THE FEMALE GAMETOPHYTE 87 



MONOSPORIC EMBRYO SACS 



Polygonum type. The monosporic 8-nucleate embryo sac, formed 

 by three divisions of the functioning megaspore, occurs in at least 

 70 per cent of the angiosperms now known. The enlargement of 

 the megaspore is always accompanied by increased vacuolation, one 

 large vacuole usually appearing on either side of the nucleus in the 

 direction of the long axis of the cell (Fig. 56 A). After the first 

 division has taken place, the two daughter nuclei move apart to 

 opposite poles. Most of the cytoplasm is aggregated around them 

 and the rest forms a thin peripheral layer, the center being occupied 

 by a large vacuole (Fig. 565). The next division gives rise to a 

 4-nucleate stage (Fig. 56C) which is followed by the 8-nucleate 

 stage comprising a micropylar and a chalazal quartet. 



Of the 8 nuclei arising in this manner, 3 at the micropylar end 

 give rise to the egg and two synergids; 3 at the chalazal end give 

 rise to antipodal cells; 2 and the remaining 2, one from each pole, 

 fuse in the center to form a secondary nucleus (Fig. 56/)). 



Occasionally embryo sacs are found with less than the normal 

 quota of 8 nuclei. This is usually because of an early degeneration 

 of the antipodals, which obscures the true nature of the embryo 

 sac. Even when the antipodals are present, they are sometimes 

 overlooked because of their being situated in the narrow chalazal end 

 of the embryo sac, which is seen only in median sections (Puri, 1939, 

 1941). 



In some cases there is a genuine reduction in the number of nuclei. 

 In certain species of Phajus, Corallorhiza, Broughtonia (Sharp, 1912), 

 Chamaeorchis, Oncidium (Afzelius, 1916), Elatine (Frisendahl, 1927) 

 (Fig. 57 A), Thesium (Rutishauser, 1937a), Calypso (Stenar, 1940), 

 and Bulbophyllum and Geodorum (Swamy, 1949a) the embryo sacs 

 are 6-nucleate owing to a suppression of division of the two chalazal 

 nuclei of the 4-nucleate stage. In Orchis morio (Afzelius, 1916) the 

 primary chalazal nucleus of the 2-nucleate stage may degenerate 

 without undergoing any division, so as to result in a 5-nucleate 

 embryo sac. 



A reduction in the number of nuclei may also be brought about in 

 a different way. In Epipactis pubescens (Brown and Sharp, 1911) 



8 In several plants, like Thesiiwi rostratum (Rutishauser, 1937a), cell formation 

 does not occur at the chalazal end and the antipodal nuclei remain free. 



