Sequoia sempervirens is the sole living species of the genus Sequoia in the cypress family Cupressaceae. Common names include coastal redwood and California redwood, it is an evergreen, long-lived, monoecious tree living 1,200 -- more. This species includes the tallest living trees on Earth, reaching up to 379 feet in height and up to 29.2 feet in diameter at breast height. These trees are among the oldest living things on Earth. Before commercial logging and clearing began by the 1850s, this massive tree occurred in an estimated 2,100,000 acres along much of coastal California and the southwestern corner of coastal Oregon within the United States; the name sequoia sometimes refers to the subfamily Sequoioideae, which includes S. sempervirens along with Sequoiadendron and Metasequoia. Here, the term redwood on its own refers to the species covered in this article, not to the other two species. Scottish botanist David Don described the redwood as the evergreen taxodium in his colleague Aylmer Bourke Lambert's 1824 work A description of the genus Pinus.
Austrian botanist Stephan Endlicher erected the genus Sequoia in his 1847 work Synopsis coniferarum, giving the redwood its current binomial name of Sequoia sempervirens. Endlicher derived the name Sequoia from the Cherokee name of George Gist spelled Sequoyah, who developed the still-used Cherokee syllabary; the redwood is one of each in its own genus, in the subfamily Sequoioideae. Molecular studies have shown that the three are each other's closest relatives with the redwood and giant sequoia as each other's closest relatives; however and colleagues in 2010 queried the polyploid state of the redwood and speculate that it may have arisen as an ancient hybrid between ancestors of the giant sequoia and dawn redwood. Using two different single copy nuclear genes, LFY and NLY, to generate phylogenetic trees, they found that Sequoia was clustered with Metasequoia in the tree generated using the LFY gene, but with Sequoiadendron in the tree generated with the NLY gene. Further analysis supported the hypothesis that Sequoia was the result of a hybridization event involving Metasequoia and Sequoiadendron.
Thus and colleagues hypothesize that the inconsistent relationships among Metasequoia and Sequoiadendron could be a sign of reticulate evolution among the three genera. However, the long evolutionary history of the three genera make resolving the specifics of when and how Sequoia originated once and for all a difficult matter—especially since it in part depends on an incomplete fossil record; the coast redwood can reach 115 m tall with a trunk diameter of 9 m. It has a conical crown, with horizontal to drooping branches; the bark can be thick, up to 1-foot, quite soft and fibrous, with a bright red-brown color when freshly exposed, weathering darker. The root system is composed of wide-spreading lateral roots; the leaves are variable, being 15–25 mm long and flat on young trees and shaded shoots in the lower crown of old trees. On the other hand, they are scale-like, 5–10 mm long on shoots in full sun in the upper crown of older trees, with a full range of transition between the two extremes.
They have two blue-white stomatal bands below. Leaf arrangement is spiral, but the larger shade leaves are twisted at the base to lie in a flat plane for maximum light capture; the species is monoecious, with seed cones on the same plant. The seed cones are ovoid, 15–32 mm long, with 15–25 spirally arranged scales; each cone scale bears three to seven seeds, each seed 3–4 mm long and 0.5 mm broad, with two wings 1 mm wide. The seeds are open at maturity; the pollen cones are 4 -- 6 mm long. Its genetic makeup is unusual among conifers, being a hexaploid and allopolyploid. Both the mitochondrial and chloroplast genomes of the redwood are paternally inherited. Coast redwoods occupy a narrow strip of land 750 km in length and 5–47 mi in width along the Pacific coast of North America; the prevailing elevation range is 98–2,460 ft above sea level down to 0 and up to 3,000 ft. They grow in the mountains where precipitation from the incoming moisture off the ocean is greater; the tallest and oldest trees are found in deep valleys and gullies, where year-round streams can flow, fog drip is regular.
