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See also: (Day 3)

The Cambrian period had biofilms and microbial mats that developed on Cambrian tidal flats and beaches about 500 mya,[fn 1] but no land plant (embryophyte) fossils have been found[3] until the Silurian period. Silurian megafossils indicate extensive terrestrial biota, in the form of moss-like miniature forests, that had developed along lakes and streams. Land fauna did not have a major impact on the Earth until it diversified in the Devonian.[4] Many Early Devonian plants did not have true roots or leaves like extant plants, nor vascular tissue. Some of the early land plants such as Drepanophycus likely spread by vegetative growth and primitive spores.[5] The earliest land plants such as Cooksonia consisted of leafless, dichotomous axes and terminal sporangia and were generally very short-statured, and grew hardly more than a few centimeters tall.[6]

Further research

Carboniferous plantsEdit

359 mya

The main Early Carboniferous flora were seed-bearing plants of gymnosperm. These were the Equisetales (Horse-tails), Sphenophyllales (scrambling plants), Lycopodiales (Club mosses), Lepidodendrales (scale trees), Filicales (Ferns), Medullosales (previously included in the "seed ferns", an artificial assemblage of a number of early gymnosperm groups) and the Cordaitales. These groups continued to dominate throughout the Mississippian age, but during late Carboniferous, several other groups, Cycadophyta (cycads), the Callistophytales (another group of "seed ferns"), and the Voltziales (related to and sometimes included under the conifers), appeared. The fronds of some Carboniferous ferns are almost identical with those of living species. Probably many species were epiphytic. Fossil ferns and "seed ferns" include Pecopteris, Cyclopteris, Neuropteris, Alethopteris, and Sphenopteris; Megaphyton and Caulopteris were tree ferns.

319 mya
Meyers b15 s0272b

Etching depicting some of the most significant plants of the Carboniferous.

A whole genome duplication event in the ancestor of seed-bearing plants occurred about 319 mya.[7] This gave rise to a series of original terrestrial biodiverse seed-producing plants that replaced the lycopsid rainforests of the tropical region.[8] [9] By far the largest group of living gymnosperms are the conifers (pines, cypresses, and relatives), followed by cycads, gnetophytes (Gnetum, Ephedra and Welwitschia), and Ginkgo biloba (a single living species). Roots in some genera have fungal association with roots in the form of mycorrhiza (Pinus), while in some others (Cycas) small specialised roots called coralloid roots are associated with nitrogen-fixing cyanobacteria.

The Carboniferous lycophytes of the order Lepidodendrales, which are cousins (but not ancestors) of the tiny club-moss of today, were huge trees with trunks 30 meters high and up to 1.5 meters in diameter. These included Lepidodendron (with its cone called Lepidostrobus), Anabathra, Lepidophloios and Sigillaria. The roots of several of these forms are known as Stigmaria. Unlike present-day trees, their secondary growth took place in the cortex, which also provided stability, instead of the xylem.[10] The Cladoxylopsids were large trees, that were ancestors of ferns, first arising in the Carboniferous.[11] The fronds of some Carboniferous ferns are almost identical with those of living species. Probably many species were epiphytic. Fossil ferns and "seed ferns" include Pecopteris, Cyclopteris, Neuropteris, Alethopteris, and Sphenopteris; Megaphyton and Caulopteris were tree ferns.

Permian flourishingEdit

300 mya

The Permian began with the Carboniferous flora still flourishing. Cordaites, a tall plant (6 to over 30 meters) with strap-like leaves, was related to the cycads and conifers; the catkin-like reproductive organs, which bore ovules/seeds, is called Cardiocarpus. These plants were thought to live in swamps. True coniferous trees (Walchia, of the order Voltziales) had appeared in the late Carboniferous, preferring higher drier ground.

About the middle of the Permian a major transition in vegetation began. The swamp-loving lycopod trees of the Carboniferous, such as Lepidodendron and Sigillaria, were progressively replaced in the continental interior by more advanced seed ferns and early conifers. At the close of the Permian, lycopod and equisete swamps reminiscent of Carboniferous flora survived only on a series of equatorial islands in the Paleo-Tethys Ocean that later would become South China.[12]

The Equisetales included the common giant form Calamites, with a trunk diameter of 30 to Template:Convert/cm and a height of up to 20 m (). Sphenophyllum was a slender climbing plant with whorls of leaves, which was probably related both to the calamites and the lycopods.

The Permian experienced many important conifer groups, including the ancestors of many present-day families. Rich forests were present in many areas, with a diverse mix of plant groups. The southern continent saw extensive seed fern forests of the Glossopteris flora. Oxygen levels were probably high there. The ginkgos and cycads also appeared during this period.

