Cycads are seed plants The spermatophytes (also known as phanerogams) comprise those plants that produce seeds. They are a subset of the embryophytes or land plants. The living spermatophytes form five groups: characterized by a large crown of compound leaves In botany, a leaf is an above-ground plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat and thin. As an evolutionary trait, the flatness of leaves works to expose the chloroplasts to more light and to increase the absorption of carbon dioxide at the expense of water loss. In the Devonian period, when carbon and a stout trunk In botany, trunk refers to the main structural member of a tree that supports the branches and is supported by and directly attached to the roots. The trunk is covered by the bark, which is an important diagnostic feature in tree identification, and which often differs markedly from the bottom of the trunk to the top, depending on the species. The. They are evergreen In botany, an evergreen plant is a plant that has leaves in all seasons. This contrasts with deciduous plants, which completely lose their foliage during winter, dioecious Dioecious species are those whose individual members can usually produce only one type of gamete; each individual organism belonging to a dioecious species is distinctly female or male . The majority of animal species (for example, all mammals and most reptiles) are dioecious. In plant sexuality, there are also dioecious species plants having large pinnately compound leaves. They are frequently confused with and mistaken for palms Arecaceae or Palmae , the palm family, is a family of flowering plants, the only family in the monocot order Arecales. There are roughly 202 currently known genera with around 2600 species, most of which are restricted to tropical, subtropical, and warm temperate climates. Most palms are distinguished by their large, compound, evergreen leaves or ferns A fern is any one of a group of about 12,000 species of plants. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants. Ferns do not have either seeds or flowers (they reproduce via spores), but are only distantly related to both, and instead belong to the division In biology, a phylum [note 1] is a taxonomic rank below Kingdom and above Class. "Phylum" is equivalent to the botanical term division Cycadophyta.
Cycads are found across much of the subtropical and tropical parts of the world. They are found in South South America is the southern continent of America, situated in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. It is bordered on the west by the Pacific Ocean and on the north and east by the Atlantic Ocean; North America and the Caribbean Sea lie to the northwest and Central America Central America is the central geographic region of the Americas. It is the southernmost, isthmian portion of the North American continent, which connects with South America on the southeast. Central America is considered to be part of the Mesoamerican biodiversity hotspot, excluding the southern portions of Panama (where the greatest diversity occurs), Mexico In Pre-Columbian Mesoamerica many cultures matured into advanced civilizations such as the Olmec, the Toltec, the Teotihuacan, the Zapotec, the Maya and the Aztec before the first contact with Europeans. In 1521, Spain conquered and colonized the territory, which was administered as the viceroyalty of New Spain which would eventually become Mexico, the Antilles The Antilles islands form the greater part of the West Indies in the Caribbean. The Antilles are divided into two major groups: the "Greater Antilles" to the north and west, including the larger islands of Cuba, Jamaica, Hispaniola (Haiti and the Dominican Republic), and Puerto Rico; and the smaller "Lesser Antilles" on the, southeastern United States ^ b. English is the de facto language of American government and the sole language spoken at home by 80% of Americans age five and older. Spanish is the second most commonly spoken language, Australia For at least 40,000 years before European settlement in the late 18th century, Australia was inhabited by indigenous Australians, who belonged to one or more of the roughly 250 language groups. After sporadic visits by fishermen from the immediate north and discovery by Dutch explorers in 1606, Australia's eastern half was claimed by Britain in 177, Melanesia Melanesia is a subregion of Oceania extending from the western end of the Pacific Ocean to the Arafura Sea, and eastward to Fiji. The region comprises most of the islands immediately north and northeast of Australia. The name Melanesia was first used by Jules Dumont d'Urville in 1832 to denote an ethnic and geographical grouping of islands, Micronesia Micronesia is a subregion of Oceania, comprising thousands of small islands in the western Pacific Ocean. It is distinct from