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INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



BISPORIC EMBRYO SAC 



Allium Type. A bisporic embryo sac was first described in Al- 

 lium fistulosum (Strasburger, 1879) and has since been confirmed in 

 several species of this genus (Weber, 1929; Messeri, 1931; Jones and 

 Emsweller, 1936; and others). The megaspore mother cell (Fig. 

 63 A) divides to form two dyad cells, of which the upper is much 

 smaller and soon degenerates (Fig. 63B). The nucleus of the lower 

 divides to form 2 (Fig. 63C), 4 (Fig. 63D) and then 8 nuclei, which 

 give rise to an embryo sac with the usual organization. 



Fig. 63. Early stages in development of embryo sac of Allium cepa. A, mega- 

 spore mother cell. B, dyad cells, upper degenerating. C, two-nucleate embryo 

 sac. D, four-nucleate embryo sac. {After Jones and Emsweller, 1936.) 



Treub and Mellink (1880) independently described the same type 

 of development in Agraphis patula (= Scilla hispanica), and this 

 has also been found to be true of other species of Scilla (see Hoare, 

 1934). The chief difference between Allium and Scilla lies in the 

 fact that while in Allium it is the lower dyad cell which gives rise 

 to the embryo sac, in Scilla it is usually the upper. The lower 

 does not degenerate at once, however, but often develops up to the 

 4-nucleate stage forming the so-called "antigone," which probably 

 serves for the nutrition of the functional embryo sac. 



During the last seven decades the Allium type has been reported 

 in several plants belonging to diverse groups and it appears to be 

 quite characteristic of certain families, viz., Podostemonaceae, Bu- 



