108 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



16 nuclei, which may form either eight groups of 2 nuclei each (in 

 the spherical embryo sacs) (Fig. 69F) or six groups of 2 and a micro- 

 pylar group of 4 nuclei (in the pear-shaped embryo sacs) (Fig. QQG). 

 In the former case the egg apparatus is usually two-celled (egg and 

 one synergid) ; 8 nuclei fuse in the center to form the secondary 

 nucleus; and 6 nuclei are cut off at the periphery (Fig. 69H). Only 

 occasionally, because of slight displacements and the small size of 

 the embryo sac, one may find another peripheral cell lying so close 

 to the egg that the egg apparatus may be said to comprise three cells 

 (Fig. 697). In the pear-shaped embryo sacs, however, a three-celled 

 egg apparatus is the rule, the fourth nucleus from the micropylar 

 end and one member from each of the six peripheral pairs form the 

 seven polars, and 6 nuclei are cut off to form the lateral cells (Fig. 

 69K-M). 



Fagerlind's observations help us to understand the slight diver- 

 gence between the account of Johnson (1900) and that of Campbell 

 (1899a,6; 1901). The former saw only one synergid, while the 

 latter believed that there were more than one. Now it appears 

 that both these conditions are possible, depending on the form 

 which the embryo sac takes during its growth and development. 

 In the pear-shaped type there are invariably two synergids; in the 

 spherical type there is usually only one synergid unless another 

 peripheral cell accidentally happens to lie so close to the egg as to 

 look like a second synergid. 



Johnson (1914) discovered a different type, however, in P. his- 

 pidula. At the 8-nucleate stage, 2 nuclei are seen at the micro- 

 pylar and 6 at the chalazal end (Fig. 70A); at the 16-nucleate 

 stage, 4 lie at the micropylar end and 12 at the chalazal (Fig. 70B). 

 Two nuclei of the micropylar group now form the egg and single 

 synergid, as in other species, but the remaining 2 nuclei of this 

 group and all the remaining 12 nuclei meet near the center and 

 fuse to form a single large secondary nucleus (Fig. 70C,D). 



The embryo sac of Gunnera (Haloragidaceae) is essentially similar 

 to that of Peperomia pcllucida. Two species have been studied: 

 G. macrophylla (Ernst, 1908; Samuels, 1912) and G. chilensis 

 (Modilewski, 1908). After the 16-nucleate stage, 3 of the micro- 

 pylar nuclei form the egg apparatus, the fourth descends and fuses 

 with 6 other nuclei tu form a large secondary nucleus, and the re- 

 maining 6 are cut off as antipodal cells. It is possible that if the 



