Vascular Plants: Evolution, Adaptations, and Diversity
Evolution and Adaptations of Vascular Plants
Plants with vascular tissues are classified in the phylum Tracheophyta. They are frequently also called vascular plants. This term highlights the most important characteristic of these plants: the presence of conducting vessels, which are true tubes comparable to the blood vessels of animals. Through these vessels, water absorbed by roots is transported into the uppermost leaves. Thus, water found under the ground level is made available to leaves that can be tens of meters high. In vascular plants, the vessels form a dense network that is distributed to all organs, ensuring a continuous supply of water. Even the most barren land in the deep strata retains a certain amount of water. The function of roots is to seek, absorb, and incorporate it into the network of vessels. Therefore, plants without conducting vessels are deprived of these organs and roots, and thus have not managed to colonize arid areas.
Water Transport and Plant Size
Through the conducting tissues, the size of vascular plants is not limited by the difficulty of transporting water, as it is in small bryophytes. They can grow freely to achieve record heights, such as the tree ferns that lived hundreds of millions of years ago, or current redwoods and eucalyptus that can reach 100 meters high.
Structural Adaptations: Mechanical Tissues and Evaporation Control
The large size of many vascular plants is made possible by the development of mechanical tissues that are arranged within the stem and root, and to a lesser extent, also in the leaves. To survive without drying quickly, the plant must find a way to control evaporation, and in extreme cases, to prevent it entirely. To this end, vascular plants develop tegumentary tissues, whose waterproofed outer surface impedes water evaporation. The only communication between the plant body and the external environment occurs through microscopic openings whose width is adjustable: the stomates.
Summarizing, to live on the Earth’s surface, plants have developed specialized tissues:
- Conductive (for water and nutrient transport)
- Mechanical (for structural support)
- Integumentary (for protection and evaporation control)
Ferns: Characteristics and Life Cycle
Ferns are plants that have evolved quite a well-developed vascular system with true roots, stems, and leaves (fronds).
Leaf Development and Morphology
Leaves are believed to have emerged in a different way than they did in lycopsids. Rather than emerging from the stem, they are likely ramifications of it. It is thought that a group of small branches could be arranged in one plane and the spaces filled with tissue. These leaves are called megaphylls, while more primitive, early developed leaves are called microphylls. Sometimes the leaves are simple, but more often they are compound, divided into numerous leaflets which can give the plant a distinctive appearance. These leaflets are called pinnae. A characteristic of fern leaves is their winding end (circinate vernation), which unrolls as the leaf matures.
Stem Structure and Growth Habits
In a few ferns, such as tree ferns of the tropics, the stem is upright and forms a trunk. However, in most temperate ferns, the stem lies on or under the ground, and long leaves or fronds are usually the only visible parts. The stem shows protective epidermis and vascular supporting tissues. The leaves have veins, collenchyma, protective epidermis, and stomates.
Reproduction and Life Cycle
Fern leaves represent the diploid stage and constitute the sporophyte. Spores are produced in sporangia grouped in formations called sori, which are located on the underside of some leaves or fronds. Leaves bearing sporangia are called sporophylls. After germination, the spores originate gametophytes that produce archegonia and antheridia. These gametophytes, or prothallia, are small (rarely more than half a centimeter wide), thin, and often heart-shaped.
The Filicina Class: Diversity and Distribution
Approximately 10,000 species of ferns are now living on Earth. They are herbaceous plants with rhizomes, but sometimes, in tropical climates, they can also be arborescent (tree-like). Ferns are distributed worldwide in diverse environments. They are found abundantly in warm, humid regions, which are inhabited by larger species and are represented by a remarkable number of species.
Classification of Tracheophytes
- Psilopsida
- Lycopsida
- Sphenopsida
- Pteropsida (Filicinae or Ferns)
- Gymnosperms (Conifers)
- Angiosperms (Flowering Plants)