43 



THE VASCULAR PLANTS 



the gametophytes and their retention within the spores is 

 also easier to understand if you study the pictures. In 

 Figure 222 you see a microspore and various stages of the 



male gameiophyte to which it 

 gives rise. In Figure 223 you 

 see the mature female gameto- 

 phyte and see how the walls of 

 the old megaspore still embrace 

 it. Of course the wall of the 

 microspore must be broken in 

 order that the sperms may es- 

 cape, and the wall of the mega- 

 spore must be broken so that 

 the sperms may enter and fer- 

 tilization occur. These break- 

 ings of the wall are accom- 

 plished by the growth of the 

 gametophytes which they in- 

 close. The sperms do not have 

 to go very far in order to reach 

 the egg for the reason that the microsporophylls are above 

 the megasporophylls, and, when the microspores are shed, 

 some of them fall and lodge upon the megasporangia. Thus, 

 evidently, when the sperms are ready to be discharged, 

 they are already near the eggs which they are to fertilize. 



There is still one more fact about Selaginella which 

 needs to be mentioned, not only because it is true of seed 

 plants, but also because it is the fact which more than 

 anything else led to the evolution of seeds themselves, as 

 you shall see. This is the fact that, in the case of the 

 megaspore, not only is the female gametophyte retained 

 within it, but the megaspore itself is retained in the mega- 



FIG. 223. Selaginella. The ma- 

 ture female gametophyte. Note 

 that it is still largely inclosed by 

 the old megaspore wall. One 

 archegonium, ready for fertil- 

 ization, may be noted at the top 

 of the gametophyte. The clear 

 space is filled with the food 

 material with which the mega- 

 spore was originally supplied. 



