FERTILIZATION IN ANGIOSPERMS. 



417 



tube, near the tip, and so is gradually brought down to the embryo-sac. Usually 

 this male cell divides into two, I nit there is no evidence to show that more than one 

 of the daughter-cells thus produced takes an actual part in fertilization. When the 

 tip of the pollen-tube reaches the micropyle (as in fig. 315 6 , though the contained 

 male cells are not shown), the male sexual cells are well up to the end of the tube. 

 The pollen-tube forces its way down the micropyle, and perforates the apex of the 

 embryo-sac. The tip of the tube is now opened, and a male sexual cell passes out, 

 and, traversing the synergidas, enters the egg-cell. The synergidae seem to promote 

 this transfer of the male cell to the egg, 

 though the precise part played by them 

 is not fully understood. With the pas- 

 sage of the male cell the synergidae col- 

 lapse and shrivel; their part is played. 

 The anterior of the two male cells 

 of the pollen-tube enters the egg-cell, 

 the other one being possibly of the 

 nature of a reserve in case of accident. 

 Occasionally, it also enters, and has been 

 observed in the egg, though probably 

 this is an accidental circumstance. After 

 the entrance of the male cell its nucleus 

 approaches the female (egg-) nucleus 

 and fuses with it. This fusion con- 

 stitutes the act of fertilization. Though 

 this nuclear fusion is the most charac- 

 teristic feature of fertilization it may 

 well be that the other elements which 

 enter the egg-cell with the male nucleus 

 likewise fuse with the protoplasm of 

 the egg. So far, observations have not 

 absolutely determined the fate of these 

 less conspicuous elements. On the 

 other hand, it is possible that these 

 subordinate elements serve in large part 

 merely as food-material for the egg. As yet the time has not arrived to speak 

 decisively on these points. 



The fertilized egg -cell, which we may now term the embryonic cell, soon gives 

 indication of the change which it has undergone. It secretes a cell-wall around 

 itself. Had it not been fertilized the egg-cell would have remained naked and 

 inactive, and ultimately would have perished. 



In the account of fertilization of the Angiosperm just given, the main facts have 

 been related; and although from time to time fresh observations come to light, it is 

 hardly probable that the main outlines as given above will be overthrown. That a 



Fig. 316.— Embryo-sac of Monotropa. 



Three stages are shown just preceding fertilization, in the 

 order > 2 . s . In each of them we see the group of antipodal 

 cells at the base, and the egg-apparatus at the apex. 1 and 

 2 show the two synergidae side by side and the egg-cell ad- 

 jacent to them. In 3 the synergidje are on the left, seen 

 through one another, both their nuclei showing; the egg- 

 cell on the right. The procesB of fusion of the so-called 

 polar nuclei is ahown in the ngs.; in i they are not yet 

 detached from their respective groups, in - they have ap- 

 proached one another, in 3 they are in process of fusion. 



TOL. II 



77 



