332 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



and also reported the occurrence of aposporic embryo sacs in P. 

 crantzii. In the latter it is the nucellar cells adjacent to the arch- 

 esporium which give rise to embryo sacs. 



Organization of Aposporic Embryo Sacs. By whichever method 

 the aposporic embryo sac may arise {i.e., by generative or by somatic 

 apospory), it is usually eight-nucleate. Sometimes fewer than 

 eight nuclei occur as in Ochna serrulata (Chiarugi and Francini, 



1930) and Atraphaxis frutescens (Edman, 1931), or more than eight 

 as in Elatostema eurrhynchum (Fagerlind, 1944), but these are occa- 

 sional deviations without any special significance. A more common 

 feature is the disturbed polarity and lack of proper organization of 

 the various elements of the embryo sac. Frequently the egg and 

 synergids are indistinguishable from one another, and sometimes 

 there are more than two polar nuclei, while the remaining elements 

 of the embryo sac remain undifferentiated. In Atraphaxis (Edman, 



1931) some embryo sacs were found to lack an egg apparatus and 

 some the antipodal cells; occasionally both egg apparatus and anti- 

 podal cells were found to be absent and all the nuclei were aggre- 

 gated in the center. However, such embryo sacs are probably 

 functionless. 



Not only the number but also the behavior of the polar nuclei is 

 extremely variable. In Zephyr anthes texana (Pace, 1913) the two 

 polar nuclei fuse with a male nucleus to give rise to a pentaploid 

 endosperm, while the embryo is formed by the unfertilized but 

 diploid egg cell. In the apomictic forms of Chondrilla (Poddubnaja- 

 Arnoldi, 1933), Taraxacum (Poddubnaja-Arnoldi and Dianowa, 

 1934), Ranunculus (Hafliger, 1943), and Elatostema there is no 

 triple fusion and the endosperm is tetraploid. In Antennaria 

 alpina (Juel, 1900) and Alchemilla arvensis (Murbeck, 1901) the 

 polar nuclei are said to divide independently and the endosperm is 

 diploid. In Ewpatorium glandulosum (Holmgren, 1919) and Ixeris 

 dentata (Okabe, 1932) the condition is variable and the endosperm 

 may be diploid or tetraploid. In Balanophora globosa, according to 

 Ernst (1913), the lower polar nucleus degenerates and only the 

 upper forms the endosperm. 



Development of Embryo in Aposporic Embryo Sacs. Theoreti- 

 cally the new sporophyte may arise from any cell or nucleus of the 

 diploid embryo sac, but usually it is only the egg which is capable 

 of such development (diploid parthenogenesis). Sometimes one or 



