7.1 Early land plants
7 min read•august 20, 2024
Early land plants revolutionized Earth's ecosystems, evolving from aquatic algae to colonize land during the Period. They developed crucial adaptations like waxy cuticles, vascular tissues, and to survive in terrestrial environments.
These pioneering plants paved the way for complex life on land. They diversified into major groups like and tracheophytes, shaping ecosystems, influencing atmospheric composition, and forming the basis of terrestrial food webs.
Origins of land plants
- Land plants evolved from aquatic green algae ancestors and colonized terrestrial environments during the Ordovician Period, approximately 470 million years ago
- The transition from aquatic to terrestrial habitats required significant adaptations to cope with challenges such as desiccation, UV radiation, and nutrient acquisition
- Early land plants played a crucial role in shaping Earth's ecosystems and paved the way for the evolution of more complex terrestrial life forms
Evolutionary adaptations for land
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- Development of a waxy to prevent water loss and protect against UV radiation
- Evolution of specialized cells and tissues for water and nutrient transport (xylem and phloem)
- Presence of stomata for gas exchange and regulation of water loss through transpiration
- Production of sporopollenin, a resistant polymer that protects reproductive structures ( and pollen)
Transition from aquatic to terrestrial
- Gradual adaptation to intermittently wet environments (e.g., mudflats and shorelines)
- Development of anchoring structures ( and root-like organs) for stability and nutrient uptake
- Evolution of upright growth forms to maximize light capture and spore dispersal
- Establishment of symbiotic relationships with fungi (mycorrhizae) for enhanced nutrient acquisition
Timeline of early land plant evolution
- Late Ordovician (450 Ma): Earliest evidence of land plant spores (cryptospores)
- Silurian (440-415 Ma): Diversification of non-vascular plants (bryophytes) and appearance of early vascular plants
- (415-360 Ma): Rapid diversification of vascular plants, including the emergence of lycophytes and euphyllophytes
- Carboniferous (360-300 Ma): Dominance of lycophytes and development of extensive coal swamp forests
Morphological characteristics
- Early land plants exhibited a range of morphological adaptations that enabled them to thrive in terrestrial environments and exploit new ecological niches
- The evolution of specialized tissues and organs, such as vascular systems, leaves, and roots, allowed for more efficient resource acquisition and distribution
- Reproductive structures and strategies played a crucial role in the dispersal and colonization of new habitats
Primitive vascular systems
- Presence of simple conducting tissues (xylem and phloem) for long-distance transport of water and nutrients
- Xylem composed of tracheids, elongated cells with reinforced walls for water conduction
- Phloem consisting of sieve cells for the transport of organic compounds (e.g., sugars and amino acids)
- Vascular tissues arranged in simple strands or bundles, often lacking secondary growth
Development of leaves and roots
- Early land plants possessed simple, undifferentiated photosynthetic structures (e.g., thalloid or leafy gametophytes)
- Evolution of true leaves (megaphylls) in vascular plants, increasing photosynthetic efficiency and gas exchange
- Emergence of root systems for anchorage, water, and nutrient uptake from the soil
- Roots originated independently in different plant lineages (lycophytes and euphyllophytes)
Reproductive structures and strategies
- Alternation of generations, with a dominant gametophyte (haploid) phase in non-vascular plants and a dominant sporophyte (diploid) phase in vascular plants
- Production of spores in specialized structures (sporangia) for dispersal and reproduction
- Evolution of heterospory, with separate male (microspores) and female (megaspores) spores
- Development of pollen grains and in more derived plant groups, enhancing reproductive success in terrestrial environments
Major early land plant groups
- Early land plants can be broadly categorized into non-vascular (bryophytes) and vascular (tracheophytes) groups
- Bryophytes, including mosses, liverworts, and hornworts, lack true vascular tissues and rely on external water for reproduction
- Tracheophytes possess vascular tissues and can be further divided into lycophytes and euphyllophytes based on their morphological characteristics
Bryophytes (non-vascular plants)
- Comprise mosses, liverworts, and hornworts
- Lack true vascular tissues, leaves, and roots
