124 



INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



One species of Leontodon, L. hispidus, also deserves mention in 

 this connection (Bergman, 1935). Ordinarily a row of four mega- 

 spores is formed, and the embryo sac is of the Polygonum type. 

 But in more than 50 per cent of the ovules of one plant the sepa- 

 rating walls between the megaspore nuclei frequently dissolved and 

 disappeared, and all the 4 nuclei divided only once to give rise to 

 the 8-nucleate stage (Fig. 80). Since here only three divisions 

 intervened between the megaspore mother cell stage and the dif- 

 ferentiation of the egg, this mode of development comes under 

 the Adoxa type. 



Fig. 79. Development of embryo sac in Tulipa tetraphylla. A-C, formation of 

 megaspore nuclei. D, 3+1 arrangement of megaspore nuclei. E, all four nuclei 

 F, mature embryo sac. (After Romanov, 1988.) 



dividing 



Plumbago Type. The embryo sac of Plumbago capensis, 

 described by Haupt (1934), may be presented as a representative 

 of the Plumbago type. The 2- and 4-nucleate stages (Fig. 81 A-B) 

 are normal, and the 4 megaspore nuclei, which are arranged in a 

 crosswise fashion, undergo a further division (Fig. 81C) resulting 

 in 8 free nuclei arranged in four pairs (Fig. 81 D), One nucleus of 

 the micropylar pair is now cut off to form the lenticular egg cell 

 (Fig. &IE). Of the remaining 7 nuclei, 4 (presumably one member 

 of each of the original four pairs) undergo a slight increase in size 

 and gradually approach one another, functioning as polar nuclei 

 (Fig. 81F). The remaining 3 nuclei degenerate at their original 

 places, but occasionally 1, 2, or all 3 of them are cut off at the 

 periphery to form cells which may persist and assume an egg-like 

 appearance; synergids are entirely absent (Fig. 81G-H). 



