Vascular Plants and Coal Age Flora
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Published: Thu, 24 May 2018
The Rhyniophyta is the most primitive group of vascular plants and appears to be the first one to most of the major divisions of vascular plants. These plants date from the Silurian about 425 million years ago and became extinct in the Devonian about 380 million years ago. Rhyniophyta relatively is one of the simplest vascular plants in structure and is seedless and leafless. The Rhyniophyta had the specialized conducting tissues xylem and phloem as modern higher plants do.
Rhynia is probably most important plant in this classification. Rhynia grew from branched rhizome, was inhabitant of marshes, had numerous lateral branches (to 18 cm) and stomata, and its specimens showed first indication of wounding by insects in the form of penetrating wounds, likely by arthropods.
I am not very sure about how they became extinct, but by the end of the Devonian, the first seed-forming plants had appeared rapidly. This rapid appearance of so many seed plant groups (called the “Devonian Explosion”) may have caused the extinction of primitive vascular plants, Rhyniophyta. Also there were climate change and asteroid impact that may affect the extinction in the Devonian.
The Coal Age Flora is replaced by cone-bearing gymnosperms (the first true seed plants) and by the first true mosses. Pteridophytes, lyciphytes, and progymnosperms-these more complex groups dated from the late Devonian through the Carboniferous, from about 375 to about 290 million years ago. The Coal Age plants include Lycophyte trees, Calamites, ferns and seed plants. For the important plant in this flora, I would say Lycophyte tree which is the dominant tropical coal swamp plants. It grew to heights of 10 to 35 meters and was sparsely branched. As the swamplands began to dry up and the climate began to change toward the end of Carboniferous, it vanished almost overnight.
Gymnosperms are primitive seed plants. They are pollinated by wind, and their seeds are exposed, rather than being enclosed within fruits. The word “gymnosperm” refers to “naked seed”. The earliest gymnosperms appeared in the Paleozoic and became the dominant plants worldwide throughout most of the Mesozoic until about 100 million years ago. Mesozoic period consists of the Triassic, the Jurassic, and the Cretaceous. Early characteristics of gymnosperms were evident in fossil progymnosperms of the time around 380 million years ago. The important fact of gymnosperms is their seed. All seeds consist of an embryo, stored food, and a seed coat covered by the integuments. Living gymnosperms comprise 4 phyla: the Pinophyta, or the Coniferophyta (Conifers), the Ginkgophyta (Ginkgo), the Cycadophyta (Cycads), the Anthophyta, and the Gnetophyta (Gnetum, Ephedra, Welwitschia).
Conifer is the most abundant group of gymnosperms that is still living with about 7 families and about 600 species. Conifer is a woody plant and most are evergreens. The leaves of conifer are long, thin and needle-like such as pine.
The seed ferns (Pteridospermales), the Cordaitales (primitive coniferlike plants) and the cycdeoids, or Bennettitales are groups of extinct Gymnosperms. Bennettitales consisted of plants with palmlike leaves and resembles the living cycads. Bennettitales are an enigmatic group of Mesozoic gymnosperms that disappeared during the Cretaceous. During the mid-Mesozoic, pollination of some extinct groups of gymnosperms was by extinct species of scorpionflies that had specialized proboscis for feeding on pollination drops.
In the Cretaceous, gymnosperms decline became peak and then extinct at the same time there was the rise of angiosperms, also known as flowering plants.
Angiosperms, known as flowering plants, appeared at least 125 million years ago in the Cretaceous period, Mesozoic era. The phylum became abundant in most plants of the world within 30 to 40 million years and has remained dominant ever since. Angiosperms constitute the phylum Anthophyta. Two largest classes of Anthophyta are the Monocotyledones and the Eudicotyledones. Flowering plants differ from other seed plants in some unique characteristics such as the presence of endosperm in the seeds. Other distinctive characteristics of Angiosperms are closed carpels, double fertilization leading to endosperm formation, stamens with two pairs of pollen sacs, and the presence of sieve tubes and companinon cells in the phloem.
