They are adapted successfully to rough land habitats. Flowering plants are also tracheophytes. They have dominated the land habitat. The vascular plants show evolution of following complex vegetative and reproductive characteristics. These characteristics have made them the predominant plants of land:
- They have root, stem and leaves.
- They have vascular systems in stems, roots and leaves.
- Tracheophytes have protected sporangia, leading to evolution of seed.
- They develop pollen tube. So, they do not require water for the transmission of male gametes to female gametes.
- They develop flower and fruits.
- The show heteromorphic alternation of generation.
Sub-divisions of Tracheophytes
The tracheophytes are further sub-divided into four sub-divisions:
- Sub-division Psilopsida (Psilopsids)
- Sub-division Lycopsida (Club Mosses)
- Sub-division Sphenopsida (Horse Tail)
- Sub-division Pteropsida (Ferns)
- Sub-division Spermopsida (Seed Plant)
They show following characteristics:
- Psilopsida has rootless sporophyte.
- The stem is divided into an underground part rhizome and an aerial part.
- The stem is branched dichotomously.
- The rhizome develops rhizoids. Both rhizome and rhizoid perform the function of roots.
- The aerial braches are green, leafless. They have small vein less outgrowths. These branches carry out photosynthesis.
- The reproductive organs of sporophyte are sporangia. These sporangia develop at tips of branches or at the lateral side of branches.
- Internal structure of stem is simple. They have narrow, central, and solid vascular tissues. They are without pith. Their cortex is wide.
- Their gametophyte is thalloid. It is colorless and underground. Its cells contain a fungus. This fungus provides food to the gametophyte and gametophyte provides it protection. Such beneficial symbiotic relationship among the two members (fungus and plants) is called mycorrhizal association.
- Classification Psilopsida is the earliest group of vascular plants. Most of the representative of this group has become extinct, for example, Rhynia, Horneophyton, Psilophyton and Cooksonia.
Psilopsida has two living genera:
Evolution of Leaf in Tracheophytes
Early vascular land plants did not have true leaves or roots. These plants had small size. They had dichotomously branched erect smooth aerial parts. They have subterranean (underground) rhizome. It was used for anchoring and absorption. Later, evolution of leaf took place.
Evolution of Microphyll (Small Leaf)
A leaf with small size and with single undivided vein (vascular supply) is called microphyll. E.g. Leaves of Lycopods (Lycopodium). Rhynia was a primitive vascular plant. It had naked stem with leaves.
Such plants started forming leaves as small outgrowths. These outgrowths were not supplied with vascular bundles. Therefore, they were not regarded as true leaves. Lycopods were the first plants that formed the true leaves and roots.
Large leaves with divided veins and veinlets and with expanded leaf blade or lamina are known as megaphylls. Such leaves are characteristics of ferns and seed plants.
It is suggested that evolution of megaphylls started from a dichotomous branching system. This evolution took place in some primitive fern like plants approximately 350 million years ago. The evolution of megaphyll took place in following steps:
There were some unequal branches on the dichotomously branched aerial, portion. Some branches remained short, while other branches grew and expanded at a much faster pace. All these branches grew in different planes. Such an unequal development of various branches is called overtopping.
In this step, the dichotomous branches were arranged in one plane. This process is called planation.
3. Fusion or Webbing
In this step, the space between dichotomous branches was filled by sheet of parenchyma cells. Parenchyma connected these branches and formed a flat lamina or leaf blade structure. This structure has many dichotomously branched veins.
After some time, the vascular strands fused to form a network or reticulation pattern. The process of evolution was very slow and gradual. It was completed in more than 15—20 million years.
- The plant body of Lycopsida is sporophyte. It is differentiated into roots, stem and true leaves.
- The leaves are small and single veined. They are also called microphylls. The arrangement of leaves is spiral or opposite.
- There are certain reproductive leaves called sporophylls. The sporangia develop singly on the upper side of the sporophylls. These sporophylls may arrange to form srobili.
- The leaves and sporophylls of certain Lycopsida like Lycopodiurn have small outgrowth called ligules. In certain other Lycopods like Selaginella, ligules are present near the base of the upper side of the leaves.
- The sporophytes of some Lycopods have one kind of sporangia like Lycopodium. They produce single type of spore. Such sporophytes are called homosporous. This condition ¡s called homospory. Some have two types of sporangia microsporangia and megasporangia as in Selaginella. They produce two types of spores. Such sporophyte is called heterosporous. This condition is called heterospory. Selaginella is a heterosporous plant and so it resembles with the seed producing plants (spermatophytes).
- Lycopsids are not mosses. Their srobili are club shaped or spike shaped. They have small leaves, resembling with leaves of mosses (bryophytes). So they are called club mosses or spike mosses. They also show resemblances with the evergreen plants. So they are, sometimes, called ground pries.
- The gametophyte of Lycopsida is mainly underground.
- The sporophyte of Sphenopsida is differentiated into roots, stem and leaves,
- The leaves may be expanded or scale like and are always arranged in whorls.
- Their whole plant body is composed of a large number of joints. So these plants are also called arthrophytes.
- Main stem is not smooth. It has a large number of ridges and furrows. It has nodes and internodes. Each node has whorl of branches.
- The sporangia are borne on structures called sporangiophores. The sporangia aggregate on sporangiophores to form cones or strobilus. Each sporangiophore has a slender stalk and an expanded disc at its free end. The sporangia are present on the underside of the disc.
- Their gametophyte is thalloid. It grows on clayey soil or on mud.
- Example: Equisetum.
- Pteropsida evolved from Psilopsida, appeared in the Devonian period live in carboniferous period decline in late Paleozoic era.
- They have true root, stem and leave with all developed vascular system.
- Leaves of fern are some the simple but more often they are compound.
- Stem are prostrate on or in the soil and large leaves are the only part normally seen.
- Large leafy fern is diploid sporopyte spore are produced in sporongia, located in cluster on the underside of see leave. A number of sporangia develop inside single sorus.
Pteropsida is divided into three classes:
- Class Filicineae
- Class Gymnospermae
Spermopsida (Seed Plant)
- They are most successful group of plant appeared in the late Devonian period live in the carboniferous period and replaced the Sphenosida and Lycopsodia.
- Gametophyte are reduced and not photosyntheic.
- Sperm are not independent free swimming flagellated cells. Seed plant can be further divided into gymonosperm (naked seed plant) and angiosperm (covered seed plant).