136 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



The antipodal cells of some members of the Ranunculaceae be- 

 come greatly enlarged and assume a glandular appearance (Fig. 

 89). Graft (1941) has shown that in Caltha palustris they attain a 



high degree of polyploidy. At first each antipo- 

 dal cell becomes binucleate. The two nuclei 

 now divide again, but the spindles fuse dur- 

 ing this process so that there are again only 

 two nuclei which, however, possess the diploid 

 number of chromosomes. This process may be 

 repeated, leading to the formation of tetraploid 

 and even octoploid nuclei. It gives an indica- 

 tion of the high metabolic activity in these cells 

 and offers a close analogy with the behavior of 

 the anther tapetum. 



Polar Nuclei. The central portion of the em- 

 bryo sac containing the polar nuclei eventually 

 gives rise to the endosperm and has therefore 

 been called the Endospermanlage or "endo- 

 sperm mother cell." Usually the two nuclei are 

 so similar to each other that once they have come 

 together it is difficult to distinguish the micro- 

 pylar from the chalazal. When there is a dif- 

 ference in size between the two, it is usually 

 the micropylar which is the larger. In embryo 

 sacs of the Fritillaria type, however, the chala- 

 zal polar nucleus is the larger (see page 118). 



The fusion of the polar nuclei may occur either 

 before, or during, or sometimes after, the entry 

 of the pollen tube inside the embryo sac. The 

 secondary nucleus formed after fusion usually 

 lies just below the egg and is separated from 

 the antipodal cells by a large vacuole. In those 

 plants in which it lies near the center, it is con- 

 nected with the egg apparatus by a conspicuous 

 cytoplasmic strand. A chalazal position is less 

 frequent except in those plants which are characterized by a Helo- 

 bial type of endosperm (see page 245). 



Embryo Sacs with Disturbed Polarity. Rarely, embryo sacs may 

 be found in which the usual polarity and organization are absent. 



Fig. 88. Embryo sac 

 of Rudbeckia bicolor, 

 showing three large 

 antipodal cells which 

 are arranged like cells 

 of egg apparatus. 

 (After Maheshwari 

 and Srinivasan, 1944-) 



