Origin of organized structures

It took a long time for plants to develop proper roots and vascular system for land dwelling. These carry water drawn through roots to all parts of their bodies – as well as taller, stiffer bodies that can evolve into ferns and trees. They would evolve ways of reproducing in the absence of water. They develop ovaries, producing ova, or eggs that are fertilized by pollen and develop into embryos within protective bodies. Plants depend on insects for reliable transfers of pollen, while wind, birds, and other animals scatter their fertile seeds.

By the Ordovician period, shallow sea covered large areas of the world. Then the primitive plants such as Boiophyton Pragense had begun to adapt themselves to life on the relatively dry land. Boiophyton had already developed a stiff stem to hold it upright against gravity. It was vascular. It had developed a system of vessels or tubes to carry water through its body

Many Devonian plants were seedless. Fern bear interesting traces of their origins. They produce no seeds but rather spore cells that fall to the soil and grow into a peculiar intergeneration plant that is not a leafy fern but a specialized fungus-like form that produces egg cells and sperm cells. Propelled by its whip-like tail, the sperm must swim a short distance through the water of moist soil to fertilize an egg cell from a neighboring plant. The fertilized egg then grows into a mature fern that produces more spores. It is as if the fern is an amphibian plant, growing on land but reverting to water to transmit its gametes.

Early Aquatic and Terrestrial ecosystem

Early aquatic ecosystem

An aquatic ecosystem is an ecosystem in a body of water. Communities of organisms that are dependent on each other and on their environment live in aquatic ecosystems. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems. Early aquatic ecosystems were mostly marine ecosystem. Some water bodies like rivers and lakes were formed due to molting of glaciers.

Most of productively of aquatic ecosystem depends on the marine life. It mostly composed of algae and fishes. Most of photosynthesis was carried out by primitive algae and cyanobacteria.

Fresh water bodies also have some life. They also have zooplanktons and phytoplankton. Some primitive floating mosses were also present in these bodies. Consumers were zooplankton and some species of fishes and amphibians.

Early terrestrial ecosystem

A community of organisms and their environment that occurs on the land masses of continents and islands. Terrestrial ecosystems are distinguished from aquatic ecosystems by the lower availability of water and the consequent importance of water as a limiting factor. Terrestrial ecosystems are characterized by greater temperature fluctuations on both a diurnal and seasonal basis than occur in aquatic ecosystems in similar climates. The availability of light is greater in terrestrial ecosystems than in aquatic ecosystems because the atmosphere is more transparent than water. Gases are more available in terrestrial ecosystems than in aquatic ecosystems. Those gases include carbon dioxide that serves as a substrate for photosynthesis, oxygen that serves as a substrate in aerobic respiration, and nitrogen that serves as a substrate for nitrogen fixation. Terrestrial environments are segmented into a subterranean portion from which most water and ions are obtained, and an atmospheric portion from which gases are obtained and where the physical energy of light is transformed into the organic energy of carbon-carbon bonds through the process of photosynthesis.

The first primitive seed plants, Pteridosperms (seed ferns) and Cordaites, both groups now extinct, appeared in the late Devonian and diversified through the Carboniferous, with further evolution through the Permian and Triassic periods. In these the gametophyte stage is completely reduced, and the sporophyte begins life inside an enclosure called a seed, which develops while on the parent plant, and with fertilization by means of pollen grains. Whereas other vascular plants, such as ferns, reproduce by means of spores and so need moisture to develop, some seed plants can survive and reproduce in extremely arid conditions.



Early seed plants are referred to as gymnosperms (naked seeds), as the seed embryo is not enclosed in a protective structure at pollination, with the pollen landing directly on the embryo. Four surviving groups remain widespread now, particularly the conifers, which are dominant trees in several biomes. The angiosperms, comprising the flowering plants, were the last major group of plants to appear, emerging from within the gymnosperms during the Jurassic and diversifying rapidly during the Cretaceous. These differ in that the seed embryo (angiosperm) is enclosed, so the pollen has to grow a tube to penetrate the protective seed coat; they are the predominant group of flora in most biomes today.

First vascular plant

Vascular plants first appeared during the Silurian period. They diversified in the Devonian and spread into many different land

environments. They have a number of adaptations that allowed them to overcome the limitations of the bryophytes. These include a cuticle resistant to desiccation, and vascular tissues which transport water throughout the organism. In most the sporophyte acts as a separate individual, while the gametophyte remains small.

Scientists widely believe that the first land plants evolved during the late Ordovician to early Silurian, although fossils from this time are incomplete and difficult to interpret. By the end of the Silurian a land flora had evolved that throughout the next 50 million years of the Devonian (410 to 360 mya [million years ago]). They continued to change, adapt to life on land exposed to air, and spread across a landscape previously devoid of vegetation. By the end of the period, small plants had given way to tree size, well-diversified vascular plants.

The lycophytes separated from the rest of the early land plants, evolved adequate reproductive, supportive, and transport systems, and, by the Carboniferous, were large swamp forest trees. Three groups of now extinct vascular plants were prevalent in Devonian times: the rhyniophytes, zosterophylls, and trimerophytes. The oldest known vascular plant is Cooksonia. It is a 6.5-centimeter­tall plant with dichotomously branched (forking into two) leafless stems with sporangia at their tips. Only bits and pieces have so far been recovered and no rhizomes or below ground parts have been found. It is a rhyniophyte and it and its relatives were extinct by mid-Devonian time.

The trimerophytes are the basal group of the lineage that gave rise to the flowering plants and are also the ancestors of the horsetails, ferns, and progymnosperms. Superficially, the trimerophytes resembled their rhyniophyte ancestors and the zosterophylls, but differed from them in bearing terminal sporangia on branch tips. At a meter or slightly less in height, these were the largest of the three groups of early land plants.

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