Melanesia to the south, and Polynesia to the east. The Philippines and Indonesia lie to the west, Japan Japan is an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south. The characters that make up Japan's name mean "sun-origin", which is why Japan is, China China is seen variously as an ancient civilization extending over a large area in East Asia, a nation and/or a multinational entity, Southeast Asia Southeast Asia is a subregion of Asia, consisting of the countries that are geographically south of China, east of India and north of Australia. The region lies on the intersection of geological plates, with heavy seismic and volcanic activity, India Home to the ancient Indus Valley Civilisation and a region of historic trade routes and vast empires, the Indian subcontinent was identified with its commercial and cultural wealth for much of its long history. Four major religions, Hinduism, Buddhism, Jainism and Sikhism originated here, while Zoroastrianism, Judaism, Christianity and Islam, Sri Lanka As a result of its location in the path of major sea routes, Sri Lanka is a strategic naval link between West Asia and South East Asia.[citation needed] It has also been a center of the Buddhist religion and culture from ancient times and is one of the few remaining abodes of Buddhism in South Asia, including Ladakh, Bhutan and the Chittagong Hill, Madagascar Madagascar, or Republic of Madagascar , is an island nation in the Indian Ocean off the southeastern coast of Africa. The main island, also called Madagascar, is the fourth-largest island in the world, and southern Southern Africa is the southernmost region of the African continent, variably defined by geography or geopolitics. Within the region are numerous territories, including the Republic of South Africa ; nowadays, the simpler term South Africa is generally reserved for the country in English and tropical Africa Africa is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area. With a billion people (as of 2009, see table) in 61 territories, it accounts for about 14.72% of the world's human population, where at least 65 species occur. Some are renowned for survival in harsh semidesert climates Climates encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elements in a given region over long periods of time. Climate can be contrasted to weather, which is the present condition of these same elements and their variations over periods up to two weeks, and can grow in sand Sand is a naturally occurring granular material composed of finely divided rock and mineral particles. The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal settings is silica , usually in the form of quartz or even on rock In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids. They are able to grow in full sun or shade, and some are salt Sodium chloride, also known as salt, common salt, table salt, or halite, is an ionic compound with the formula Na tolerant. Though they are a minor component of the plant kingdom today, during the Jurassic The Jurassic is a geologic period and system that extends from about 199.6± 0.6 Ma to 145.5± 4 Ma, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic Era, also known as the "Age of Reptiles". The start of the period is marked by the major Triassic– period they were extremely common.
They have very specialized pollinators A pollinator is the biotic agent that moves pollen from the male anthers of a flower to the female stigma of a flower to accomplish fertilization or syngamy of the female gamete in the ovule of the flower by the male gamete from the pollen grain. Though the terms are sometimes confused, a pollinator is different from a pollenizer, which is a plant and have been reported to fix nitrogen Nitrogen fixation generally refers to the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia. This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g. nucleotides for DNA and amino acids for proteins. Formally, nitrogen in association with a cyanobacterium Cyanobacteria is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria (Greek: κυανός (kyanós) = blue) living in the roots. These blue-green algae Cyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria (Greek: κυανός = blue). They are a significant component of the marine nitrogen cycle and an important primary produce a neurotoxin A neurotoxin is a toxin that acts specifically on nerve cells , usually by interacting with membrane proteins such as ion channels. Some sources are more general, and define the effect of neurotoxins as occurring at nerve tissue. Bungarotoxin, which is considered a neurotoxin, has its effect at the motor end plate called BMAA that is found in the seeds A seed ( /ˈsiːd/ ) is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant. The formation of the seed completes the process of reproduction in seed of cycads.
