120 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



the secondary 2-nucleate and secondary 4-nucleate stages show no 

 appreciable difference in size, and rarely the chalazal nuclei are 

 smaller than the micropylar in spite of the triploid nature of the 

 former. 



Finally, the basal nucleus of the secondary 4-nucleate stage some- 

 times fails to divide, resulting in a 7-nucleate gametophyte with 

 two antipodal cells instead of three, as in some species of Gagea 

 (Romanov, 1936) ; or, both the basal as well as the subbasal nucleus 

 remain undivided and the embryo sac is 6-nucleate, as in Statice 

 (Fagerlind, 19396). In Tulipa maximovitii (Romanov, 1939) the 

 3 chalazal megaspore nuclei undergo an abnormal division in which 

 all the telophase chromosome groups become included in a common 

 membrane, so that the mature embryo sac is 5-nucleate. In one 

 genus, Clintonia (R. W. Smith, 1911; F. H. Smith, 1943; Walker, 

 1944), the chalazal megaspore nuclei degenerate as soon as they are 

 formed, without undergoing any division at all. 



Plumbagella Type. In this type also, which has so far been re- 

 ported only in Plumbagella micrantha (Fagerlind, 19386; Boyes, 

 1939), the 4 megaspore nuclei take up a 1+3 arrangement (Fig. 

 77 A-C), and a large vacuole separates the 3 chalazal nuclei from the 

 micropylar nucleus (Fig. 77 D). The former gradually approach 

 one another and eventually fuse to give rise to a single triploid 

 nucleus (Fig. 77 E). This results in a secondary 2-nucleate stage, 

 followed by a secondary 4-nucleate one, in which the 2 micropylar 

 nuclei are haploid and the chalazal are triploid (Fig. 77 F-G). 

 There are no further divisions. The nucleus nearest the micro- 

 pylar end organizes into the egg; the triploid nucleus nearest the 

 chalazal end forms the single antipodal cell; and the remaining^ 

 nuclei, one haploid and the other triploid, fuse to form a tetraploid 

 secondary nucleus (Fig. 77/7-7). 



This mode of development shows an evident relationship with 



Fig. 77. Development of embryo sac in Plumbagella micrantha. A, megaspore 

 mother cell. B, second meiotic division in megaspore mother cell. C, megaspore 

 nuclei showing 1+3 arrangement; the three chalazal nuclei are of a smaller size. 

 D, chalazal nuclei in process of fusion. E, fusion of the three chalazal nuclei is 

 completed, resulting in formation of secondary two-nucleate stage. F,G, forma- 

 tion of secondary four-nucleate sLage. H, wall formation in embryo sac. /, 

 mature embryo sac showing egg, secondary nucleus, and single antipodal cell. 

 (After Fagerlind, 1938b.) 



