APOMIXIS 327 



variation in several species of the genus Hieracium, subgenus 

 Archier actum. In some cases the division is of the mitotic type 

 and the megaspore mother cell develops directly into the embryo 

 sac (Fig. 187 A-H). In others both the meiotic divisions take place 

 but several chromosomes remain unpaired, resulting in disturbances 

 in tetrad formation and consequent sterility (Fig. 187 1-P). In 

 still other cases there is a production of unreduced dyad cells (Fig. 

 1S7Q-T) either by the formation of a restitution nucleus or by a 

 pseudohomotypic division. Rarely, the first meiotic division takes 

 place in the usual way, but this is followed by a prolonged inter- 

 kinetic stage in which the chromosomes divide longitudinally so 

 that the diploid number is restored. In all three cases one of the 

 dyad cells functions and gives rise to the embryo sac. 



Somatic Apospory. Rosenberg (1907) described the occurrence 

 of somatic apospory in three species of the genus Hieracium (sub- 

 genus Pilosella), viz., H. excellens (Fig. 188 A-E), H.flagellare (Fig. 

 188F-H), and H. aurantiacum. The megaspore mother cell goes 

 through the usual meiotic divisions, but at just about this stage a 

 somatic cell situated in the chalazal region begins to enlarge and 

 becomes vacuolated. This cell gradually increases in volume, en- 

 croaching upon the megaspores and finally crushing them. The 

 aposporic embryo sac, arising from it, has the unreduced chromo- 

 some number and is able to function without fertilization. In H. 

 excellens the normal and reduced embryo sac, as well as the aposporic 

 and unreduced embryo sac, sometimes develops simultaneously but 

 this is rare in the other two species. 9 H. aurantiacum is peculiar 

 in that the aposporic embryo sac usually originates from a cell of 

 the nucellar epidermis. 



Aposporic embryo sacs have also been reported in several other 

 genera like Malus (Dermen, 1936), Crepis (Stebbins and Jenkins, 

 1939), Hypericum (Noack, 1939), Ranunculus (Hafliger, 1943), and 

 Poa (Hakansson, 1943; Nielsen, 1945, 1946). The cell giving rise 

 to the embryo sac may belong either to the integument or to some 

 part of the nucellar epidermis. In every case, however, the develop- 

 ment is characterized by two common features — an increase in the 



9 As remarked by Rosenberg, it is because of this combination of typical as well 

 as unreduced embryo sacs that H. excellens can give rise to hybrids in spite of being 

 an apomict. 



