Plant

There was little life on land 500 million years ago, although the oceans abounded with diverse photosynthetic organisms, as well as species in the Monera, Protoctista, and Animalia kingdoms. Land plants appear to have evolved from photosynthetic, aquatic ancestors about 500 million years ago, probably from the Chlorophyta, or green algae. Both groups use chlorophyll-a and chlorophyll-b as photosynthetic pigments, store their energy reserves as starch, and have cellulose in their cell walls.

The evolution of the terrestrial habit required special adaptations of reproductive and vegetative tissues for protection against desiccation. The most significant adaptation of the reproductive tissues is enclosure of the sex cells (egg and sperm) within specialized tissues, and retention of the fertilized egg as it develops into a multicellular embryo. The most significant adaptation of the vegetative tissue is development of a parenchymatous cell organization, in which unspecialized cells (parenchyma) are embedded in a dense matrix of cells. This reduces water loss by reducing the overall surface area of the plant per cell, and also provides the plant with a body matrix for differentiation of specialized tissues.

The life cycle of all plants consists of an alternation of generations, in which a haploid gametophyte (tissue in which each cell has one copy of each chromosome) alternates with a diploid sporophyte (tissue in which each cell has two copies of each chromosome). A major trend in plant evolution has been the increasing dominance of the sporophyte. Chlorophyta (green algae), the ancestors of land plants, have a dominant gametophyte and greatly reduced sporophyte. Bryophyta, the most primitive land plants, have a more elaborate sporophyte than Chlorophyta, although their gametophyte is still dominant. Free-sporing vascular plants (Filicinophyta, Lycopodophyta, and Sphenophyta) have a somewhat more dominant sporophyte phase than gametophyte phase. However, seed plants, the most advanced of the land plants, have a greatly reduced gametophyte, and a dominant sporophyte.