THE FEMALE GA3JET0PHYTE 



ST 



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

 tion (Trent) 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 Carpinus ( 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. 34), 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 •'), Erio- 

 botrya (Guignard 12 ), and Alchemilla (Murbeek 9 *); and the 

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

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

 Pyrethrum, and Mottier 2 ' 1 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. 



Fig. 34. — Delphinium tricorne. Two ma- 

 ture embryo-sacs lying side by side in 

 one ovule ; x 250. — After Mottier. 36 



