The Devonian Period (approximately 419 to 359 million years ago) is known as the “Age of Fishes,” but it was also a transformative time for plant evolution. Plants embarked on a journey to colonize terrestrial environments, fundamentally reshaping the Earth’s surface and setting the stage for future ecosystems.
The Rise of Complex Plant Life
A major evolutionary innovation during the Devonian was the development of vascular tissue, which allowed plants to grow taller and transport resources more efficiently. This internal transport system, consisting of xylem (moving water and dissolved minerals upwards) and phloem (distributing sugars), provided structural support and a circulatory system, enabling plants to achieve greater heights and move beyond low-lying, damp environments.
Building upon this, the emergence of true roots provided plants with stable anchorage and improved their ability to absorb water and nutrients from deeper within the soil. Early Devonian plants lacked true roots, relying instead on shallow rhizoids or symbiotic relationships with fungi to acquire nutrients. True leaves also evolved, offering a larger surface area for photosynthesis and gas exchange, further enhancing the plant’s ability to produce energy.
Early reproductive strategies also saw advancements, moving beyond simple spore dispersal. While many Devonian plants still reproduced via spores, the Late Devonian saw the appearance of the first seed-bearing plants. Seeds offered increased protection for the developing embryo and contained food reserves, allowing for greater survival rates and dispersal over longer distances compared to spores. This shift marked a significant step towards plants dominating terrestrial landscapes.
Major Devonian Plant Groups
The Devonian landscape witnessed the diversification of several distinct plant groups, each showcasing the evolutionary innovations of the period. Among the earliest vascular plants were the Rhyniophytes, present from the mid-Silurian into the Lower Devonian. These simple, leafless, and rootless plants, such as Cooksonia and Rhynia, had upright, branching stems with spore cases (sporangia) at their tips. Cooksonia grew only a few centimeters tall and showed evidence of vascular tissue.
Zosterophylls were another prominent group of early Devonian vascular plants. Unlike rhyniophytes, zosterophylls bore their kidney-shaped sporangia on short stalks along the sides of their stems, rather than at the tips. These plants, which included Zosterophyllum, lacked true leaves and roots, with photosynthesis occurring primarily in their stems.
By the Late Devonian, more complex forms had emerged, including tree-like lycopods (clubmosses) such as Lepidodendron. These tree-like lycopods could grow into towering trees over 35 meters (115 feet) tall, forming some of the earliest forests. Sphenophytes, the ancestors of modern horsetails, also diversified, characterized by their distinctive jointed stems. Early ferns, with their more complex leaf structures, also made their appearance in the mid-Devonian.
A significant plant group of the Late Devonian was Archaeopteris, a progymnosperm. This tree-like plant, growing up to 30 meters (98 feet) tall with a trunk diameter exceeding 1 meter (3 feet), possessed true wood similar to conifers but reproduced by spores and had fern-like leaves. Archaeopteris represents an important transitional form, showcasing characteristics of both ferns and the more advanced seed plants that would follow.
Global Impact of Devonian Flora
The widespread growth and diversification of Devonian plants had significant environmental consequences, altering Earth’s systems. Through photosynthesis, these plant communities absorbed significant amounts of carbon dioxide from the atmosphere and released oxygen, leading to a considerable increase in atmospheric oxygen levels. This oxygen enrichment paved the way for the evolution and diversification of larger animal life, particularly terrestrial arthropods and early vertebrates.
The development of true roots in Devonian plants also played a major role in the formation of the first true soils. As roots penetrated deeper into the rock, they initiated the process of weathering, breaking down minerals and contributing organic matter from decaying plant material. This transformation of barren rock into fertile ground created new habitats and nutrient cycles on land.
Plant roots further stabilized riverbanks, influencing water flow and significantly reducing erosion. This stabilization led to the development of more defined river channels and extensive floodplains, altering the Earth’s hydrological cycle. The increased input of nutrients from land into marine environments also impacted oceanic chemistry.
The extensive “greening” of the continents by Devonian flora also influenced global climate patterns. The large-scale removal of atmospheric carbon dioxide by plants, a greenhouse gas, contributed to a cooling trend towards the end of the Devonian Period. This climate modification, coupled with other geological events, played a role in the significant extinction events observed at the close of the Devonian.