一 origin of plants

     •Abillion years ago, plants had not yet begun to appear on land.

     •Theshallow waters covered much of Earth’s surface at that time , and there weremany bacteria,algae, corals, sponges, jellyfish and worms.

     •Evidenceindicates that green algae became adapted to life on land.

• The earliest plants undoubtedly evolved from an aquatic green algae.

• Different plant groups have becomeadapted to terrestrial life to varying degrees.

• Desiccation

Drastic changes in temperature

• There are an estimated 390,900different species of plants known to science, andnew species are continually being described, particularly from previouslyunexplored tropical areas of the world.

• Plants evolved from aquaticancestors and have subsequently migrated over the entire surface of Earth, inhabiting tropical, Arctic,desert, and Alpine regions.

• Some plants have returned to anaquatic habitat in either fresh or saltwater.

二 adaptation of plants

1 Some plants float in a pond, so they do not need to conserve water.

2 For most land plants, the only available supply of water and minerals is in the soil.