THE FEMALE GAMETOPHYTE 



131 



the chalazal nucleus. The total number of nuclei in the mature 

 embryo sac may therefore be 10 or 9 or even as few as 6, depending 

 upon two divisions, or a single division, or a complete failure of 

 division, of this nucleus (Fig. 84E-G). 



Fig. 84. Two modes of development of embryo sac of Chrysanthemum cinerariae- 

 folium. A-C, formation of the four megaspore nuclei. D-G, first type of develop- 

 ment, in which the two central megaspore nuclei remain undivided and function 

 directly as polar nuclei. H-K, second type of development, in which two central 

 megaspore nuclei fuse to form diploid nucleus which undergoes two divisions to 

 give rise to four nuclei; of these, one functions as polar nucleus and three form anti- 

 podal cells. For details, see text. (Adapted from Martinoli, 1939.) 



ORGANIZATION OF MATURE EMBRYO SAC 



Although the origin of the mature embryo sac may differ, its 

 eventual organization shows a surprisingly uniform pattern in the 

 majority of angiosperms. The Polygonum, Allium, Fritillaria, and 

 Adoxa types of embryo sacs all have a similar appearance at the 

 time of fertilization (three -celled egg apparatus, three antipodals, 

 and two polar nuclei). Even in the remaining types an egg ap- 

 paratus, at least, is almost always present and it is only in a few 

 genera like Peperomia, Plumbago, Plumbagella, and Acalypha indica 

 that we see a radical departure from the basic plan. Ignoring for 