- Dominant gametophyte phase in the life cycle
- Reproduce via spores and require external water for fertilization
- Adapted to moist environments and play important roles in nutrient cycling and soil stabilization
Tracheophytes (vascular plants)
- Possess true vascular tissues (xylem and phloem) for long-distance transport of water and nutrients
- Dominant sporophyte phase in the life cycle
- Include lycophytes and euphyllophytes
- Adapted to a wide range of terrestrial environments and exhibit diverse growth forms (e.g., herbs, shrubs, trees)
Lycophytes vs euphyllophytes
- Lycophytes (clubmosses, spikemosses, and quillworts) possess microphylls, simple leaves with a single vein
- Euphyllophytes (ferns, horsetails, and seed plants) have megaphylls, larger leaves with complex venation patterns
- Lycophytes have roots that develop from modified stem structures (rhizophores), while euphyllophytes have true roots that arise from the embryo
- Lycophytes reproduce via homosporous or heterosporous spores, while euphyllophytes exhibit a trend towards heterospory and seed production
Ecological impact
- Early land plants played a significant role in shaping terrestrial ecosystems and influencing global biogeochemical cycles
- Their surfaces led to the development of soil profiles, increased weathering rates, and changes in atmospheric composition
- Early land plants formed the basis of terrestrial food webs and provided habitats for a diverse array of organisms
Roles in early terrestrial ecosystems
- Primary producers, converting solar energy into organic compounds through photosynthesis
- Formed the foundation of terrestrial food webs, supporting the evolution and diversification of herbivores and decomposers
- Provided habitats and microenvironments for other organisms (e.g., insects, fungi, and microbes)
- Contributed to the development of complex ecological interactions and coevolutionary relationships
Contributions to soil formation
- Accelerated physical and chemical weathering of rocks through root penetration and exudation of organic acids
- Stabilized soil particles and prevented erosion through the binding action of roots and rhizoids
- Contributed organic matter to the soil through the decomposition of plant litter
- Facilitated the development of soil profiles and the establishment of diverse soil microbial communities
Influence on atmospheric composition
- Increased oxygen levels in the atmosphere through photosynthesis and burial of organic carbon
- Reduced atmospheric CO2 concentrations, potentially contributing to global cooling events (e.g., Late Ordovician glaciation)
- Influenced the global water cycle through transpiration and the formation of cloud-nucleating aerosols
- Modulated the Earth's albedo and energy balance through changes in land surface properties (e.g., roughness, reflectivity)
Fossil record
- The fossil record provides crucial insights into the evolution and diversification of early land plants
- Plant fossils are preserved through various processes, including permineralization, compression, and charcoalification
- Key fossil localities and assemblages document the morphological and ecological changes in early land plant communities over time
Preservation of early land plants
- Permineralization: Infiltration of plant tissues by mineral-rich solutions, resulting in the preservation of cellular details (e.g., Rhynie Chert)
- Compression: Flattening of plant remains between sediment layers, preserving external morphology (e.g., Devonian shales)
- Charcoalification: Preservation of plant fragments as charcoal due to incomplete combustion (e.g., Devonian wildfire deposits)
- Spores and pollen: Resistant structures that can be preserved in sediments and used for biostratigraphic and paleoecological studies
Key fossil localities and assemblages
- Rhynie Chert (Scotland, Early Devonian): Exceptionally preserved early vascular plants and associated biota
- Gilboa Fossil Forest (New York, Middle Devonian): In situ tree stumps and rooting systems of early forests
- Coal swamp deposits (Carboniferous): Extensive accumulations of plant remains, documenting the diversity and ecology of lycophyte-dominated forests
- Permian-Triassic boundary sections: Record the response of plant communities to mass extinction events
Techniques for studying plant fossils
- Light and electron microscopy: Examination of morphological and anatomical details
- Thin sectioning: Preparation of thin slices of permineralized fossils for microscopic analysis
- Palynology: Study of spores and pollen preserved in sediments
- Geochemical analysis: Investigation of stable isotope ratios and biomarker compounds to reconstruct paleoenvironmental conditions and plant physiology
