CHAPTER VII. ANGIOSPERMS. 



Since the classical researches of De Bary ('49) and Strasburger ('78, 

 '79, '84), especially the latter, the nature of the sexual process in the 

 Angiosperms has been a matter of common knowledge among botanists. 

 It is considered beyond the purpose of this work to discuss the subject 

 historically, and no attempt will be made to present a summary of the 

 various theories that have been advanced from time to time during the 

 past half century upon the homologies of the female gametophyte or 

 embryo-sac. The view held here is that pollen grains and embryo- 

 sacs are respectively micro- and macrospores. The author is of the 

 opinion, as will be seen from what follows, that the preponderance of 

 morphological and cytological evidence indicates clearly that the pollen 

 mother-cell and the embryo-sac mother-cell are undeniably homologous 

 with the micro- and macrospore mother-cells of the archegoniates. 

 The fact that the embryo-sac mother-cell is not provided with a special 

 or well-differentiated cell-wall is almost without significance in deter- 

 mining homologies. 



THE EMBRYO-SAC OR FEMALE GAMETOPHYTE. 



Although many variations occur among Angiosperms in the develop- 

 ment of the embryo-sac, yet in the vast majority of cases this process 

 may be reduced to two forms or types. In the one case a readily 

 distinguishable hypodermal cell of the nucellus, either with or without 

 giving rise to a tapetum, divides into an axial row of four (sometimes 

 three ?) cells, or potential macrospores, the lowermost one developing 

 usually into the embryo-sac. In the second case, which is typified by 

 various species of Lilium, the hypodermal cell becomes at once the 

 macrospore. As illustrating these two types respectively, the process 

 of development will be described in Helleborus fcetidus, one of the 

 Ranunculaceae, and Lilium martagon. 



The macrospore mother-cell of Helleborus fcetidus increases 

 greatly in size, becoming much longer than broad in keeping pace 

 with the growth in length of the nucellus. Its nucleus, which lies 

 usually in the upper end of the cell, increases in size simultaneously, as a 

 preparation for the first nuclear division. This period of growth of both 

 cell and nucleus corresponds to the period of growth immediately pre- 

 ceding the first nuclear division in the pollen mother-cell (Fig. 70, A). 

 The nucleus now divides, and, as a rule, there follows a division of 



