THE EMBRYO SAC 299 



Polygonum type, but, because the nuclei of four spores are involved, the 

 number is attained by the third, rather than the fifth, division. Mature 

 embryo sacs of the Adoxa and Polygonum types are much alike in 

 nuclear arrangement. 



(For the sake of simplicity in the discussion above, no distinction 

 has been made between nuclei and cells in the sac.) 



Minor variations in number of nuclei in the sac are occasional, espe- 

 cially in the antipodals, where chalazal nuclei may fail to divide, but 

 some reports of lesser numbers are perhaps in error, because of failure 

 to observe all the nuclei. Nuclei from surrounding nucellar cells are re- 

 ported to enter the sac, suggesting a higher number of nuclei. 



Specialization of the embryo sac has followed that seen in the 

 gametophyte — both male and female — of the major taxa of vascular 

 plants; reduction of the vegetative tissue and earlier and earlier forma- 

 tion of gametes occur in the ontogeny of the gametophyte. In angio- 

 sperms, the female gametophyte is reduced from the many cells of the 

 gymnosperm gametophytes to eight as a basic number — the Polygonum 

 type. In the Oenothera type, the monosporic gametophyte shows reduc- 

 tion from three to two divisions in its development, and an accompany- 

 ing reduction of prothallial nuclei from seven to three. In the specializa- 

 tion of the embryo sac, the Oenothera type is the least modified of the 

 advanced types. Still monosporic, the number of cell divisions occurring 

 between the spore mother cell and the mature gametophyte is four, a 

 reduction of one from that in the basic Polygonum type; the egg is 

 formed at the second division from the spore [the fourth division from 

 the megaspore mother cell (Fig. 114)]. The reduction in number of 

 divisions naturally reduces the remaining (prothallial) cells from seven 

 to three. 



In the Allium type — bisporic but 8-nucleate, like the basic type — the 

 gametophyte is mature after the fourth division from the spore mother 

 cell, as in the Oenothera type, and, similarly, the egg is formed at the 

 second division from the spore. The Allium and Oenothera t)'pes are 

 alike in the number of cell divisions intervening between the spore 

 mother cell and the spore and the formation of the egg, but the mature 

 sac of the Allium type has seven prothallial cells, in contrast to three 

 in the Oenothera type, because two spores build the sac in the Allium 

 type. The Allium type sac resembles the Polygonum type in form and 

 number of cells, but has a different origin. As in the Oenothera type, the 

 number of cell divisions occurring in the formation of the gametophyte 

 has been shortened by one, though the sac resembles the basic type in 

 structure and in number of prothallial cells. 



In the 8-nucleate, tetrasporic types of embryo sac — Adoxa, Plumbago — 

 the number of nuclear divisions before egg formation is shortened by two 



