420 FERTILIZATION AND FORMATION OF FRUIT IN PHANEROGAMS. 



After the entrance of the pollen-grains into its mouth the micropyle con- 

 tracts somewhat, so that the pollen is, so to speak, imprisoned. In Gymnosperms 

 the active development of pollen-tubes only occurs some considerable time after 

 pollination. In the Pine the pollen-grains put out short tubes soon after poUina- j 

 tion, but these tubes remain dormant from the spring in which pollination takes 

 place through the summer, autumn, and winter, and only continue their growth 

 after the lapse of about a year. Meanwhile changes take place within the ovule | 

 leading to the production of the archegonia with mature egg-cells. Actual fertili- 

 zation occui's about thirteen months after pollination. 



The contents of the pollen-cell, before it leaves the anther (Pines and Firs), or 

 shortly after its reception in the micropyle {Taxus and Cupressus), divides several 

 times, a number of small cells being cut off at one side of the grain and their 

 substance being separated from the rest of the contents of the grain by cell-mem- | 

 branes. Of these small cells one is the male sexual cell, and ultimately effects ferti- ' 

 lization. The big cell (known as the "vegetative cell") produces the pollen-tube. 

 The male sexual cell ("generative cell"), becoming free from its attachments , 

 (membranes), passes into the pollen-tube, where it divides into two cells. Ulti- ' 

 mately one of these cells fertilizes an egg-cell (the other not being required). In the 

 Juniper, where one pollen-tube fertilizes more than one archegonium, both these 

 generative cells — and possibly others, the result of their further division — would 

 appear to be utilized. The generative cells are carried along with the tube near , 

 its growing tip — much as in Angiosperms. Bit by bit the pollen-tube penetrates 

 deeper into the substance of the ovule which forms the floor of the micropyle. 

 Ultimately the tube reaches the neck of an archegonium, and pushes in between 

 the neck-cells, carrying the male cells to the mature egg-cell. In the Cypress 

 and Juniper, where several egg -cells are clustered close together, the tip of the 

 tube widens out, sending a little branch to each of the egg -cells (archegonia), 

 every one of which it is competent to fertilize. In the Pine, Firs, and other | 

 Gymnosperms, distinct tubes from separate pollen-grains penetrate to the several \ 

 archegonia. \ 



Fertilization happens much as in Angiosperms (described on p. 417); the male |j 

 cell enters the egg-cell, and the male nucleus fuses with the female nucleus. 

 Probably the other elements of the male cell are also taken up by the egg-cell. 

 Indeed, the whole process of pollen-tube development and fertilization is exceed- 

 ingly similar to these events as described in the Angiosperms. A chief point of 

 difference consists in the absence of distinct cell -walls bet\veen the cells which arise 

 in the pollen-grain of the last-named group. 



The development of the embryo from the fertilized egg-cell is different in Angio- 

 sperms and Gymnosperms. In Angiosperms the egg-cell, after surrounding itself 

 with a cell-wall, becomes partly attached to the apex of the embryo-sac. It divides 

 by a transverse wall into two cells, one directed towards the micropyle, the other 

 towards the base (chalazal end) of the embryo-sac. The upper {i.e. micropylar) of 



