298 EVOLUTION OF PLANTS 
some plants have succeeded in establishing themselves, 
and from the equator to the poles no district is com- 
pletely wanting in some types of vegetable life. 
Starting from indifferent unicellular organisms, in- 
termediate in character between plants and animals, 
we have seen how there has been a steady progression 
in the direction of the more specialized plants. This 
progression consists in specialization of both vegetative 
and reproductive parts, which do not, however, neces- 
sarily advance equally. In the lower forms there is 
no clear distinction between the sexual and non-sexual 
plants, but in the highest green alge this becomes 
recognizable, but is most clearly seen in the Archegoni- 
ates, where the alternation of generations is very con- 
spicuous. 'In the lower Archegoniates the sexual phase, 
or gametophyte, is the more important, but in the higher 
ones the sporophyte becomes more and more prominent 
until, in the seed-bearing plants, the gametophyte is 
exceedingly rudimentary and may be reduced to a very 
few cells and is never capable of independent growth. 
The angiospermous flowering plants are the most 
modern and specialized members of the vegetable 
kingdom, and have largely superseded the earlier plant 
types, although remnants of the latter persist, espe- 
cially among aquatic forms, which have been subjected 
to less marked changes of environment and less keen 
competition in the struggle for existence. 
