This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.
Rhyniophytes are a group of fossil plants, which compose the first land flora. These seedless vascular plants that developed during the early Devonian period and during the Paleozic era, occurred in the Rhynie Chert deposit. Some signs of this appeared as early as in the Silurian period as well. The first land flora is a clasification of three extinct early vascular plant groups (the Rhyniophytes, the Zosterophyllophytes and the Trimerophytes), that flourished around 425 to 370 million years ago. Despite the fact that these were very simple plants containing no seeds, flowers, and were even leafless, they bear a very striking resemblance to the plants we see today. The vascular tissue was comprised of a protostele or of one vascular bundle. A number of unicellular organisms were preserved in this location, such as fungi primitive plants and arthropods, leading researchers to believe that conditions were perfect for the process. The Rhyniophyta are notorious thanks to the prominent fossil record held in the village of Rhynie, Aberdeenshire, Scotland. Rhynia, one of the most notable plant groups, are primarily characterized by their moderately small structure and dichotomous branches, with various lateral branches, most of which was common in all three groups with minor differences in size and location of sporangia. In the Rhynia, the sporangia were located mainly at the top of the main branches, but were usually overtaken by the growth of lateral branches. The sporangia present in the fossils found, contain enough cellular detail to tell which plants are the sporophyte generation. Significant unique features, like reproductive structures, can be seen connected to their parent plants, thanks to the fact the plants were buried in-situ. Fossils also showed first indication of wounding by insects in the form of penetrating wounds, likely by arthropods. Another important characteristic to take note of is homospory, each plant having the same size spores. This gives researchers more information into the kind of ecosystems these early plants evolved.
During the second land flora or the coal age in specific regions of the globe, such as North America and Europe, were covered by shallow seas and swamps where favorable conditions yielded year-round growth with a tropical climate. Given these conditions five groups of plants thrived during this period, three of which were seedless vascular plants ferns, lycophytes, and equisetophytes. The other two dominante plants were of the gymnospermous type, the seed fern and the cordaites. These more complex groups dated from the late Devonian (375 million years ago) through the Carboniferous (290 million years ago). During the carboniferous period plants such as these grow high it the sky producing forests, but became extinct due to a time of increasing tropical drought during the Late Paleozoic. Leaving behind only a few relatives such as the herbaceous, several groups of ferns, and the conifers; some of these plants such as the fern and the herbaceous still exist today. The plants of this period were classified by having more modern characteristics, things like pseudomonopodial branching, monopodial branching, microphyllous leaves, and sporophylls leaf that covered the sporangia. Even ferns, had developed megaphyllous leaves and eusteles.
The third flora more commonly known as the gymnosperm flora signifies a sequence of evolutionary lines of seed baring plants. Late into the Paleozoic era the progymnosperms existed, which is considered to be the intermediate between the seedless vascular plants and the gymnosperns that predominantly characterize the later period. The progymnosperms carried some of the traits for their predecessors as well as their successors, with the production of secondary xylem similar to living conifers and reproducing by means of freely dispersed spores, but most importantly the presence of a bifacial vascular cambrium. Progymnosperms became extinct around 340 million years ago in the Mississippian period. Evidence suggests that seed plants developed from similar plants such as these. Gymnosperm, which means naked seed, is a broad expressive term for plants like conifers, which have seeds that are borne naked, the primary characteristic of the classification. They are pollinated by wind. It was the global climate change that brought an end to the Carboniferous Period and the succeeding growth of the third major land flora, set apart by forests of gymnosperms. The extinct gymnosperms existed from Devonian period up until the Jurassic and range from a number of groups and characteristics. The Ptericdospermales or seed ferns and Cordaitles varied in form, from plants that looked like tree ferns to smaller slender branch plants. There are also a number of extinct Mesozic plants that are included with the gymnosperms on occasion, with seed ferns as well as a series of other Carboniferous and Devonian seed ferns being inclded. There is also some debate as to the lineage of the Bennettitales, which was an enigmatic group of Mesozoic gymnosperms characterized by its palm like leaves that faded out during the Cretaceous period.
