7.2 Seedless vascular plants
10 min read•august 20, 2024
Seedless vascular plants are ancient land dwellers with specialized tissues for water and nutrient transport. They lack seeds, reproducing via spores, and include , sphenophytes, and pteridophytes. These plants played a crucial role in early terrestrial ecosystems.
These plants dominated during the , forming vast that contributed to coal formation. Though they declined in the Permian, seedless vascular plants persist today, showcasing unique adaptations like alternating generations and diverse ecological roles in modern habitats.
Characteristics of seedless vascular plants
- Seedless vascular plants are a diverse group of ancient land plants that have evolved specialized tissues for the transport of water and nutrients
- These plants lack seeds and instead reproduce using spores, which are small, lightweight reproductive structures that can be dispersed by wind or water
Vascular tissue for transport
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- Seedless vascular plants possess specialized vascular tissues called xylem and phloem that enable efficient transport of water, minerals, and nutrients throughout the plant body
- Xylem tissue consists of tracheids and vessel elements that conduct water and dissolved minerals from the to the
- Phloem tissue is composed of sieve elements and companion cells that transport sugars and other organic compounds from the leaves to other parts of the plant
- The presence of vascular tissue allows seedless vascular plants to grow taller and colonize a wider range of terrestrial habitats compared to non-vascular plants (bryophytes)
Lack of seeds for reproduction
- Unlike seed plants (gymnosperms and angiosperms), seedless vascular plants do not produce seeds as their primary means of reproduction
- The absence of seeds limits the ability of seedless vascular plants to survive in dry or harsh environments, as they require moist conditions for spore germination and fertilization
- Seedless vascular plants rely on other reproductive strategies, such as the production of spores and the alternation of generations, to ensure their survival and dispersal
Spores as reproductive units
- Spores are the primary reproductive units of seedless vascular plants and are produced by specialized structures called
- Spores are haploid (containing a single set of chromosomes) and can be dispersed by wind, water, or animals to new locations
- Upon landing in a suitable environment, spores can germinate and develop into small, independent gametophyte plants that produce male and female reproductive structures (antheridia and archegonia)
- The fusion of male and female gametes produced by the gametophytes gives rise to a diploid sporophyte plant, completing the life cycle of the seedless vascular plant
Major groups of seedless vascular plants
- Seedless vascular plants are classified into three main groups based on their morphological and anatomical characteristics: lycophytes, sphenophytes, and pteridophytes
- These groups represent distinct evolutionary lineages that have adapted to various terrestrial environments and have played significant roles in the development of Earth's ecosystems
Lycophytes (club mosses)
- Lycophytes, commonly known as club mosses, are the oldest and most primitive group of seedless vascular plants, with a fossil record dating back to the Silurian period (440-415 million years ago)
- Modern lycophytes are small, herbaceous plants that typically grow in moist, shaded habitats and have a creeping or climbing growth habit
- Lycophytes are characterized by the presence of microphylls, which are small, simple leaves with a single vein that are arranged in a spiral or whorled pattern along the stem
- Examples of lycophytes include Lycopodium (ground pines) and Selaginella (spike mosses)
Sphenophytes (horsetails)
- Sphenophytes, also known as or scouring rushes, are a group of seedless vascular plants that were dominant during the Carboniferous period (359-299 million years ago) but have since declined in diversity
- Modern sphenophytes are characterized by jointed, hollow that are often ribbed and have whorls of small, scale-like leaves at each node
- Sphenophytes reproduce using cone-like structures called strobili that produce spores, which are dispersed by wind
- The most common extant genus of sphenophytes is Equisetum, which includes species such as the field horsetail (Equisetum arvense) and the giant horsetail (Equisetum giganteum)
Pteridophytes (ferns)
- Pteridophytes, or , are the most diverse and abundant group of seedless vascular plants, with over 10,000 extant species worldwide
- Ferns are characterized by large, compound leaves called fronds that are typically divided into leaflets (pinnae) and have a coiled vernation (fiddle-heads) when young
- Ferns reproduce using sporangia that are usually clustered into sori on the underside of the fronds or on specialized fertile fronds
