MORPHOLOGY OP REPRODUCTIVE ORGANS 233 



tissue either of the megasporangium itself as in the Gymno- 

 sperms, or to the sporophyll or carpel which bears the mega- 

 sporangium (ovule) as in the Angiosperms. The prothallium 

 produced from the microspore, which we have seen is a long 

 tubular outgrowth known as the pollen-tube, bores its way 

 through the tissue on which it iinds itself and makes its way 

 into the interior of the megaspore, now containing its gameto- 

 phyte (figs. 483 and 484, p.t.). The male gamete fuses with 

 the oosphere, forming a zygote, as in other oases. As usual, 

 the latter surrounds itself with a cell-wall. In the Angiosperms 

 it very shortly afterwards divides into two cells, of which the 

 upper one becomes the suspensor, consisting of a chain of cells 

 in consequence of repeated transverse divisions. The lower 

 develops into the embryo or young sporophyte. In the Gymno- 

 sperms the suspensor and the embryo are produced somewhat 

 similarly, but from only part of the zygote. The embryo after 

 a short period of active growth becomes quiescent and remams 

 inside the spore for a, considerable period. The megaspore, 

 still enclosed in its sporangium, now contains the gametophyte 

 and the young sporophyte to which the latter hag given rise, 

 and constitutes the body which is called the seed. No further 

 development takes place until the latter becomes detached irom 

 the parent sporophyte. 



In some of the Gymnosperms the process is somewhat 

 different. There are several archegonia in the prothallus or 

 endosperm, and each contains its oosphere. More than one 

 may become fertilised, and consequently more than one young 

 sporophyte may be originated. In many genera also the zygote 

 gives rise to more than one embryo. This phenomenon is 

 known as poVyemhryony . Though many embryos are thus 

 produced, only one as a rule matures. 



It is evident from the history of development that only such 

 plants can produce seeds as have all the stages of the life of the 

 gametophyte carried out while the latter is attached to the 

 parent sporophyte. 



