THE EMBRYO SAC 301 



sac. The less common types of sac are not restricted to definite lines of 

 relationship. The mature sac is closely the same in structure in the 

 major types — 8-nucleate, with the egg apparatus and antipodals at the 

 two ends and other nuclei in the center — though derived from different 

 numbers of spores. 



The union of spore protoplasts to form the gametophyte introduces a 

 unique feature in the elaboration of alternation of generations in vas- 

 cular plants. Simplification is continued — spores or dyads abort — in the 

 tetrasporic types; and, in the Adoxa, Plumbago, and Plumbagella types, 

 the egg is formed at the germination (first division) of the spore. In- 

 terpretation of the reduction shown in the bisporic and tetrasporic 

 types is not strictly comparable with that in the monosporic types, be- 

 cause of the morphologically compound natiu-e of the bisporic and 

 tetrasporic gametophyte. 



Despite the remarkable morphological difference in origin, the three 

 major types of embryo sac are much alike in general structure. The 

 Adoxa, Allium, and Fritillaria types much resemble the Polygonum type, 

 but the Polygonum is clearly the basic one from which the bisporic and 

 tetrasporic types have been derived; the general structural similarity 

 probably represents parallel development. In these specialized types, 

 the number of successive divisions in the formation of the female 

 gametophyte is reduced — in the Adoxa, Plumbago, and Plumbagella 

 types, by two. Among seed plants, the angiosperms show the last stage 

 in the reduction of the gametophytic generation. The female gameto- 

 phyte has many fewer cells than that of any of the gymnosperms. The 

 male gametophyte similarly reaches extreme reduction — to a generative 

 and a tube cell — a simplicity attained in gymnosperms only in the 

 highest conifers, the Cupressaceae. 



Theories of the Morphological Nature of the Embryo Sac 



Interpretation of the embryo sac in terms of the female gametophytes 

 of lower taxa is difficult, because of its great reduction. Progressive 

 reduction of the gametophytes is characteristic of the vascular-plant 

 series, but tlie steps from the gymnosperms to the angiosperms is a big 

 one, even if the multinucleate gametophytes of Gnetum and Welwitschio 

 are included. Several theories of the underlying structure of the angio- 

 sperm embryo sac, based on comparisons with gymnosperm gameto- 

 phytes, have been proposed and discussed. In all theories, the sac is 

 looked upon as the result of extreme reduction in that it consists of 

 "archegonia and their contents," or of "vestiges of archegonia," with 

 few or no prothallial cells. 