Evolutionary significance
- The evolution of early land plants represents a major transition in Earth's history, setting the stage for the colonization of terrestrial environments and the diversification of life on land
- Early land plants served as precursors to modern plant lineages and provided the foundation for the development of complex terrestrial ecosystems
- The coevolution of early land plants with terrestrial fauna, such as arthropods and vertebrates, shaped the evolutionary trajectories of both groups
Diversification and adaptive radiation
- Early land plants underwent rapid diversification during the Devonian Period, giving rise to a wide range of morphologies and ecological strategies
- Adaptive radiation in response to new terrestrial niches and opportunities for ecological specialization
- Evolution of key innovations, such as vascular tissues, leaves, roots, and seeds, enabling the exploitation of diverse environments
Precursors to modern plant lineages
- Bryophytes represent the earliest diverging lineages of land plants and provide insights into the transition from aquatic to terrestrial life
- Lycophytes and euphyllophytes (ferns, horsetails, and seed plants) form the two major clades of vascular plants
- Early land plants established the evolutionary framework for the subsequent diversification of modern plant groups (e.g., conifers, flowering plants)
Coevolution with early terrestrial fauna
- The evolution of early land plants provided new food sources and habitats for terrestrial animals
- Herbivory and pollination syndromes emerged as a result of plant-animal interactions
- Coevolutionary arms races between plants and herbivores drove the evolution of defensive mechanisms (e.g., lignification, secondary metabolites) and specialized feeding strategies
- Development of complex multi-trophic interactions involving plants, herbivores, predators, and decomposers in early terrestrial ecosystems
Key Terms to Review (18)
Bryophytes: Bryophytes are non-vascular plants that include mosses, liverworts, and hornworts. These early land plants play a crucial role in the evolution of terrestrial ecosystems, as they were among the first organisms to colonize land, paving the way for more complex plant life. Bryophytes are characterized by their simple structure and reproduction through spores, which makes them distinct from vascular plants that have specialized tissues for water and nutrient transport.
Carbon cycling: Carbon cycling refers to the continuous movement of carbon through the Earth's atmosphere, oceans, soil, and living organisms. This cycle is crucial for regulating the Earth's climate and supporting life, as it involves processes such as photosynthesis, respiration, decomposition, and combustion. In the context of early land plants, carbon cycling played a pivotal role in transforming the Earth's atmosphere and supporting diverse ecosystems by facilitating the storage and release of carbon compounds.
Climate adaptation: Climate adaptation refers to the adjustments made by organisms, including plants, in response to changes in climate conditions in order to survive and thrive. This concept is essential for understanding how early land plants developed features that enabled them to cope with varying environmental factors, such as temperature fluctuations, moisture levels, and sunlight exposure. Effective climate adaptation ensures the continuation of species and their roles in ecosystems amidst shifting climates.
Colonization of land: The colonization of land refers to the process by which organisms, particularly plants and animals, establish themselves and adapt to terrestrial environments. This critical event marked a significant shift in the evolution of life on Earth, as species transitioned from aquatic habitats to land, leading to new ecosystems and diverse biological innovations.
Cooksonia: Cooksonia is an early land plant from the Silurian period, known for its simple, branching structure and reproductive features. This primitive plant is significant as it represents one of the first groups of vascular plants to transition from aquatic environments to terrestrial ecosystems, showcasing important adaptations that allowed for life on land.
Cuticle: A cuticle is a protective layer of waxy substance that covers the surfaces of land plants, primarily on leaves and stems. This adaptation plays a crucial role in reducing water loss, making it essential for early land plants that faced the challenge of drying out in terrestrial environments. The cuticle not only helps retain moisture but also serves as a barrier against pathogens and herbivores, contributing to the overall survival and success of these plants on land.
