🐾General Biology II Unit 17 – Plant Diversity and Evolution

Key Concepts

• Plants are multicellular eukaryotes that have adapted to live on land and undergo photosynthesis to produce their own food
• Plant cells have cell walls made of cellulose, a tough polysaccharide that provides structure and support
• Vascular tissue consists of xylem and phloem, which transport water, minerals, and sugars throughout the plant body
• Alternation of generations is a key feature of plant life cycles, involving the alternation between a haploid gametophyte generation and a diploid sporophyte generation
• Bryophytes (mosses, liverworts, and hornworts) are non-vascular plants that lack true roots, stems, and leaves
• Seedless vascular plants, such as ferns and horsetails, have vascular tissue but reproduce using spores rather than seeds
• Gymnosperms are seed plants that produce naked seeds not enclosed in an ovary (conifers, cycads, and ginkgos)
• Angiosperms, or flowering plants, are the most diverse group of land plants and produce seeds enclosed within an ovary

Plant Evolution Timeline

• The first land plants, believed to be similar to modern liverworts, appeared during the Ordovician Period approximately 470 million years ago
• Vascular plants evolved during the Silurian Period, around 430 million years ago, allowing for the transport of water and nutrients throughout the plant body
• The Carboniferous Period, which began around 360 million years ago, saw the rise of extensive forests dominated by lycophytes, ferns, and early gymnosperms
• Gymnosperms, including conifers and cycads, became dominant during the Mesozoic Era (252-66 million years ago)
• Angiosperms, or flowering plants, first appeared in the fossil record during the Early Cretaceous, around 130 million years ago
• The rapid diversification of angiosperms, known as the Cretaceous Terrestrial Revolution, occurred between 125-80 million years ago
• The Cenozoic Era (66 million years ago to present) has been characterized by the continued diversification and global spread of angiosperms

Major Plant Groups

• Bryophytes are non-vascular plants that include mosses, liverworts, and hornworts
  • They lack true roots, stems, and leaves and are typically small in size
  • Bryophytes require moist environments for reproduction, as they rely on water for the dispersal of sperm
• Seedless vascular plants include ferns, horsetails, and lycophytes
  • They have vascular tissue for the transport of water and nutrients but reproduce using spores rather than seeds
  • Ferns are the most diverse group of seedless vascular plants, with over 10,000 living species
• Gymnosperms are seed plants that produce naked seeds not enclosed in an ovary
  • Major groups include conifers (pines, spruces, firs), cycads, ginkgos, and gnetophytes
  • Many gymnosperms are adapted to cold or dry environments and are often the dominant plants in boreal forests and temperate rainforests
• Angiosperms, or flowering plants, are the most diverse and abundant group of land plants
  • They produce seeds enclosed within an ovary and have flowers for sexual reproduction
  • Angiosperms are divided into monocots (grasses, lilies, orchids) and eudicots (roses, sunflowers, oaks), based on differences in seed, leaf, and flower structure

Adaptations to Land

• Cuticle is a waxy layer that covers the aerial parts of plants, reducing water loss and providing protection against UV radiation and pathogens
• Stomata are pores in the leaf surface that allow for gas exchange (uptake of CO2 and release of O2) while minimizing water loss
  • Guard cells surrounding the stomata can open and close in response to environmental conditions, regulating gas exchange and water loss
• Roots anchor the plant to the substrate, absorb water and minerals from the soil, and store food reserves
  • Root hairs increase the surface area for water and mineral absorption
• Stems provide support, elevate leaves for optimal light capture, and transport water and nutrients between roots and leaves
  • Vascular tissue (xylem and phloem) allows for efficient long-distance transport of water, minerals, and sugars
• Leaves are the primary sites of photosynthesis, with a broad, flattened surface area to maximize light capture
  • Leaf arrangement and morphology can vary depending on environmental conditions (shade leaves vs. sun leaves)
• Seeds contain an embryo and food reserves, allowing for dispersal and survival in a variety of environments
  • Seed dormancy enables seeds to remain viable for extended periods until conditions are favorable for germination