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Origins
The cycad fossil record dates to the early Permian The Permian[note 1] is a geologic period and system characterized among land vertebrates by the diversification of the early amniotes into the ancestral groups of the mammals, turtles, lepidosaurs and archosaurs. The Permian Period follows the Carboniferous and extends from 299.0 ± 0.8 to 251.0 ± 0.4 Ma . It is the last period of the Paleozoic, 280 mya In astronomy, geology, and paleontology, mya or "m.y.a." is an abbreviation for "million years ago". Like the related unit bya, mya is traditionally written in lower case. The deprecated abbreviation m.yr. is still used informally to refer to remote time intervals. There is controversy over older cycad fossils that date to the late Carboniferous The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Ma , to the beginning of the Permian Period, about 299.0 ± 0.8 Ma (ICS, 2004, period, 300–325 mya. One of the first colonizers of terrestrial habitats, this clade probably diversified extensively within its first few million years, although the extent to which it radiated is unknown because relatively few fossil specimens have been found. The regions to which cycads are restricted probably indicate their former distribution on the supercontinents Laurasia Laurasia was a supercontinent that most recently existed as a part of the split of the Pangaea supercontinent in the late Mesozoic era. It was located in the north after Pangaea split into two followed by Gondwanaland in the south. It included most of the landmasses which make up today's continents of the northern hemisphere, chiefly Laurentia ( and Gondwana Gondwana , originally Gondwanaland, is the name given to a southern precursor supercontinent. Its final geological suturing occurred between ca. 570 and 510 million years ago (Ma), joining East Gondwana to West Gondwana. It later separated from Laurasia 180-200 million years ago during the breakup of the Pangaea supercontinent that existed about 50.
The family Stangeriaceae (named for Dr. William Stanger, 1812(?)–1854), consisting of only three extant species Extant is a term commonly used in biology to refer to taxa that are still in existence (living). The term extant contrasts with extinct. For example, Brandt's Cormorant is an extant species, while the Spectacled Cormorant is an extinct species. Likewise, of the group of molluscs known as the cephalopods, there are approximately 600 extant species, is thought to be of Gondwanan origin, as fossils have been found in Lower Cretaceous deposits in Argentina The Argentine claims in Antarctica along with the Falkland Islands, South Georgia, and the South Sandwich Islands (administered by the United Kingdom) shown in light green, dating to 70–135 mya. Zamiaceae is more diverse, with a fossil record extending from the middle Triassic The Triassic is a geologic period that extended from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events. The extinction event that closed the Triassic Period has recently been more accurately to the Eocene The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Palaeogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Paleocene Epoch to the beginning of the Oligocene Epoch. The start of the Eocene is marked by the emergence of the first (54–200 mya) in North North America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific Ocean; South America lies to the southeast and South America, Europe Europe is one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally divided from Asia to its east by the water divide of the Ural Mountains, the Ural River, the Caspian Sea, the Caucasus region (Specification of borders) and the Black Sea to the southeast. Europe is bordered by the Arctic Ocean and, Australia, and Antarctica The Antarctic is the region around the Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica and the ice shelves, waters and island territories in the Southern Ocean situated south of the Antarctic Convergence. The region covers some 20% of the Southern Hemisphere, of which 5.5% (1, implying that the family was present before the break-up of Pangea Pangaea, Pangæa, or Pangea was the supercontinent that presumably existed during the Paleozoic and Mesozoic eras about 250 million years ago, before the component continents were separated into their current configuration. Cycadaceae are thought to be early offshoots from other cycads, with fossils from Eocene deposits (38–54 mya) in Japan Japan is an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south. The characters that make up Japan's name mean "sun-origin", which is why Japan is and China China is seen variously as an ancient civilization extending over a large area in East Asia, a nation and/or a multinational entity, indicating that this family originated in Laurasia. Cycas is the only genus in the family and contains 99 species, the most of any cycad genus. Molecular data have recently shown that Cycas species in Australasia and the east coast of Africa are recent arrivals, suggesting adaptive radiation may have occurred. The current distribution of cycads may be due to radiations from a few ancestral types sequestered on Laurasia and Gondwana, or could be explained by genetic drift following the separation of already evolved genera. Both explanations account for the strict endemism across present continental lines.