The trees above the fog layer, above about 2,296 ft, are shorter and smaller due to the drier and colder conditions. In addition, Douglas fir and tanoak crowd out redwoods at these elevations. Few redwoods grow close to the ocean, due to intense salt spray and wind. Coalescence of coastal fog accounts for a considerable part of the trees' water needs; the northern boundary of its range is marked by groves on the Chetco River on the western fringe of the Klamath Mountains, near the California-Oregon border. The largest populations are in Redwood National and State Parks (Del Norte and Humbo
Fish are gill-bearing aquatic craniate animals that lack limbs with digits. They form a sister group to the tunicates. Included in this definition are the living hagfish and cartilaginous and bony fish as well as various extinct related groups. Tetrapods emerged within lobe-finned fishes, so cladistically they are fish as well. However, traditionally fish are rendered paraphyletic by excluding the tetrapods; because in this manner the term "fish" is defined negatively as a paraphyletic group, it is not considered a formal taxonomic grouping in systematic biology, unless it is used in the cladistic sense, including tetrapods. The traditional term pisces is considered a typological, but not a phylogenetic classification; the earliest organisms that can be classified as fish were soft-bodied chordates that first appeared during the Cambrian period. Although they lacked a true spine, they possessed notochords which allowed them to be more agile than their invertebrate counterparts. Fish would continue to evolve through the Paleozoic era.
Many fish of the Paleozoic developed external armor. The first fish with jaws appeared in the Silurian period, after which many became formidable marine predators rather than just the prey of arthropods. Most fish are ectothermic, allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature. Fish can communicate in their underwater environments through the use of acoustic communication. Acoustic communication in fish involves the transmission of acoustic signals from one individual of a species to another; the production of sounds as a means of communication among fish is most used in the context of feeding, aggression or courtship behaviour. The sounds emitted by fish can vary depending on the stimulus involved, they can produce either stridulatory sounds by moving components of the skeletal system, or can produce non-stridulatory sounds by manipulating specialized organs such as the swimbladder.
Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams to the abyssal and hadal depths of the deepest oceans, although no species has yet been documented in the deepest 25% of the ocean. With 33,600 described species, fish exhibit greater species diversity than any other group of vertebrates. Fish are an important resource for humans worldwide as food. Commercial and subsistence fishers hunt fish in wild fisheries or farm them in ponds or in cages in the ocean, they are caught by recreational fishers, kept as pets, raised by fishkeepers, exhibited in public aquaria. Fish have had a role in culture through the ages, serving as deities, religious symbols, as the subjects of art and movies. Fish do not represent a monophyletic group, therefore the "evolution of fish" is not studied as a single event. Early fish from the fossil record are represented by a group of small, armored fish known as ostracoderms. Jawless fish lineages are extinct.
An extant clade, the lampreys may approximate ancient pre-jawed fish. The first jaws are found in Placodermi fossils; the diversity of jawed vertebrates may indicate the evolutionary advantage of a jawed mouth. It is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors. Fish may have evolved from a creature similar to a coral-like sea squirt, whose larvae resemble primitive fish in important ways; the first ancestors of fish may have kept the larval form into adulthood, although the reverse is the case. Fish are a paraphyletic group: that is, any clade containing all fish contains the tetrapods, which are not fish. For this reason, groups such as the "Class Pisces" seen in older reference works are no longer used in formal classifications. Traditional classification divides fish into three extant classes, with extinct forms sometimes classified within the tree, sometimes as their own classes: Class Agnatha Subclass Cyclostomata Subclass Ostracodermi † Class Chondrichthyes Subclass Elasmobranchii Subclass Holocephali Class Placodermi † Class Acanthodii † Class Osteichthyes Subclass Actinopterygii Subclass Sarcopterygii The above scheme is the one most encountered in non-specialist and general works.
Many of the above groups are paraphyletic, in that they have given rise to successive groups: Agnathans are ancestral to Chondrichthyes, who again have given rise to Acanthodiians, the ancestors of Osteichthyes. With the arrival of phylogenetic nomenclature, the fishes has been split up into a more detailed scheme, with the following major groups: Class Myxini Class Pteraspidomorphi † Class Thelodonti † Class Anaspida † Class Petromyzontida or Hyperoartia Petromyzontidae Class Conodonta † Class Cephalaspidomorphi † Galeaspida † Pituriaspida † Osteostraci † Infraphylum Gnathostomata Class Placodermi † Class Chondrichthyes Class Acanthodii † Superclass Osteichthy
Australia the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania and numerous smaller islands. It is the world's sixth-largest country by total area; the neighbouring countries are Papua New Guinea and East Timor to the north. The population of 25 million is urbanised and concentrated on the eastern seaboard. Australia's capital is Canberra, its largest city is Sydney; the country's other major metropolitan areas are Melbourne, Brisbane and Adelaide. Australia was inhabited by indigenous Australians for about 60,000 years before the first British settlement in the late 18th century, it is documented. After the European exploration of the continent by Dutch explorers in 1606, who named it New Holland, Australia's eastern half was claimed by Great Britain in 1770 and settled through penal transportation to the colony of New South Wales from 26 January 1788, a date which became Australia's national day; the population grew in subsequent decades, by the 1850s most of the continent had been explored and an additional five self-governing crown colonies established.