Plant Life Cycles

• The life cycle of plants involves the alternation of generations between a haploid gametophyte and a diploid sporophyte
• In bryophytes, the gametophyte is the dominant stage of the life cycle
  • Sperm produced by the male gametophyte swim through water to fertilize eggs produced by the female gametophyte, forming a zygote that develops into the sporophyte
  • The sporophyte remains attached to and dependent on the gametophyte for nutrients
• In vascular plants (seedless vascular plants, gymnosperms, and angiosperms), the sporophyte is the dominant stage of the life cycle
  • The sporophyte produces spores through meiosis, which develop into gametophytes
  • In seedless vascular plants, sperm from the male gametophyte swim through water to fertilize eggs produced by the female gametophyte
  • In seed plants (gymnosperms and angiosperms), pollen grains (male gametophytes) are transported by wind or animals to the ovules (female gametophytes), where fertilization occurs
• Double fertilization is unique to angiosperms and involves the fusion of one sperm cell with the egg to form a zygote and the fusion of a second sperm cell with two polar nuclei to form endosperm, which provides nutrition for the developing embryo

Ecological Importance

• Plants are the primary producers in most terrestrial ecosystems, converting solar energy into chemical energy through photosynthesis
  • This energy is then transferred to other trophic levels through food chains and food webs
• Plant roots help to stabilize soil and prevent erosion, while also improving soil structure and fertility through the addition of organic matter
• Plants provide habitat and food for a wide variety of animals, including insects, birds, and mammals
  • Many plant-animal interactions, such as pollination and seed dispersal, are mutually beneficial and have co-evolved over time
• Forests and grasslands play a crucial role in the global carbon cycle by sequestering atmospheric carbon dioxide and storing it in biomass and soils
• Vegetation cover influences local and regional climate through transpiration, albedo (reflectivity), and surface roughness
  • Transpiration from plants can increase humidity and contribute to cloud formation and precipitation
• Plants are a source of numerous ecosystem services, such as timber, food, medicine, and recreation
  • The loss of plant diversity can have cascading effects on ecosystem functioning and human well-being

Diversity Hotspots

• Biodiversity hotspots are regions with exceptionally high levels of plant endemism (species found nowhere else) that are also under threat from habitat loss and degradation
• The Tropical Andes (Colombia, Ecuador, Peru, Bolivia) is home to over 30,000 plant species, half of which are endemic
  • This region is threatened by deforestation, agriculture, and mining
• The Atlantic Forest (Brazil, Paraguay, Argentina) has over 20,000 plant species, with 40% endemism
  • Less than 10% of the original forest cover remains due to urbanization, agriculture, and plantation forestry
• The Cape Floristic Region (South Africa) has over 9,000 plant species, with 69% endemism, in an area of just 78,555 km²
  • Threats include urban expansion, agriculture, and invasive species
• The Caribbean Islands are home to over 11,000 plant species, with 72% endemism
  • The main threats are habitat destruction, invasive species, and climate change
• The Sundaland region (Indonesia, Malaysia, Singapore, Brunei) has over 25,000 plant species, with 60% endemism
  • Rapid deforestation for palm oil plantations and logging are major threats to biodiversity in this region

Current Research and Future Directions

• Climate change impacts on plant diversity and distribution are a major focus of current research
  • Studies aim to predict species' responses to changing temperature, precipitation, and CO2 levels, and to identify vulnerable populations and regions
• Plant-microbe interactions, including mycorrhizal associations and nitrogen-fixing symbioses, are being investigated for their potential to improve plant growth and resilience under stress
• The development of crop varieties with improved resistance to pests, diseases, and abiotic stresses (drought, salinity, heat) is a priority for ensuring food security in a changing climate
• Advances in genomics and biotechnology are enabling the rapid identification and manipulation of genes controlling key traits, such as yield, nutritional content, and stress tolerance
• Restoration ecology seeks to develop effective strategies for restoring degraded ecosystems and conserving threatened plant species
  • Research focuses on factors influencing seedling establishment, community assembly, and ecosystem functioning in restored habitats
• The role of traditional ecological knowledge in plant conservation and sustainable use is gaining recognition, with efforts to integrate indigenous perspectives and practices into research and management
• Citizen science initiatives, such as iNaturalist and Project BudBurst, engage the public in collecting data on plant phenology, distribution, and interactions, contributing to our understanding of plant ecology and conservation