- Examples of ferns include the bracken fern (Pteridium aquilinum), the maidenhair fern (Adiantum), and the tree ferns (Cyatheaceae)
Evolutionary history of seedless vascular plants
- Seedless vascular plants have a long and complex evolutionary history that spans over 400 million years, from their emergence in the Silurian period to their decline in the Permian period
- The evolution of seedless vascular plants is closely tied to the colonization of land by plants and the development of terrestrial ecosystems
Emergence in Silurian period
- The earliest known fossils of seedless vascular plants date back to the Silurian period (440-415 million years ago), when plants first began to colonize land
- The Silurian period saw the evolution of the first vascular tissues (xylem and phloem) in plants, which allowed them to grow taller and transport water and nutrients more efficiently
- Early seedless vascular plants, such as Cooksonia and Rhynia, were small, herbaceous plants with simple branching patterns and no true leaves or roots
Dominance in Carboniferous period
- Seedless vascular plants reached their peak diversity and ecological dominance during the Carboniferous period (359-299 million years ago)
- The warm, humid climate and high atmospheric CO2 levels of the Carboniferous period favored the growth of extensive forests dominated by lycophytes (scale trees) and sphenophytes (giant horsetails)
- These forests played a crucial role in the formation of coal deposits, as the dead plant material accumulated in swamps and was buried and compressed over millions of years
Decline in Permian period
- The diversity and abundance of seedless vascular plants began to decline during the Permian period (299-252 million years ago), as the climate became drier and cooler
- The evolution of seed plants (gymnosperms) during the Permian period also contributed to the decline of seedless vascular plants, as seed plants were better adapted to survive in drier conditions and could outcompete seedless vascular plants in many habitats
- Despite their decline, seedless vascular plants have persisted to the present day and continue to play important ecological roles in various terrestrial ecosystems
Reproduction in seedless vascular plants
- Seedless vascular plants have a unique reproductive cycle that involves the alternation of generations between a diploid sporophyte phase and a haploid gametophyte phase
- The reproductive strategies of seedless vascular plants have evolved to ensure their survival and dispersal in various terrestrial environments
Alternation of generations
- Seedless vascular plants undergo an alternation of generations, where two distinct phases (sporophyte and gametophyte) alternate in the life cycle
- The sporophyte phase is the dominant, diploid (2n) phase that produces spores through meiosis in specialized structures called sporangia
- The gametophyte phase is the smaller, haploid (n) phase that develops from the spores and produces male and female reproductive structures (antheridia and archegonia)
- Fertilization occurs when sperm from the antheridia swim to and fuse with the eggs in the archegonia, giving rise to a new diploid sporophyte
Sporophyte vs gametophyte
- The sporophyte and gametophyte phases of seedless vascular plants differ in their size, morphology, and function
- The sporophyte is typically larger, more complex, and longer-lived than the gametophyte, and is responsible for producing spores and conducting
- The gametophyte is usually smaller, simpler, and short-lived, and its primary function is to produce gametes (sperm and eggs) for sexual reproduction
- In some seedless vascular plants (e.g., ferns), the gametophyte is free-living and independent of the sporophyte, while in others (e.g., lycophytes), the gametophyte is reduced and dependent on the sporophyte for nutrition
Homospory vs heterospory
- Seedless vascular plants can be classified as homosporous or heterosporous based on the type of spores they produce
- Homosporous plants produce only one type of spore that develops into a bisexual gametophyte, which contains both male and female reproductive structures
- Heterosporous plants produce two types of spores: microspores (male) and megaspores (female), which develop into unisexual gametophytes
- Heterospory is considered an evolutionary step towards the development of seeds, as it allows for the specialization of male and female gametophytes and the provisioning of the female gametophyte with nutrients
Ecological importance of seedless vascular plants
- Seedless vascular plants have played a significant role in shaping Earth's ecosystems throughout their evolutionary history and continue to be important components of many terrestrial habitats
- The ecological importance of seedless vascular plants is multifaceted and includes their contributions to early terrestrial ecosystems, coal formation, and soil development
Role in early terrestrial ecosystems
