FERTILIZATION IN ANGIOSPERMS. 



417 



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

 cell divides into two, and both are brought down to the embryo-sac; these two male 

 cells are both of them used in the process of fertilization. When the tip of the 

 pollen-tube reaches the micropyle (as in fig. 315«, 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 the male sexual cells pass 

 out, and, traversing the synergidae, enter the embryo-sac. The synergidaj seem 

 to promote this transfer of the male 

 sexual cells, though the precise part 

 played by them is not fully understood. 

 With the passage of the male cell the 

 synergidae collapse and shrivel; their 

 part is played. One of the two male 

 cells enters the egg-cell, and its nucleus 

 approaches and fuses with the nucleus 

 of the egg. The other approaches the 

 polar nuclei (shown in process of fusion 

 in figs. 316^ and ^) and with them takes 

 part in an act of triple fusion. The 

 result of this triple fusion is the de- 

 finitive nucleus of the embryo-sac from 

 which the endosperm is produced. Thus 

 it seems that two several processes of 

 fertilization take place in the embryo- 

 sac. From the one the embryo is pro- 

 duced, from the other (the result of 

 a triple fusion) the food-tissue at the 

 cost of which the embryo develops. 



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 fei'tilized the egg-cell would have remained 

 naked and inactive, and ultimately would have perished. 



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

 have been related in their barest outline; the discovery of the triple fusion is still 

 quite recent, but its existence has been confirmed in a sufficient number of families 

 of both Monocotyledons and Dicotyledons to arouse the confident expectation that 

 it will prove to occur generally throughout the whole series of Angiospermous 

 plants. The significance of the process is hardly appreciated as yet at its true 

 value, and it will be necessary to wait some years before we see it in its true light. 

 Investigation into the minutest features and processes of ovular morphology and 



Fig. 316.— Embryo-sac of ifonotiopa. 



Three stages are shown just preceding fertilization, in the 

 order i, 2, s. in each of thera we see the group of antipodal 

 cells at the base, and the egg-apparatus at tlie apex. • and 

 a show the two synergidaj side by side and the egg-cell ad- 

 jacent to them. In s the synergidae are vu the left, seen 

 through one another, both their nuclei sh<nving; the egg- 

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

 polar nuclei is shown in the flgs.; in i they are not yet 

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

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