On 1 January 1901, the six colonies federated. Australia has since maintained a stable liberal democratic political system that functions as a federal parliamentary constitutional monarchy, comprising six states and ten territories. Being the oldest and driest inhabited continent, with the least fertile soils, Australia has a landmass of 7,617,930 square kilometres. A megadiverse country, its size gives it a wide variety of landscapes, with deserts in the centre, tropical rainforests in the north-east and mountain ranges in the south-east. A gold rush began in Australia in the early 1850s, its population density, 2.8 inhabitants per square kilometre, remains among the lowest in the world. Australia generates its income from various sources including mining-related exports, telecommunications and manufacturing. Indigenous Australian rock art is the oldest and richest in the world, dating as far back as 60,000 years and spread across hundreds of thousands of sites. Australia is a developed country, with the world's 14th-largest economy.
It has a high-income economy, with the world's tenth-highest per capita income. It is a regional power, has the world's 13th-highest military expenditure. Australia has the world's ninth-largest immigrant population, with immigrants accounting for 26% of the population. Having the third-highest human development index and the eighth-highest ranked democracy globally, the country ranks in quality of life, education, economic freedom, civil liberties and political rights, with all its major cities faring well in global comparative livability surveys. Australia is a member of the United Nations, G20, Commonwealth of Nations, ANZUS, Organisation for Economic Co-operation and Development, World Trade Organization, Asia-Pacific Economic Cooperation, Pacific Islands Forum and the ASEAN Plus Six mechanism; the name Australia is derived from the Latin Terra Australis, a name used for a hypothetical continent in the Southern Hemisphere since ancient times. When Europeans first began visiting and mapping Australia in the 17th century, the name Terra Australis was applied to the new territories.
Until the early 19th century, Australia was best known as "New Holland", a name first applied by the Dutch explorer Abel Tasman in 1644 and subsequently anglicised. Terra Australis still saw occasional usage, such as in scientific texts; the name Australia was popularised by the explorer Matthew Flinders, who said it was "more agreeable to the ear, an assimilation to the names of the other great portions of the earth". The first time that Australia appears to have been used was in April 1817, when Governor Lachlan Macquarie acknowledged the receipt of Flinders' charts of Australia from Lord Bathurst. In December 1817, Macquarie recommended to the Colonial Office. In 1824, the Admiralty agreed that the continent should be known by that name; the first official published use of the new name came with the publication in 1830 of The Australia Directory by the Hydrographic Office. Colloquial names for Australia include "Oz" and "the Land Down Under". Other epithets include "the Great Southern Land", "the Lucky Country", "the Sunburnt Country", "the Wide Brown Land".
The latter two both derive from Dorothea Mackellar's 1908 poem "My Country". Human habitation of the Australian continent is estimated to have begun around 65,000 to 70,000 years ago, with the migration of people by land bridges and short sea-crossings from what is now Southeast Asia; these first inhabitants were the ancestors of modern Indigenous Australians. Aboriginal Australian culture is one of the oldest continual civilisations on earth. At the time of first European contact, most Indigenous Australians were hunter-gatherers with complex economies and societies. Recent archaeological finds suggest. Indigenous Australians have an oral culture with spiritual values based on reverence for the land and a belief in the Dreamtime; the Torres Strait Islanders, ethnically Melanesian, obtained their livelihood from seasonal horticulture and the resources of their reefs and seas. The northern coasts and waters of Australia were visited s
A birch is a thin-leaved deciduous hardwood tree of the genus Betula, in the family Betulaceae, which includes alders and hornbeams. It is related to the beech-oak family Fagaceae; the genus Betula contains 30 to 60 known taxa of which 11 are on the IUCN 2011 Red List of Threatened Species. They are a rather short-lived pioneer species widespread in the Northern Hemisphere in northern areas of temperate climates and in boreal climates. Birch species are small to medium-sized trees or shrubs of northern temperate and boreal climates; the simple leaves are alternate, singly or doubly serrate, feather-veined and stipulate. They appear in pairs, but these pairs are borne on spur-like, two-leaved, lateral branchlets; the fruit is a small samara. They differ from the alders in that the female catkins are not woody and disintegrate at maturity, falling apart to release the seeds, unlike the woody, cone-like female alder catkins; the bark of all birches is characteristically marked with long, horizontal lenticels, separates into thin, papery plates upon the paper birch.
