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 dicecious 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 : — 
Mo—>arG—> fg Mo —>-arG—-+» fg. 
ae > 00s—> 8. 4 > 00s etc, 
mo—>anG—> mg 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 oéspore, and if this were a gametophyte 
with its limited number of archegonia, multiplication would 