Taxonomy
There are about 305 described species, in 10–12 genera and 2–3 families of cycads (depending on taxonomic viewpoint). The classification below, proposed by Dennis Stevenson in 1990, is based upon a hierarchical structure based on cladistic analyses of morphological, anatomical, karyological, physiological and phytochemical data.
The number of species in the clade is low compared to the number of species in most other plant phyla. However, paleobotanical and molecular research indicates that diversity was higher in the history of the phylum. Fossil evidence shows that structural diversity in Mesozoic cycad pollen "considerably exceeds that seen in surviving genera today". The impacts of extinction on diversity are highlighted below. The disparity in molecular sequences is very high between the three main lineages of cycads, implying that genetic diversity in the clade was once high, but this fact has led to major disagreements about the divisions within the Cycadales.
The number of described cycad species has doubled in the past 25 years, mostly due to improved sampling and further exploration. Experts assume there may still be about 100 undescribed species, based on the rate of discovery. These are likely to be in Asia and South America, where areas of endemism are highest. Diversity hotspots also occur in Australia, South Africa, Mexico, China and Vietnam, which together account for more than 70% of the world’s cycad species. The taxonomy of the Cycadophyta is, however, now stabilizing.
Cycad systematists reject the biological species concept, as clearly defined cycad species can interbreed and produce fertile offspring; this character is thus not disproportionately weighted when determining species barriers. The phenetic species concept, which states that a species is defined based on overall similarities with other individuals of the same species combined with a significant gap in variation with other species, is also rejected. Most cycad taxonomists agree on a modified version of the evolutionary species concept, termed the ‘morphogeographic’ species concept, which recognises the combined effects of geographical isolation and morphological disparity. Thus, the presence of large geographical gaps in cycad distribution has greatly affected the way cycads are classified.
- Suborder Cycadineae
- Family Cycadaceae
- Subfamily Cycadoideae
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- Cycas. About 105 species in the Old World from Africa east to southern Japan, Australia and the western Pacific Ocean islands; type: C. circinalis L.; see also C. pruinosa and C. revoluta
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- Subfamily Cycadoideae
- Suborder Zamiineae
- Family Stangeriaceae
- Subfamily Stangerioideae
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- Stangeria. One species in southern Africa; type: S. eriopus (Kunze) Baillon
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- Subfamily Bowenioideae
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- Bowenia. Two species in Queensland, Australia; type: B. spectabilis Hook. ex Hook. f.
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- Subfamily Stangerioideae
- Family Zamiaceae
- Subfamily Encephalartoideae
- Tribe Diooeae
- Tribe Encephalarteae
- Subtribe Encephalartinae
- Encephalartos. About 66 species in southeast Africa; type: E. friderici-guilielmi Lehmann, E. transvenosus (Modjadji cycad)
- Subtribe Macrozamiinae
- Macrozamia. About 41 species in Australia; type: M. riedlei (Fischer ex Gaudichaud) C.A. Gardner
- Lepidozamia. Two species in eastern Australia; type: L. peroffskyana Regel
- Subtribe Encephalartinae
- Subfamily Zamioideae
- Tribe Ceratozamieae
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- Ceratozamia. 26 species in southern Mexico and Central America; type: C. mexicana Brongn.
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- Tribe Zamieae
- Subtribe Microcycadinae
- Microcycas. One species in Cuba; type: M. calocoma (Miquel) A. DC.
- Subtribe Zamiinae
- Subtribe Microcycadinae
- Tribe Ceratozamieae
- Subfamily Encephalartoideae
Cycads are most closely related to the extinct Bennettitales, and are also relatively close relatives to the Ginkgoales, as shown in the following phylogeny (Crepet 2000):
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Mon, 27 Apr 2009 13:25:24 GM
Image of the . Cycad. (Cycas cairnsiana) on ARKive: Female cone of Cycas cairnsiana with large seeds (image G51469)
