Chapter 31 Notes: Green Algae and Land Plants

Chapter 31: Green Algae and Land Plants

Introduction

• This chapter focuses on the diversification of green algae and land plants.
• Key themes include:
  • Why and how biologists study these organisms.
  • Major themes in their diversification.
  • Key lineages of green algae and land plants.
  • The transition to life on land and the adaptations required, such as controlling water loss, surviving intense sunlight, growing upright, and reproducing without water.
• These adaptations allowed green algae and land plants to colonize and dominate the land.

Origin of Land Plants

• The earliest plant fossils are microscopic.
• These fossils exhibit reproductive cells called spores and a waxy coating called a cuticle.
• Several Characteristics support the hypothesis that these early plants grew on land:
  • The cuticle is a watertight barrier that prevents desiccation.
  • The spores are surrounded by a sheetlike coating almost identical to sporopollenin that encases spores and pollen in modern land plants.
  • Early spores are found with spore-producing structures called sporangia, similar to those in modern nonvascular plants.

Silurian-Devonian Explosion

• In rocks dated 416 to 359 mya, fossils from most major plant lineages are found.
• Adaptations allowing plants to occupy dry, terrestrial habitats are present, including:
  • Water-conducting vascular tissue.
  • Roots.
  • Stomata.
  • Leaves.
• Plants colonized land with symbiotic fungi.

The Carboniferous Period

• Extensive coal deposits are found in sediments dated from about 359 to 299 mya.
• Coal is a carbon-rich rock packed with fossil spores, branches, leaves, and tree trunks, derived mainly from seedless vascular plants.
• Coal formation requires water, indicating the presence of extensive forested swamps during the Carboniferous period.

Diversification of Gymnosperms

• Gymnosperms were prominent in the fossil record from 299 mya to 145 mya.
• Five major groups of gymnosperms living today include:
  • Ginkgophyta (ginkgoes).
  • Cycadophyta (cycads).
  • Cupressophyta (redwoods, junipers, and yews).
  • Pinophyta (pines, spruces, and firs).
  • Gnetophyta (gnetophytes).
• Gymnosperms grow readily in dry habitats, and both wet and dry environments became blanketed with green plants during this time.

Diversification of Angiosperms

• The first flowering plants appear in the fossil record about 150 mya and continue to diversify today.
• These plants are the ancestors of today’s grasses, orchids, daisies, oaks, maples, and roses.

Summary of the Land Plant Fossil Record

• According to the fossil record, green algae appear first, followed by nonvascular plants, seedless vascular plants, and seed plants.
• Organisms appearing later in the fossil record are often less dependent on moist habitats than earlier groups.
• The data support the hypotheses that:
  • Green plants evolved from green algae.
  • Land plants evolved to colonize dry habitats.

Evaluating Molecular Phylogenies

• The phylogenetic tree of green plants shows that:
  • The green plants are monophyletic, meaning a single common ancestor gave rise to all green algae and land plants.
  • Green algae are paraphyletic, including some, but not all, descendants of a single common ancestor.
  • Charophyceae (stoneworts) are the closest living relative to land plants.
  • Land plants evolved from a multicellular green alga in freshwater habitats.

Land Plants and Nonvascular Plants

• Land plants are monophyletic, indicating a single successful transition from freshwater to land.
• Nonvascular plants are the earliest-branching groups among land plants and the most ancient living group of land plants.
• Nonvascular plants are paraphyletic, including some, but not all, descendants of a single common ancestor.

Seedless Vascular Plants

• The seedless vascular plants are paraphyletic, but the vascular plants as a whole are monophyletic, implying that vascular tissue evolved only once.
• Seed plants (gymnosperms plus angiosperms) are monophyletic, with the seed evolving only once.
• Gymnosperms and angiosperms are monophyletic groups, with a major divergence in seed development.

Themes in the Diversification of Land Plants

• The evolution of land plants required adaptations for photosynthetic organisms to move from aquatic to terrestrial environments.
• Plants had to adapt to living and reproducing in a dry environment.

Adaptations to Dry Conditions

• Plants adapted to conditions where only a portion of their tissues are wet, as tissues exposed to air tend to dry out.
• Natural selection favored early plants with adaptations that solved the drying problem.
• Resources like light and carbon dioxide were more plentiful once plants survived out of water.
• The adaptations that solved the water problem arose in three steps:
  • Preventing water loss from cells.
  • Providing protection from harmful ultraviolet (UV) radiation.
  • Moving water from tissues with direct access to water to tissues without direct access.

Preventing Water Loss: Cuticle and Stomata

• The cuticle is a watertight sealant covering aboveground parts, enabling survival in dry environments but also keeping necessary $CO_2$ out.
• Another critical adaptation was the stoma (plural: stomata), consisting of a pore surrounded by specialized guard cells.
• The pore opens and closes as the guard cells change shape, facilitating gas exchange.
• Stomata are present in all land plants except liverworts, which have pores but lack guard cells.

The Importance of Upright Growth

• Early land plants lacked rigidity and grew low to the ground, staying in contact with moist soil to obtain water, leading to intense competition for space and light.
• Terrestrial plants capable of growing erect have better access to light.
• Erect growth presented two problems:
  • Transporting water against gravity.
  • Lack of rigidity, making them susceptible to gravity and wind.

The Origin of Vascular Tissue

• Fossils from the Rhynie Chert formation in Scotland include early land plants that grew upright and contained elongated cells organized into tissues along the plant's length.
• Biologists hypothesized these cells were part of water-conducting tissue because some fossilized cells had:
  • Simple, cellulose-containing cell walls like water-conducting cells in today’s mosses.
  • Cell walls with thickened rings containing lignin, an extraordinarily strong polymer.

Lignin

• The presence of lignin in cell walls of water-conducting cells defines vascular tissue.
• The evolution of vascular tissue allowed early plants to:
  • Transport water from roots to aboveground tissues.
  • Support erect stems.

Elaboration of Vascular Tissue: Tracheids and Vessels

• Simple water-conducting tissues evolved into more complex, efficient supportive and water-conducting tissues via natural selection.
• Long, thin, tapering, water-conducting cells called tracheids evolved about 380 mya.

Tracheids

• Tracheids have:
  • A thickened, lignin-containing secondary cell wall in addition to a cellulose-based primary cell wall.
  • Pits in the sides and ends of the cell, allowing water to flow efficiently between tracheids.
• The secondary cell wall increased structural support, and water could still move easily through the cells because of the pits.
• Today, all vascular plants contain tracheids.

Vessel Elements

• Vessel elements, the most specialized type of water-conducting cell, appeared about 250 to 270 mya.
• Vessel elements:
  • Are shorter and wider than tracheids.
  • Have gaps on both ends where both cell walls are missing.
• These characteristics:
  • Reduce resistance.
  • Make water movement extremely efficient.

Structure of Vessel Elements

• In vascular tissue, vessel elements line up end to end to form a continuous pipelike structure.
• In the stems and branches of some vascular plants, tracheids or a combination of tracheids and vessels form wood, a strong support material.

Mapping Evolutionary Changes on the Phylogenetic Tree

• Cuticle, stomata, and vascular tissue were key adaptations that allowed early plants to colonize land.
• Fundamental adaptations to dry conditions (cuticle, pores, stomata, vascular tissue, and tracheids) evolved just once.
• Convergent evolution also occurred, as vessels evolved independently in gnetophytes and angiosperms.