298 EVOLUTION OF PLANTS 



some plants have succeeded in establishing themselves, 

 and from the equator to the poles no dfstrict 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 algse 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 kingdom, and have largely superseded the earlier plant 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. 