David Ackerly: David Ackerly is a prominent biologist known for his research on plant ecology and evolution, particularly in relation to how plants adapt to their environments. His work has provided significant insights into the evolution of early land plants and their physiological responses to changing climates, helping to understand the historical context of plant development and biodiversity.
Devonian: The Devonian is a geologic period and system that spans from approximately 419 to 359 million years ago, known for significant developments in Earth's history, particularly the evolution of early land plants and the diversification of fish. This period is often referred to as the 'Age of Fishes' due to the vast variety of fish species that thrived in the oceans, while also marking the transition of some organisms from water to land.
Katherine J. Willis: Katherine J. Willis is a notable figure in the field of paleobotany, particularly recognized for her research on early land plants and their evolutionary significance. Her work has contributed to the understanding of how plants adapted to terrestrial environments and the ecological changes that occurred during the transition from aquatic to terrestrial habitats. Through her studies, she has highlighted the importance of early land plants in shaping ecosystems and their role in the Earth's history.
Ordovician: The Ordovician is a geological period that lasted from about 485 to 443 million years ago, following the Cambrian period and preceding the Silurian period. It is marked by significant diversification of marine life and the first evidence of land plants, showcasing a time of great evolutionary change and ecological development in the oceans and on land.
Pteridophytes: Pteridophytes are a group of vascular plants that reproduce via spores and do not produce seeds or flowers. This group includes ferns, horsetails, and clubmosses, which were some of the earliest land plants to evolve, showcasing key adaptations to terrestrial life such as vascular tissue for nutrient transport and a dominant sporophyte generation in their life cycle.
Rhizoids: Rhizoids are root-like structures found in certain non-vascular plants, such as mosses and liverworts, that anchor the plant to the substrate. They help in stabilizing the plant, while also assisting in the absorption of water and nutrients. Unlike true roots, rhizoids do not have vascular tissue, which means they do not transport water and nutrients as effectively as roots do in higher plants.
Rhynia: Rhynia is an extinct genus of early land plants that lived during the Devonian period, known for its simple structure and significant role in the evolution of terrestrial flora. This plant is often recognized for its rhizomatous growth form and spore-based reproduction, which laid the groundwork for more complex land plants that followed. Its discovery provided crucial insights into the transition of life from aquatic environments to terrestrial ecosystems.
Seeds: Seeds are reproductive structures that contain a developing embryo and a supply of nutrients, protected by a seed coat. They represent a crucial evolutionary advancement in plants, allowing them to reproduce and disperse efficiently in diverse environments, ultimately contributing to the establishment of early land plants.
Soil formation: Soil formation is the process through which soil develops from the weathering of rocks and the accumulation of organic matter, resulting in a layered structure that supports plant life. This process involves various factors such as climate, organisms, topography, parent material, and time, which all interact to create diverse soil types. The presence of early land plants played a significant role in enhancing soil development by contributing organic material and promoting nutrient cycling.
Spores: Spores are reproductive units produced by various organisms, particularly in fungi, algae, and plants, that can develop into a new individual without fusion with another reproductive cell. In the context of early land plants, spores played a crucial role in their reproductive strategies and adaptation to terrestrial environments, allowing them to disperse and colonize new habitats efficiently.
Stomata: Stomata are small openings on the surfaces of leaves and stems that facilitate gas exchange between the plant and its environment. They play a crucial role in photosynthesis and respiration by allowing carbon dioxide to enter the plant while enabling oxygen and water vapor to escape. The presence of stomata is a significant adaptation that enabled early land plants to thrive in terrestrial environments, where efficient gas exchange is essential for survival.
Vascular tissue: Vascular tissue is a specialized system of plant cells that facilitates the transport of water, nutrients, and sugars throughout the plant. This tissue is essential for the growth and survival of land plants, as it allows them to efficiently manage resources and maintain structural integrity. It consists primarily of two main types: xylem, which transports water and minerals from the roots to the rest of the plant, and phloem, which distributes organic compounds like sugars produced during photosynthesis.