Distinctive colors give the common names gray, black and yellow birch to different species. The buds form early and are full grown by midsummer, all are lateral, no terminal bud is formed; the wood of all the species is close-grained with a satiny texture and capable of taking a fine polish. The flowers are monoecious, opening with or before the leaves and borne once grown these leaves are 3–6 millimetres long on three-flowered clusters in the axils of the scales of drooping or erect catkins or aments. Staminate aments are pendulous, clustered or solitary in the axils of the last leaves of the branch of the year or near the ends of the short lateral branchlets of the year, they remain rigid during the winter. The scales of the staminate aments when mature are broadly ovate, yellow or orange color below the middle, dark chestnut brown at apex; each scale bears two bractlets and three sterile flowers, each flower consisting of a sessile, membranaceous two-lobed, calyx. Each calyx bears four short filaments with one-celled anthers or two filaments divided into two branches, each bearing a half-anther.
Anther cells open longitudinally. The pistillate aments are pendulous, solitary; the pistillate scales are oblong-ovate, three-lobed, pale yellow-green tinged with red, becoming brown at maturity. These scales bear each flower consisting of a naked ovary; the ovary is compressed, two-celled, crowned with two slender styles. Each scale bears a single small, winged nut, oval, with two persistent stigmas at the apex. Betula species are organised into five subgenera. Birches native to Europe and Asia include Betula albosinensis – Chinese red birch Betula alnoides – alder-leaf birch Betula ashburneri – Betula baschkirica – Betula bomiensis – Betula browicziana – Betula calcicola – Betula celtiberica – Betula chichibuensis – Betula chinensis – Chinese dwarf birch Betula coriaceifolia – Betula corylifolia – Betula costata – Betula cylindrostachya – Betula dahurica – Betula delavayi – Betula ermanii – Erman's birch Betula falcata – Betula fargesii – Betula fruticosa – Betula globispica – Betula gmelinii – Betula grossa – Japanese cherry birch Betula gynoterminalis – Betula honanensis – Betula humilis or Betula kamtschatica – Kamchatka birch platyphylla Betula insignis – Betula karagandensis – Betula klokovii – Betula kotulae – Betula litvinovii – Betula luminifera – Betula maximowiczii – monarch birch Betula medwediewii – Caucasian birch Betula megrelica – Betula microphylla – Betula nana – dwarf birch ) Betula pendula – silver birch Betula platyphylla – —Siberian silver birch Betula potamophila – Betula potaninii – Betula psammophila – Betula pubescens – downy birch known as white, European white or hairy birch Betula raddeana – Betula saksaren
A fungus is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separate from the other eukaryotic life kingdoms of plants and animals. A characteristic that places fungi in a different kingdom from plants and some protists is chitin in their cell walls. Similar to animals, fungi are heterotrophs. Fungi do not photosynthesize. Growth is their means of mobility, except for spores, which may travel through the water. Fungi are the principal decomposers in ecological systems; these and other differences place fungi in a single group of related organisms, named the Eumycota, which share a common ancestor, an interpretation, strongly supported by molecular phylogenetics. This fungal group oomycetes; the discipline of biology devoted to the study of fungi is known as mycology. In the past, mycology was regarded as a branch of botany, although it is now known fungi are genetically more related to animals than to plants.
Abundant worldwide, most fungi are inconspicuous because of the small size of their structures, their cryptic lifestyles in soil or on dead matter. Fungi include symbionts of plants, animals, or other fungi and parasites, they may become noticeable when fruiting, either as molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange in the environment, they have long been used in the form of mushrooms and truffles. Since the 1940s, fungi have been used for the production of antibiotics, more various enzymes produced by fungi are used industrially and in detergents. Fungi are used as biological pesticides to control weeds, plant diseases and insect pests. Many species produce bioactive compounds called mycotoxins, such as alkaloids and polyketides, that are toxic to animals including humans; the fruiting structures of a few species contain psychotropic compounds and are consumed recreationally or in traditional spiritual ceremonies.
