352 PRACTICAL COURSE IN BOTANY 



asexual spores (o of the formula) into two kinds, large and 

 small, known respectively as megaspores and microspores. 

 The prothallia developed by the former bear archegonia 

 containing female gametes only; those by the latter, antheri- 

 dia containing male gametes while in the dioecious bryo- 

 phytes, the archegonial and antheridial thalli are produced 

 by spores of the same kind. 



The differentiation of the asexual spores in the higher 

 pteridophytes gives rise to corresponding changes in the 

 sporangia that bear them, and even in the sporophylls them- 

 selves, one kind bearing microsporangia only, the other 

 megasporangia. In this way the differentiation of sex is 

 pushed back, step by step, until it virtually begins with the 

 sporophyte, or asexual generation. 



Using the same terms as before, and representing the mi- 

 crospores by the abbreviation mo, the megaspores by Mo, 

 the archegonial gametophyte by arG, the antheridial by 

 anG, the formula may be modified to express this more com- 

 plicated process of alternation, as follows : - 



> arG > fg^ , Mo > arG > /9k 



\. / ^V 



y oos-+S<( ^> oos->Setc. 



*-anG > nig/ ^ mo >anG >mg' 



Comparing this formula with the preceding, it will be seen 

 that the increased complexity affects the sporophyte at the 

 expense of the gametophyte, which has now become a mere 

 dependent on the former. 



410. Advantages of alternation. -- This roundabout mode 

 of reproduction would hardly have been developed unless it 

 had been of some benefit to the plants in which it occurs. 

 The chief advantage seems to be in more rapid multiplication 

 and consequently better chance to propagate the species, as 

 compared with the slow process of sexual reproduction were 

 the plant confined to that method alone. Only one plant is 

 produced by each oospore, and if this were a gametophyte 

 with its limited number of archegonia, multiplication would 



