Early Plants

Characteristics

Psilotum

Lycopodium

Selaginella

Ancient Lycopods

Equisetum

Ferns


Plants and the colonization of land:

As we discusssed earlier, many new structures had to evolve to deal with environmental conditions on land

  • Vascular system: rings of xylem and phloem had to evolve : once a vascular system developed then only could TRUE leaves and roots could evolve....
  • They had to evolve anti-dehydrating surface to stems and leaves: a waxy substance called cuticle
  • They had to evolve stomata for controlled gas exchange to help conserve water while gaining carbon dioxide.
  • They had to evolve lignin to strengthen cell walls since water no longer supported them...
  • Alternation of generations: Main life stage reverted from primarily gametophytic (haploid) to sporophytic (diploid) dominance. Why?

Only then could the Questions of dispersals really be dealt with.

  • Eventually the need for water to move the male gametes would be resolved, but not successfully till the evolution of the gymnosperms
  • Eventually the production of flowers to entice pollinators who would carry those male gametes would evolve in the angiosperms
  • eventually the production of fruit to help disperse the embryos would occur with the evolution of the flower

    Obviously all these sturctures didn't happen over night: We can characterize three major revolutions in the plant world.

    Stage I. First period of evolution was during the Paleozoic with the evolution of vascular tissues

    Stage II. Second with diversification- still primarily non-seed types with separate gametophytic/sprophytic generations but with sporophyte gaining dominance

    Stage III. Third period involved the origination of the seed with sporophyte retaining the female gametophyte and male gametophytes reduced to pollen grains. Eventual production of flowers and fruits

Before we delve in to the major phyla, let's go over the characteristics of these early land plants...

 

Early Plants

Characteristics

Psilotum

Lycopodium

Selaginella

Ancient Lycopods

Equisetum

Ferns