17.01

• Charophytes


  Green algae thought to have evolved from a common ancestor with plants
• Charophytes


  • Photosynthetic eukaryotes
  • Many species have complex, multicellular bodies
• Adaptations that evolved after plants diverged from algae facilitated survival and reproduction on dry land
• Some plant groups, including water weeds such as Anacharis, returned to aquatic habitats during their evolution
• Most present-day plants live in terrestrial environments
• Algal ancestors of plants may have carpeted moist fringes of lakes or coastal salt marshes more than 500 million years ago
• These shallow-water habitats were subject to occasional drying, and natural selection would have favored algae that could survive periodic droughts
• Adaptations making life on dry land possible had accumulated by about 470 million years ago, the age of the oldest-known land plant fossils
• The evolutionary novelties of these first land plants opened the new frontier of a terrestrial habitat
• Early plant life would have thrived in the new environment due to bright sunlight, abundance of carbon dioxide, and relatively few pathogens and plant-eating animals
• Challenges of life on land

  • Maintaining moisture inside cells
  • Supporting the body in a nonbuoyant medium
  • Reproducing and dispersing offspring without water
  • Anchoring bodies in the soil and obtaining resources from both soil and air
• Algae


  Anchored by a holdfast, generally have no rigid tissues, supported by surrounding water, obtain CO2 and minerals directly from water, most of the organism receives light and can perform photosynthesis, flagellated sperm swim to fertilize an egg, offspring dispersed by water
• Land plants


  Aboveground parts covered by waxy cuticle that prevents water loss, gas exchange through stomata, discrete organs (roots, stems, leaves) to obtain resources from soil and air, vascular tissue to conduct water, minerals, and sugars, thickened and lignified cell walls to provide support, gametes and embryos protected from drying out, dispersal by spores or seeds
• Mosses lack a complex transport system and are severely restricted in height
• Mosses and ferns can only reproduce in a moist environment
• Pines and flowering plants have pollen grains that allow sperm and egg to come together without the need for water
• All plants have a life cycle involving the alternation of a haploid generation (producing gametes) and a diploid generation (producing spores)
• The earliest land plants relied on tough-walled spores for dispersal, a trait retained by mosses and ferns today
• Pines and flowering plants have seeds for launching their offspring, which are well protected from the elements and dispersed by wind or animals