Plant Evolution and Terrestrial Adaptations

• Land plants include mosses and worts.
• The age of the Earth is 4.6 billion years.
• The origin of land plants was about 475 million years ago.
• Vascular plants appeared approximately 420 million years ago.
• Seed plants originated about 305 million years ago.
• Chara species are pond organisms.
• Coleochaete orbicularis is a disk-shaped charophyte that lives in ponds.

Storming the Beaches

• Charophytes possess a layer of sporopollenin, a durable polymer that prevents exposed zygotes from drying out.
• Moving onto land provided charophyte ancestors with unfiltered sun, more readily available CO2, nutrient-rich soil, and fewer herbivores or pathogens.
• Land presented a scarcity of water and lack of structural support.

Key Plant Traits

• There are four key traits that appear in nearly all land plants but are absent in the charophytes:
  • Alternation of generations, where plants alternate between multicellular haploid and diploid stages.
  • Walled spores produced in sporangia, which protect spores.
  • Multicellular gametangia, which enclose and protect gametes.
  • Apical meristems, regions of cell division at the root and shoot tips for linear growth.
• Additional plant traits:
  • Cuticles provide protection and control moisture loss.
  • Secondary compounds defend against herbivory and UV protection.
• Nonvascular plants include:
  • Bryophytes
  • Liverworts are in the phylum Hepatophyta
  • Hornworts, are in the phylum Anthocerophyta
  • Mosses are in the phylum Bryophyta
• Vascular plants are either seedless or bear seeds.

Alternation of Generations

• The gametophyte is haploid and produces haploid gametes by mitosis.
• Fusion of the gametes results in a diploid sporophyte, which produces haploid spores through meiosis.
• The diploid embryo is retained within the female gametophyte tissue.
• Nutrients are transferred from parent to offspring via placental transfer cells.
• Land plants are called embryophytes because of the embryo's dependency on the parent.

Haplodiplontic Life Cycle

• Includes a multicellular diploid stage (sporophyte) and a multicellular haploid stage (gametophyte).
• The sporophyte produces haploid spores by meiosis. Diploid spore mother cells (sporocytes) undergo meiosis in sporangia to produce 4 haploid spores, the first cells of the gametophyte generation.
• The gametophyte produces gametes by mitosis that fuse to form a diploid zygote, the first cell of the next sporophyte generation.

Multicellular Gametangia

• Gametes are produced within organs called gametangia.
• Female gametangia, called archegonia, produce eggs and are the site of fertilization.
• Male gametangia, called antheridia, are the site of sperm production and release.
• Bryophyte sporophytes grow out of archegonia and are the smallest, simplest among all extant plant groups.
• A sporophyte consists of a foot, a seta (stalk), and a sporangium (capsule), which discharges spores through a peristome.
• Hornwort and moss sporophytes have stomata for gas exchange.

Hornworts

• Earliest fossils are in Cretaceous.
• There are about 100 known species.

Ecology of Mosses

• Sphagnum or "peat moss," forms extensive deposits of partially decayed organic material known as peat.
• Sphagnum is an important global reservoir of organic carbon.
• Peat bogs make up an estimated 70 years of industrial carbon and carbon is released at an increasing rate.
• Peat is rich in carbon.
• Peat is a beginning source of coal deposits.
• Over 20% of homes in Ireland use peat bricks to heat their houses.
• Peat is composed of many plants but Sphagnum moss is common in peat bogs.
• 2% of global landmass is peat.
• 8 billion terajoules of energy are stored in the peat bogs.

This section covers the evolution of land plants from charophytes, key adaptations to terrestrial life, and the alternation of generations. Main topics include the origins of vascular and seed plants and the challenges and benefits of moving onto land.