296 MORPHOLOGY OF THE ANGIOSPERMS 



typical sac, monosporic and 8-nucleate, was shown to be tetrasporic. 

 Study of embryo-sac structure continues to some extent, but the basic 

 types are probably now known, although only a comparatively few of 

 the vast number of living species have been studied. Embryo-sac t^pe 

 often characterizes genera and families, and study of additional taxa 

 may be of help in establishing natural relationships. 



Embryo-sac Types 



Types Based on Origin. The embryo sac formed by the germination of 

 one of the spores of the tetrad is a monosporic embryo sac. In some 

 taxa, after dyad formation, only one of the dyads divides; the other 

 degenerates. The surviving dyad enlarges and divides to form two 

 spores, not separated by a wall. The two spore protoplasts, within the 

 wall of the surviving dyad, enlarge and form a bisporic embryo sac. 

 Where four spores are formed by division of the megaspore mother cell 

 but remain unseparated by walls, and the protoplasts of all enter into 

 the formation of an embryo sac, the embryo sac is tetrasporic. The 

 female gametophyte of the angiosperms has three types of origin: from 

 cells that represent, morphologically, one, two, or four spores. In the 

 development of the bisporic and tetrasporic embryo sac, the protoplasts 

 of the spores, two and four, respectively, unite to form the gameto- 

 phyte. The origin of a single gametophyte by the united growth of two 

 or four spores is extraordinary and, morphologically, probably unique. 

 It implies the existence, in some taxa, of gametophytes basically con- 

 sisting of two or four individuals, a specialization in the gametophyte 

 as great as any in the sporophyte, and it supports the evidence from the 

 sporophyte for great age of the angiosperms. 



Types Based on Mature Structure of the Embryo Sac, As the structure 

 and development of the embryo sacs of large numbers of taxa became 

 known, ten subtypes were distinguished, on the basis of number of 

 nuclear divisions intervening between the megaspore mother cell and 

 the mature sac, and the number and arrangement of the nuclei in the 

 mature sac. The characters of the subtypes are best shown by a dia- 

 grammatic chart (Fig. 114). Each subtype has been given the name of 

 the genus in which it was first clearly described. Within each subtype, 

 minor variations occur. (The embryo sacs of a few genera, as described, 

 do not fit into any of these subtypes and remain as aberrant or un- 

 classified. ) 



The common type, long- and well-termed the normal, is now termed 

 the Polygonum type. Here, the egg is formed at the fifth division, the 

 third from the spore. The eight nuclei are distributed as follows: three 

 at the micropylar end of the sac, those of the egg and two synergids; 

 three at the chalazal end, those of the antipodals; and two in the central 



