THE EMBRYO SAC 



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largely absorbed, and often, in part also, of adjacent nucellar tissue 

 (Fig. 112E). Nutrition of the embryo sac comes also, in some taxa, 

 from a uniseriate palisade layer formed by the epidermis of the enclos- 

 ing integument (Fig. 112E to H). Structural development of the typical 

 8-nucleate, monosporic embryo sac is by successive divisions: into two; 

 each of these two into two; and each of the four so formed into two. 



Fig. 112. Megasporogenesis in Sarracenia purpurea. A, megaspore mother cell, 

 dyad forming; B, basal dyad divided, micropylar dyad degenerating; C, tetrad of 

 megaspores, the chalazal spore enlarging, the micropylar spores degenerating; D, 

 longitudinal section of ovule with tetrad of spores surrounded by uniseriate nucellus; 

 E, 2-nucleate embryo sac with collapsed nucellar cells; F, 4-nucleate embryo sac; 

 G, mature, 8-nucleate embryo sac, one antipodal cell out of plane of section; H, 

 embryo sac showing polar nuclei fused. ( After Shreve. ) 



