APOMIXIS 



331 



towards meiosis but several unreduced embryo sacs arise by mitotic 

 divisions in the central cells of the archesporium (generative apos- 

 pory). The same condition also occurs in E. machaerophyllum 

 except that here the division may be mitotic or pseudohomotypic, 

 or there may be a semiheterotypic division followed by the forma- 

 tion of a restitution nucleus. 



Fig. 190. Apospory and polyembryony in Atraphaxis frutescens. A, normal 

 embryo sac with embryo and endosperm; additional aposporic embryo sac at 

 chalazal end showing inverted polarity. BiB 2 , consecutive sections of ovule show- 

 ing well-developed embryo in upper embryo sac and aborted embryos in chalazal 

 region. C, twin embryo sacs — one on left with two overlapping embryos, that on 

 right with one embryo. (After Edman, 1931.) 



In some species of Potentilla (Rutishauser, 1943) there is a multi- 

 cellular archesporium without any well-marked distinction between 

 the axial and the lateral cells of the sporogenous tissue. Embryo 

 sacs may arise either from such cells by ordinary mitotic division as 

 in P. verna (generative apospory), or from the chalazal cells of the 

 nucellus as in P. canescens, P. praecox, and P. argentea (somatic 

 apospory). Hakansson (1946) has confirmed this in P. argentea 