The four phyla of gymnosperms that came about still have existing representatives, which including the Coniferophyta, Cycadophyta, Gnetophyta and Ginkgophyta. The relationship between the four groups is still uncertain. Gymnosperm forests grew dominate for almost 150 million years, well into the Cretaceous Period where they began to diminish in size, around 145 million years ago. They became overshadowed by the flowering plants that emerged and ultimately rose to dominance around 90 million years ago. What resemble modern day cycads and cycadeoids first emerged at the end of the Paleozoic Era, approximately 290 million years ago and became plentiful in the Mesozoic Era, characterized by the dominate trait of the period the cone. Cycadeoids fell to extinct before the end of the Cretaceous period, but conifers being the most abundant group of gymnosperms, still live on today with around 7 families and ap 600 species. Conifer is a woody plant and most are evergreens. The leaves of conifer are long, thin and needle-like such as pine. Ginkgophyta is a monotypic phylum that lives on through one species, Ginkgo biloba or more commonly known as Ginkgo, which has spanned over 150 million years. It has reproductive qualities like the cycads and vegetative qualities much like the conifers. Gnetophytes are considered to be the closest existing relations to the flowering plant, dating back all the way to the early Cretaceous.
Angiosperm, which literally means covered seed, is a broad expressive term for flowering plants whose ovules or seed are covered by the ovary. The angiosperms comprise only a single phylum, while gymnosperms consist of several phyla. The phylum consists of two major classes, Monocotyledones and the Eudicotyledones as well as a much less sophisticated and primitive group called Magnoliids, all of which encompass approximately 235,000 different species of plants. Flowering plants started to appear during the Cretaceous period, about 125 million years ago. They are considered to be the most successful vascular plants to evolve, since they are the largest, most widespread, and diverse. Some of the possible causes for the flowering plants success include a range of adaptations for drought confrontation, together with the development of the deciduous habit, as well as, possibly the most important, the evolution of resourceful and specific systems for pollination and seed distribution. The first recognized flowering plant was the Archaefructus, which was only recently discovered. It has many of the major characteristics of angiosperms including slight roots, floral axes with sealed carpels on top as well as sealed stamens at the bottom, and bisected leaves,
Angiosperms major characteristics consist of the most concentrated megagametophytes and microgametophytes of the vascular plants, fruits, flowers, ovules with two integuments, and "double fertilization". Vessel elements are present in flowering plants xylem and in their phloem there are sieve tube elements. Some plants like the primitive class mentioned earlier lack these cell types. Sporophylls are leaves that folded to enclose the sporangia, which was most likely a defense mechanism to protect the reproductive materials. Different sporophylls undertake different jobs, while some produced male sporangia which became the stamens, others produced the female reproductive structure or pistil and some that don't produce reproductive sporophylls became the petals and sepals. The anther, which are the folded sporophyll that are positioned at the tip of the stalk or filament, are the location of the male sporangia. In the sporangium, spores are formed which divide almost instantaneously to generate gametophytes. Once mature, the anthers open to discharge the mature male gametophytes or pollen grains. The pollen grain is carried by wind or by animal to the female gametophyte. When the male gametophytes find a well-matched stigma, a pollen tube develops to reach the female gametophytes, through the pistil and into the carpel. Flowering plants rely predominantly on insects for pollen transfer. Insects get a sweet nectar reward and as flowering plants discharge odors to draw bees, beetles, butterflies, and wasps to their blossoms. Other larger pollinators include bats, mothhawks, and hummingbirds. There has been a co-evolution of flowering plants and their pollinators as modifications of plants and pollinators have been a key factor in the success of the flowering plant, which can be seen with an look at the flowering plant visitors.
"Angiosperm". 2011. The Columbia Encyclopedia, Sixth Edition. May 15, 2011. http://www.encyclopedia.com/doc/1E1-angiospe.html
Conrad, Jim. "Gymnosperms". 1929, August 08. The Backyard Nature Website. May 15, 2011. http://www.backyardnature.net/gymnos.htm.
Cummings, Robert J. and Larry Jon Friesen. 2007. Botany 121 Plant Diversity. May 15, 2011. http://www.naturejournal.net/plants
"Geologic time scale". 2011, April 22. Wikipedia, The Free Encyclopedia. May 15, 2011. http://en.wikipedia.org/w/index.php?title=Geologic_time_scale&oldid=374824571
"Gymnosperm". 2011, May 6. Wikipedia, The Free Encyclopedia. May 15, 2011. http://en.wikipedia.org/w/index.php?title=Gymnosperm&oldid=373096484
"Flowering plant". 2011, May 6. Wikipedia, The Free Encyclopedia. May 15, 2011. http://en.wikipedia.org/w/index.php?title=Flowering_plant&oldid=374748990
"Rhyniophyta". 2011. Botany Research and Development Group of Vietnam. May 15, 2011. http://www.botanyvn.com/cnt.asp?param=edir&v=Rhyniophyta&list=divisio&lg=en
"Rhyniophyta". 2011, May 2. Wikipedia, The Free Encyclopedia. May 15, 2011. http://en.wikipedia.org/w/index.php?title=Rhyniophyta&oldid=360330161