308 



PRACTICAL BOTANY 



285. The gametophyte and fertilization. 1 It was stated in 

 Sect. 283 that the megaspore does not escape from the mega- 

 sporangium or ovule. It germinates therein and produces 



the gametophyte, which 

 sometimes well-nigh fills 

 the interior of the ovule. 

 The old megaspore wall 

 is now known as the 

 embryo sac. At the end 

 toward the open tip (the 

 little gate or micropyle) 

 of the ovule, this game- 

 tophyte develops arche- 

 gonia, in each of which 

 an egg is formed. This 

 is therefore a female ga- 

 metophyte. 



Obviously new diffi- 

 culties in fertilization 

 are presented in the 

 pine, for here the egg is 

 within the female ga- 

 metophyte, which itself 

 is inclosed by layers of 

 ovule tissue. This in- 

 closing tissue prevents 

 the sperm from swim- 

 ming directly to the 

 egg, as in the pterido- 

 phytes. After the microspore or pollen grain falls upon the 

 megasporangium its contents begin to produce several cells. 

 From the wall of the pollen grain there grows a tube into which 



1 Under ordinary circumstances it is not wise to attempt with secondary- 

 school students to do detailed individual laboratory work upon the game- 

 tophytes and upon fertilization in gymnosperms and angiosperms. If im- 

 portant structures can be shown under a demonstration microscope, they 

 will prove of interest and value. 



FIG. 255. The tip of a pollen tube of a pine 

 at the time it has almost reached the egg 



Just back of the two nourishing cells are the 



two darkly colored male cells, either of which 



may fertilize an egg. Much magnified 