Seed-bearing plantsEdit

See also: (Day 3)
Gymnosperm life cycle (en)

Example of gymnosperm lifecycle

Gymnosperms are seed-bearing plants that have a sporophyte-dominant life cycle. They spend most of their life cycle with diploid cells, while the gametophyte (gamete-bearing phase) is relatively short-lived. Two spore types, microspores and megaspores, are typically produced in pollen cones or ovulate cones, respectively. Gametophytes, as with all heterosporous plants, develop within the spore wall. Pollen grains (microgametophytes) mature from microspores, and ultimately produce sperm cells. Megagametophytes develop from megaspores and are retained within the ovule.

Gymnosperms produce multiple archegonia, which produce the female gamete. During pollination, pollen grains are physically transferred between plants from the pollen cone to the ovule. Pollen is usually moved by wind or insects. Whole grains enter each ovule through a microscopic gap in the ovule coat (integument) called the micropyle. The pollen grains mature further inside the ovule and produce sperm cells. Two main modes of fertilization are found in gymnosperms. Cycads and Ginkgo have motile sperm that swim directly to the egg inside the ovule, whereas conifers and gnetophytes have sperm with no flagella that are moved along a pollen tube to the egg. After syngamy (joining of the sperm and egg cell), the zygote develops into an embryo (young sporophyte). More than one embryo is usually initiated in each gymnosperm seed. The mature seed comprises the embryo and the remains of the female gametophyte, which serves as a food supply, and the seed coat.[13]

NotesEdit

  1. Cambrian microbes, forming microbial Earth ecosystems comparable with modern soil crust of desert regions, contributed to soil formation.[1][2]

See alsoEdit

ReferencesEdit

  1. Retallack, G.J. (2008). "Cambrian palaeosols and landscapes of South Australia". Alcheringa 55 (8): 1083–1106. doi:10.1080/08120090802266568. Bibcode2008AuJES..55.1083R. 
  2. "Greening of the Earth pushed way back in time". http://phys.org/news/2013-07-greening-earth.html. 
  3. Schieber et al., 2007, pp. 53–71.
  4. Munnecke, Axel; Calner, Mikael; Harper, David A.T.; Servais, Thomas (2010). "Ordovician and Silurian sea–water chemistry, sea level, and climate: A synopsis". Palaeogeography, Palaeoclimatology, Palaeoecology 296 (3–4): 389–413. doi:10.1016/j.palaeo.2010.08.001. 
  5. Zhang, Ying-ying; Xue, Jin-Zhuang; Liu, Le; Wang, De-ming (2016). "Periodicity of reproductive growth in lycopsids: An example from the Upper Devonian of Zhejiang Province, China". Paleoworld 25 (1): 12–20. doi:10.1016/j.palwor.2015.07.002. 
  6. Gonez, Paul; Gerrienne, Philippe (2010). "A new definition and a lectotypification of the genus Cooksonia Lang 1937". International Journal of Plant Sciences 171 (2): 199–215. doi:10.1086/648988. 
  7. Jiao Y, Wickett NJ, Ayyampalayam S, Chanderbali AS, Landherr L, Ralph PE, Tomsho LP, Hu Y, Liang H, Soltis PS, Soltis DE, Clifton SW, Schlarbaum SE, Schuster SC, Ma H, Leebens-Mack J, Depamphilis CW (2011) Ancestral polyploidy in seed plants and angiosperms. Nature
  8. Sahney, S.; Benton, M.J.; Falcon-Lang, H.J. (2010). "Rainforest collapse triggered Pennsylvanian tetrapod diversification in Euramerica" (PDF). Geology 38 (12): 1079–1082. doi:10.1130/G31182.1. Bibcode2010Geo....38.1079S. http://geology.geoscienceworld.org/cgi/content/abstract/38/12/1079. 
  9. Campbell and Reece; Biology, Eighth edition
  10. A HISTORY OF PALAEOZOIC FORESTS - Part 2 The Carboniferous coal swamp forests
  11. C.Michael Hogan. 2010. Fern. Encyclopedia of Earth. National council for Science and the Environment Template:Webarchive. Washington, DC
  12. Xu, R. & Wang, X.-Q. (1982): Di zhi shi qi Zhongguo ge zhu yao Diqu zhi wu jing guan (Reconstructions of Landscapes in Principal Regions of China). Ke xue chu ban she, Beijing. 55 pages, 25 plates.
  13. Walters, Dirk R Walters Bonnie By (1996). Vascular plant taxonomy. Dubuque, Iowa: Kendall/Hunt Pub. Co.. p. 124. ISBN 978-0-7872-2108-9. https://books.google.com/?id=ZbaNxSnNoecC&pg=PA124&dq=Gymnosperm+seeds.