294 MORPHOLOGY OF THE ANGIOSPERMS 



Of the eight nuclei so formed, three He in the micropylar end, two near 

 the center, and three in the chaL^zal end. The three micropylar nuclei, 

 with surrounding cytoplasm — the egg and the two sijnergids — become 

 walled by delicate membranes. The three at the chalazal end, the 

 antipodals, usually also become walled, but the two median nuclei, the 

 polar nuclei, remain unwalled. The synergids and the antipodals are 

 usually ephemeral, but the antipodals may persist and even multiply. 

 The antipodals and occasionally the synergids may become haustorial, 

 forming tubes that extend into surrounding tissues. In some taxa, the 

 ends of the sac form extensive haustoria. The sac itself, as it matures, 

 enlarges haustorially, at the expense of surrounding tissues and the 

 degenerating spores. No tapetum like that about the microsporogenous 

 tissue is present. A tapetumlike layer is present in a few taxa but 

 represents the inner epidermis of the inner integument, sometimes 

 strongly modified. Destruction of tissues surrounding the embryo sac 

 as the sac develops may be extensive, involving all the nucellus and 

 parts of the integuments. An embryo sac without nucellus has been 

 termed naked. 



The nature of the embryo-sac wall — whether or not a part of the 

 gametophyte — has received some attention, as has the nature of the 

 pollen-tube wall. Ontogenetically, the pollen-tube wall is clearly a 

 proliferation of the microspore wall; similarly, in the monosporic types 

 of embryo sac, the embryo-sac wall is an enlarged spore wall. In the 

 dyad type, it is the wall of a dyad; in the tetrasporic types, it is the 

 megaspore-mother-cell wall. In both the embryo sac and the pollen 

 tube, the wall forms a container for the gametophyte, which consists of 

 cells and naked protoplasts. Stages in the enclosure of gametophytes 

 within the spore wall are seen in Selaginella, Isoetes, the heterosporous 

 ferns, and the gymnosperms. 



An 8-nucleate embryo sac, with this ontogenetic history and number 

 and arrangement of nuclei is typical of the great majority of the taxa 

 that have been critically studied, and was, at first, called the normal 

 type (Figs. 113A and 118A). Bisporic and tetrasporic embryo sacs are 

 probably specialized variations of this apparently basic type. 



The monosporic, 8-nucleate type was, at first, considered character- 

 istic of all angiosperms, and only slowly were other types recognized. 

 In the earlier part of the twentieth century, the description and in- 

 terpretation of sacs with more or less than eight nuclei stimulated 

 critical study of many new taxa and the reexamination of supposedly 

 well-known forms. The existence of bisporic and tetrasporic types was 

 established, and reexamination of the embryo sacs of some supposedly 

 well-known taxa made changes in their interpretation necessary. The 

 embryo sac of Lilium, often used in teaching as an example of the 



