106 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



the "Podostemon type", has, however, always been considered 

 doubtful, and the four plants named above deserve to be re- 

 investigated. 



An even more doubtful case is that of Dicraea elongata (Magnus, 

 1913), in which the chalazal dyad cell is said to divide transversely 

 to form two cells (Fig. 68G). Of these the upper, which is larger, 

 again divides in the same plane (Fig. 68//,/) to produce one syn- 

 ergid and an egg cell, and the lower divides anticlinally to form 

 two antipodal cells (Fig. 68/). According to this interpretation 

 the polar nuclei are absent, and all the cells except the zygote 

 degenerate after fertilization (Fig. 68/v). These observations need 

 to be confirmed before they can be accepted. 



TETRASPORIC EMBRYO SACS 8 



Peperomia Type. Campbell (1899«,o; 1901) and Johnson (1900) 

 reported that in Peperomia pellucida each of the 4 megaspore nuclei 

 divides twice, resulting in a total of 16 nuclei which become more 

 or less uniformly distributed in the rather thick layer of cytoplasm 

 lying at the periphery of the embryo sac. According to Johnson, 

 2 nuclei at the micropylar end now become organized to form the 

 egg and a synergid, 8 fuse to form the secondary nucleus, and the 

 remaining 6 are cut off at the periphery of the embryo sac. Ac- 

 cording to Campbell, on the other hand, 1 to 3 nuclei in the vicinity 

 of the egg show a more or less evident aggregation of cytoplasm 

 around them and are to be regarded as the equivalents of syn- 

 ergids; approximately 8 nuclei enter into the formation of the sec- 

 ondary nucleus; and the remaining 4 to 6 nuclei are cut off as 

 antipodal cells. 



Subsequent studies, made by others on several species of 

 Peperomia, have confirmed Johnson's account. The chief varia- 

 tions concern the number of nuclei which fuse to form the secondary 

 nucleus, and the number left over to form the antipodals. In 

 every case only one synergid was observed. 



A recent study of Peperomia pellucida (Fagerlind, 1939a) has 

 shown that after the meiotic divisions are over (Fig. 69 A-C), the 

 coenomegaspore 9 may either retain its more or less spherical form 

 or become slightly pear-shaped with a little protuberance at the 



8 See Fagerlind (1944) for fuller information on tetrasporic embryo sacs. 



9 This term is used to denote the cell containing the four free megaspore nuclei. 