- Seedless vascular plants were among the first plants to colonize land during the Silurian and Devonian periods (440-359 million years ago)
- The evolution of vascular tissues and other adaptations in seedless vascular plants allowed them to grow taller, transport water and nutrients more efficiently, and colonize a wider range of terrestrial habitats
- Seedless vascular plants played a crucial role in the development of early terrestrial ecosystems by providing habitats and food sources for early land animals and by contributing to the formation of soil
Contribution to coal formation
- During the Carboniferous period (359-299 million years ago), seedless vascular plants (particularly lycophytes and sphenophytes) formed extensive forests in lowland swamps and
- As these plants died and accumulated in the swamps, their biomass was buried and compressed over millions of years, leading to the formation of coal deposits
- Coal is a valuable fossil fuel that has been used by humans for energy production and industrial processes, and its formation is largely attributed to the abundance of seedless vascular plants during the Carboniferous period
Influence on soil development
- Seedless vascular plants have contributed to the development and stabilization of soils through their root systems and the accumulation of organic matter
- The rhizoids and roots of seedless vascular plants help to anchor the plants in the soil, prevent erosion, and facilitate the uptake of water and nutrients
- As seedless vascular plants die and decompose, they add organic matter to the soil, which improves soil structure, fertility, and water retention
- The presence of seedless vascular plants in an ecosystem can also influence the pH and chemical composition of the soil, creating unique microhabitats for other organisms
Adaptations of seedless vascular plants
- Seedless vascular plants have evolved various adaptations that enable them to survive and reproduce in terrestrial environments
- These adaptations include specialized structures for water retention, gas exchange, and anchorage, which have allowed seedless vascular plants to colonize a wide range of habitats
Cuticle for water retention
- Many seedless vascular plants have a waxy cuticle covering their aerial surfaces (stems and leaves) that helps to prevent water loss and protect against UV radiation
- The cuticle is composed of cutin, a hydrophobic polymer that forms a barrier between the plant and the environment
- The thickness and composition of the cuticle can vary among different species of seedless vascular plants, reflecting their adaptations to specific environmental conditions (e.g., drought, high light intensity)
Stomata for gas exchange
- Seedless vascular plants have small pores called stomata on their aerial surfaces (usually leaves) that allow for gas exchange between the plant and the atmosphere
- Stomata are typically composed of two guard cells that can open and close to regulate the exchange of gases (CO2 and O2) and the loss of water vapor
- The density and distribution of stomata on the plant surface can vary among different species of seedless vascular plants, reflecting their adaptations to specific environmental conditions (e.g., humidity, temperature)
Rhizoids for anchorage and absorption
- Many seedless vascular plants (particularly ferns and lycophytes) have small, root-like structures called rhizoids that help to anchor the plant to the substrate and absorb water and nutrients
- Rhizoids are typically unicellular or multicellular filaments that grow from the base of the stem or from specialized organs called rhizophores
- The morphology and abundance of rhizoids can vary among different species of seedless vascular plants, reflecting their adaptations to specific substrates (e.g., soil, rock, bark)
- In some aquatic ferns (e.g., Azolla), the rhizoids have evolved into specialized structures called root pockets that house nitrogen-fixing cyanobacteria, providing the plant with a source of fixed nitrogen
Fossil record of seedless vascular plants
- The fossil record of seedless vascular plants provides valuable insights into their evolutionary history, paleoecology, and the development of terrestrial ecosystems
- Fossils of seedless vascular plants are preserved in various sedimentary rocks and can be studied using techniques from paleobotany and paleoecology
Preservation in sedimentary rocks
- Fossils of seedless vascular plants are typically preserved in sedimentary rocks, such as shales, sandstones, and limestones
- The quality and completeness of the fossil preservation depend on factors such as the type of plant tissue, the depositional environment, and the diagenetic processes that occur after burial
- Common modes of preservation for seedless vascular plant fossils include compressions (flattened remains), impressions (surface features), and permineralizations (three-dimensional structure)