Possible reasons for their success may be various adaptations for drought resistance, including the evolution of the deciduous habit, and the evolution of efficient and specialized mechanisms for pollination and seed dispersal.
Other factors that may have influenced its distribution and success can include the pollination interactions with more specialized groups of insects and animals. Angiosperms are pollinated by many kinds of pollinators such as bees, beetles, butterflies, and wasps. The bees especially have the greatest effect on evolution of angiosperms flowers. Bees are the most specialized and constant of flower visiting insects. Hummingbirds, hawkmoths, and bats are also groups of animals that visit and pollinate the plants, but eat lots of nectar of the flowers so that plants have to produce more nectar. Whereas insects and animals can carry pollen greater distances from plant to plant, wind pollination is the best way for individual plants to grow together in a large group. The earliest well known flowering plant is Archaefructus, recently discovered in China. It has slender roots, dissected leaves, and floral axes with closed carpels above, and closed stamens below, as the characteristics of angiosperms.
Typical ferns have Haplodiplontic in their life cycle. Mature ferns have sori underside of their leaves. By meiosis, spores are produced in the sporangia. Each spore germinates by itself and becomes gametophyte. In the lower surface, mature gametophyte makes immature antheridium which produces a sperm, and immature archegonium which produces a egg. When there is enough water and the sperm is mature, the antheridium bursts and releases the sperm into archegonium where the sperm and the egg fertilize. By fertilization, zygote begins to divide immediately, becoming a young embryo. The young embryo grows and differentiates by itself into the adult sporophyte, soon obtaining nutrition by photpsynthesis. Once the young sporophyte sprouts in the soil, the gametophyte collapses.
Pine trees have two kinds of branch: one with pollen-bearing cones (male) and one with aged ovulate cones (female).
Each scale of the male cone bears a microsporangium which later produces microspore mother cells. By meiosis, microspore mother cells produce tetrad of microspores that develop into pollen grains which are made by prothallial cells, tube cells, and generative cells. Pollen grains germinate to produce sperm.
Female cone has two denuded ovules in ovuliferous scales, but has no ovary. There is megaspore mother cell in the ovule and by meiosis it is divided into 4 megaspores. Each megaspore grows and becomes archegonia with two egg cells.
When the pollen grains are transferred by wind to the female gametophyte, the sperm swims into pollen tube and reaches to the egg. After fertilization, the ovule matures and becomes the seed. While the embryo is developed, the suspensor which connects the endosperm to an embryo disintegrates. The pine seed, made up of an embryo, seed coat, and stored food, is completed.
The significant similarity of fern and conifer is that they reproduce spores and they both experience fertilization and meiosis. The significant difference is that conifers are heterosporous, whereas ferns are homosporous. Also conifers have the gametophyte in life cycle whereas ferns have the sporophyte.
The significant asset of life cycle with seeds is that pollen needs no special condition to reach the egg since insects, animals or weather (natural pollinators) can help pollination, while seedless plants need moist condition since they are unable to fertilize in dry area.
The significant asset of life cycle without seeds is that they can reproduce by themselves, not depending on others. Plants with seeds cannot fertilize if there is a shortage of pollinators.
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Conrad, Jim. Last updated 09.8.29. 7:54:52 . Page title: Gymnosperms. Retrieved from The Backyard Nature Website at http://www.backyardnature.net/gymnos.htm.
Flowering plant. (2010, July 21). In Wikipedia, The Free Encyclopedia. Retrieved 13:35, July 25, 2010, from http://en.wikipedia.org/w/index.php?title=Flowering_plant&oldid=374748990
“angiosperm.” The Columbia Encyclopedia, Sixth Edition. 2008. Encyclopedia.com. (July 25, 2010). http://www.encyclopedia.com/doc/1E1-angiospe.html
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