother cell elongating and functioning directly as diploid 

 spore. Note vacuolation of its cytoplasm and disorganization of the nucellar 

 epidermis, accompanied by a differentiation of the integumentary tapetum. E, 

 binucleate embryo sac with both nuclei in division. F, four-nucleate embryo sac. 

 (After Esau, 1946.) 