Significance in paleobotany
- The fossil record of seedless vascular plants is of great significance in paleobotany, as it provides evidence for the evolution and diversification of land plants
- Paleobotanists study the morphology, anatomy, and reproductive structures of fossil seedless vascular plants to reconstruct their evolutionary relationships and adaptations
- The fossil record also allows paleobotanists to track the geographic distribution and ecological preferences of seedless vascular plants through time, providing insights into past climates and biogeography
Insights into paleoenvironments
- Fossils of seedless vascular plants can provide valuable information about the paleoenvironments in which they lived
- The presence and abundance of certain species of seedless vascular plants in a fossil assemblage can indicate specific environmental conditions, such as humidity, temperature, and soil type
- For example, the abundance of lycophyte and sphenophyte fossils in Carboniferous coal swamps suggests a warm, humid climate and waterlogged soils
- The study of fossil seedless vascular plants in the context of their associated sediments and other fossil organisms (e.g., animals, fungi) can help to reconstruct ancient ecosystems and understand their functioning
Key Terms to Review (20)
Carboniferous Period: The Carboniferous Period, spanning from about 359 to 299 million years ago, is a significant geological time frame marked by extensive forest ecosystems and the proliferation of seedless vascular plants. This period saw the development of vast swampy regions that contributed to the formation of extensive coal deposits, which played a crucial role in Earth's carbon cycle and the evolution of terrestrial life. The lush vegetation and complex ecosystems created an environment that facilitated the transition from aquatic to terrestrial habitats for many organisms.
Desiccation tolerance: Desiccation tolerance is the ability of certain organisms, particularly plants, to withstand extreme dehydration and survive in dry environments. This trait allows them to enter a dormant state during periods of water scarcity, where metabolic processes slow down or halt, enabling the organism to survive until conditions improve. This adaptation is especially relevant for seedless vascular plants, which have evolved mechanisms to cope with fluctuating moisture levels in their habitats.
Devonian Period: The Devonian Period, often referred to as the 'Age of Fishes,' lasted from about 419 to 359 million years ago. It is notable for significant evolutionary developments, particularly in fish and the emergence of early amphibians from their aquatic ancestors. Additionally, it marked the diversification of seedless vascular plants, which colonized land, contributing to major shifts in terrestrial ecosystems.
Ferns: Ferns are seedless vascular plants that belong to the division Pteridophyta. They are characterized by their feathery leaves, known as fronds, and their ability to reproduce via spores rather than seeds. Ferns thrive in a variety of environments, particularly in moist and shaded areas, and are an important group in the evolution of land plants, showing a complex life cycle that includes both a gametophyte and a sporophyte stage.
Forests: Forests are large areas dominated by trees and other woody vegetation, which create complex ecosystems that provide habitat, food, and resources for a wide range of organisms. In relation to seedless vascular plants, forests serve as important environments where these plants thrive, contributing to the overall biodiversity and ecological balance.
Gametophyte generation: Gametophyte generation refers to the haploid phase in the life cycle of plants, where gametes are produced through mitosis. This generation is crucial for sexual reproduction in seedless vascular plants, as it develops from spores and gives rise to gametes that will eventually fuse to form a diploid zygote. In seedless vascular plants, such as ferns, the gametophyte is typically independent and photosynthetic, highlighting its vital role in the plant's life cycle.
Habitat formation: Habitat formation refers to the processes and factors that create and structure a living environment for organisms. It involves the development of physical and biological characteristics that define an ecosystem, influencing which species can thrive in that environment. The formation of habitats is critical for the survival of various plant groups, particularly in understanding their ecological roles and interactions.
Horsetails: Horsetails are a group of seedless vascular plants belonging to the genus Equisetum, characterized by their jointed stems and whorled leaves. These ancient plants have a rich history, dating back to the Carboniferous period, and are often found in wetland habitats. They play an important role in the ecosystem as they can thrive in nutrient-poor soils and contribute to soil stabilization.
