APOMIXIS 



325 



lead to the formation of an eight-nucleate embryo sac, organized 

 in the usual manner. 



The diploid species of the genus Taraxacum go through the usual 

 meiotic divisions and tetrad formation, followed by syngamy, but 

 the polyploid species show a semiheterotypic division and dyad 

 formation, followed by the development of an unreduced egg cell 

 into the embryo (Osawa, 1913; Sears, 1922; Poddubnaja-Arnoldi 

 and Dianowa, 1934; Gustafsson, 1935; Fagerlind, 1947a). Usually 

 it is the chalazal dyad cell which functions (Fig. 186), but in T. 

 laevigatum (Sears, 1922) it is frequently the upper. 



Fig. 186. Development of unreduced embryo sac in Taraxacum albidum. A U A 2) 

 consecutive sections of megaspore mother cell. B, megaspore mother cell in divi- 

 sion. C, formation of dyad cells, of which the lower functions and gives rise to 

 embryo sac. D, two-nucleate embryo sac. E, mature embryo sac, showing 

 endosperm formation. {After Osawa, 1913.) 



In Erigeron some species are sexual, others partially apomictic, 

 and still others almost entirely apomictic. In E. annum (Fager- 

 lind, 19476), which belongs to the third category, a restitution 

 nucleus is formed during the first meiotic division. This divides 

 without wall formation to give rise to the mature embryo sac, which 

 is usually eight-nucleate. Ordinarily the development of the em- 

 bryo and endosperm go hand in hand (Holmgren, 1919), but some- 

 times the endosperm may lag behind, so that occasionally a many- 

 celled embryo is associated with an undivided endosperm nucleus 

 (Tahara, 1921). 



Bergman (1941) has recently reported a considerable range of 



