THE FEMALE GAMETOPHYTE 



87 



(Treub 15 ) and in Casuarina (Treub 25 ) ; in Viscum articula- 

 tum (Treub 13 ) all of the four or five megaspores reach the 

 two-nucleate stage; in Salix (Chamberlain 46 ) occasionally two 

 embryo-sacs are found in the fertilization stage ; in Fagus, 

 Corylus, and Carpirius (Miss Benson 31 ) two or more completed 

 sacs have been observed; in Juglans cordiformis (Karsten 115 ) 

 two embryo-sacs often occur; in Delphinium (Mottier 36 ) two 

 completed embryo-sacs have been found ( Fig. 3i), and in Ra- 

 nunculus (Coulter 51 ) several sacs develop to the two or four- 

 nucleate stage (Fig. 25) ; among the Rosaceae, several embryo- 

 sacs have been observed to start in Rosa (Strasburger 5 ), Erio- 

 botrya (Guignard 12 ), and Alchemilla (Murbeck 94 ); and the 

 same is true of Astilbe (Webb 111 ). Among the Compositae, 

 Marshall- Ward 8 observed three sacs enlarging side by side in 

 Pyrethrum, and Mottier 29 reports two completed sacs in 

 Senecio. 



The history of the gametophyte from the megaspore to the 

 completion of the egg-apparatus is remarkably uniform. Atten- 

 tion has been focused upon it for 



many years, and almost every 



description is a reiteration of 

 the preceding one. The mega- 

 spore and its nucleus usually en- 

 large very much before division, 

 and the daughter nuclei migrate, 

 one to each end of the sac (Figs. 

 35—37). The subsequent divi- 

 sions proceed rapidly and simul- 

 taneously, resulting in a group 

 of four nuclei at each end of the 

 embryo-sac. The antipodal po- 

 lar nucleus and the micropylar 

 polar nucleus (sister to the egg) 



then move toward one another and fuse in the general central re- 

 gion of the sac. forming the primary endosperm nucleus.* The 

 three remaining micropylar nuclei enter into the formation of 

 the cells of the egg-apparatus, while the three remaining antipo- 

 dal nuclei enter into the formation of the antipodal cells. Such 



* A discussion of the participation of one of the male cells in the forma- 

 tion of this nucleus will be found in Chapter VII. 



7 



Fig. 34. — Delphinium tricorne. Two ma- 

 ture embryo-sacs lying side by side in 

 one ovule; x -250. — After Mottier. sa 