Leaves: Leaves are the primary sites of photosynthesis in plants, typically green in color due to chlorophyll. In the context of seedless vascular plants, leaves play a crucial role in capturing sunlight and exchanging gases, which is essential for the plant's survival and growth. They come in various shapes and sizes, influencing how plants adapt to their environments and how they reproduce through spores.
Lycophytes: Lycophytes are a group of seedless vascular plants that include clubmosses, spikemosses, and quillworts. They are characterized by their small, scale-like leaves called microphylls and their ability to reproduce via spores instead of seeds. These ancient plants played a significant role in Earth's history, particularly during the Carboniferous period, when they thrived in lush forests and contributed to the formation of coal deposits.
Photosynthesis: Photosynthesis is the biochemical process by which green plants, algae, and some bacteria convert light energy into chemical energy, primarily glucose, using carbon dioxide and water. This process is essential for life on Earth as it provides the organic compounds and oxygen necessary for the survival of most living organisms. It connects to early life forms that began to influence the atmosphere, evolving through complex plant groups over geological time.
Roots: Roots are essential structures in vascular plants that anchor the plant to the soil and absorb water and nutrients necessary for growth. They play a critical role in the overall health of a plant, as they not only secure it in place but also facilitate the uptake of essential minerals and water from the soil, which are vital for photosynthesis and other metabolic processes.
Soil stabilization: Soil stabilization refers to the process of improving the physical properties of soil to enhance its strength and load-bearing capacity. This technique is essential for ensuring the stability of the ground in various ecological contexts, including the habitats of seedless vascular plants. By altering soil characteristics, it helps in maintaining the integrity of ecosystems that depend on these plants for structure and nutrient cycling.
Sporangia: Sporangia are specialized structures in plants that produce and contain spores, which are crucial for reproduction and dispersal. These structures are found in various plant groups, particularly in seedless vascular plants, where they play a key role in the alternation of generations, facilitating the transition between haploid and diploid phases in their life cycles.
Spore production: Spore production is the biological process through which certain plants, fungi, and bacteria create and release spores as a means of reproduction and dispersal. In the context of seedless vascular plants, this process is crucial as these organisms do not produce seeds, relying instead on spores to propagate their species and ensure survival in various environments.
Stems: Stems are the structural components of vascular plants that support leaves, flowers, and fruits, while facilitating the transport of water, nutrients, and photosynthates throughout the plant. They play a crucial role in maintaining the plant's upright structure and overall growth, allowing for effective light capture and reproductive success.
Transpiration: Transpiration is the process by which water vapor is released from plant leaves into the atmosphere, primarily through small openings called stomata. This process not only helps regulate water loss in plants but also plays a crucial role in nutrient uptake and maintaining plant health. Transpiration creates a negative pressure that draws water and nutrients from the roots up through the plant, making it essential for the survival of vascular plants.
Vascularization: Vascularization refers to the development and organization of vascular tissues, which are specialized for the transport of water, nutrients, and food in plants. This process is crucial for the growth and survival of seedless vascular plants, as it allows them to efficiently transport resources throughout their structures. Vascularization is characterized by the presence of two main types of tissues: xylem and phloem, which work together to facilitate the movement of substances and support the plant's overall structure.
Wetlands: Wetlands are ecosystems that are characterized by saturated soil conditions and the presence of water, either permanently or seasonally. They serve as critical habitats for a variety of organisms, including seedless vascular plants, which thrive in these moist environments and play essential roles in nutrient cycling and water filtration.
Xylem and Phloem Structure: Xylem and phloem are the two types of vascular tissues found in plants, crucial for transporting water, nutrients, and food throughout the organism. Xylem is primarily responsible for the upward movement of water and dissolved minerals from the roots to the rest of the plant, while phloem transports organic compounds, mainly sugars produced during photosynthesis, from the leaves to other parts of the plant. Together, these structures play a vital role in the survival and growth of seedless vascular plants.