172 STERILE AND FERTILE REGIONS 
process, as regards the whole life-cycle, as that in the simplest sporophytes. 
All the vegetative machinery which precedes and delays it, is, from our 
point of view, a phase intercalated between the two constant and cyto- 
logically complementary events of sexuality and spore-production. 
Once fully differentiated the sterile and fertile regions may vary independ- 
ently of one another. This is already seen in some degree in those species 
of Lycopodium or Selaginella in which the strobilus is strictly circumscribed ; 
but it becomes a more prominent feature in the higher Flowering Plants, 
where the flower often differs in marked degree from the vegetative system 
of the same plant. Still, even where the sterile and fertile regions are 
the most divergent a comparison of the life-histories as a whole points 
to the conclusion that their genetic relation has ultimately been as it is 
seen in the less advanced Archegoniatae: that the larger part at least, 
if not indeed’ the whole of the vegetative system is referable in its origin 
to progressive sterilisation of parts originally fertile. The question whether 
the whole is thus referable involves embryological discussion, which must 
be reserved for the next chapter. 