Fungi can break down manufactured materials and buildings, become significant pathogens of humans and other animals. Losses of crops due to fungal diseases or food spoilage can have a large impact on human food supplies and local economies; the fungus kingdom encompasses an enormous diversity of taxa with varied ecologies, life cycle strategies, morphologies ranging from unicellular aquatic chytrids to large mushrooms. However, little is known of the true biodiversity of Kingdom Fungi, estimated at 2.2 million to 3.8 million species. Of these, only about 120,000 have been described, with over 8,000 species known to be detrimental to plants and at least 300 that can be pathogenic to humans. Since the pioneering 18th and 19th century taxonomical works of Carl Linnaeus, Christian Hendrik Persoon, Elias Magnus Fries, fungi have been classified according to their morphology or physiology. Advances in molecular genetics have opened the way for DNA analysis to be incorporated into taxonomy, which has sometimes challenged the historical groupings based on morphology and other traits.
Phylogenetic studies published in the last decade have helped reshape the classification within Kingdom Fungi, divided into one subkingdom, seven phyla, ten subphyla. The English word fungus is directly adopted from the Latin fungus, used in the writings of Horace and Pliny; this in turn is derived from the Greek word sphongos, which refers to the macroscopic structures and morphology of mushrooms and molds. The word mycology is derived from the Greek logos, it denotes the scientific study of fungi. The Latin adjectival form of "mycology" appeared as early as 1796 in a book on the subject by Christiaan Hendrik Persoon; the word appeared in English as early as 1824 in a book by Robert Kaye Greville. In 1836 the English naturalist Miles Joseph Berkeley's publication The English Flora of Sir James Edward Smith, Vol. 5. Refers to mycology as the study of fungi. A group of all the fungi present in a particular area or geographic region is known as mycobiota, e.g. "the mycobiota of Ireland". Before the introduction of molecular methods for phylogenetic analysis, taxonomists considered fungi to be members of the plant kingdom because of similarities in lifestyle: both fungi and plants are immobile, have similarities in general morphology and growth habitat.
Like plants, fungi grow in soil and, in the case of mushrooms, form conspicuous fruit bodies, which sometimes resemble plants such as mosses. The fungi are now considered a separate kingdom, distinct from both plants and animals, from which they appear to have diverged around one billion years ago; some morphological and genetic features are shared with other organisms, while others are unique to the fungi separating them from the other kingdoms: Shared features: With other euka
Complex early seral forest
Complex early seral forests, or snag forests, are ecosystems that occupy forested sites after a stand-replacement disturbance and before re-establishment of a closed forest canopy. They are generated by natural disturbances such as wildfire or insect outbreaks that reset ecological succession processes and follow a pathway, influenced by biological legacies that were not removed during the initial disturbance. Complex early seral forests develop with rich biodiversity because the remaining biomass provides resources to many life forms and because of habitat heterogeneity provided by the disturbances that generated them. In this and other ways, complex early seral forests differ from simplified early successional forests created by logging. Complex early seral forest habitat is threatened from fire suppression and post-fire or post-insect outbreak logging. Complex early seral forests are structurally more complex, contain more large trees and snags, have more diverse understories, more functional ecosystem processes, more diverse gene pools than areas of timber harvest.
These characteristics provide greater resilience in the face of climate change than that provided by the simplified early seral forests produced by logging. Complex early seral forest attributes promote a high level of species richness bird communities that utilize these forests extensively; the residual biomass of snags reduces disturbance stress and provides for the rapid proliferation of new life For example, seed banks and live vegetation tissue gives rise to dense forb cover, abundant grasses, shrubs – nitrogen fixers and ectomycorrhizal associates that facilitate conifer growth. Closed cone conifers like giant sequoia do well in these forests. Other plants that can abundantly colonize burns, such as conifers and fireweed, arrive by wind or animal dispersed seed. Plant species richness of snag forests can be much higher than in unburned forests. Bird and small mammal communities that utilize complex early seral forests forage on the abundant insects and increased abundance of seeds in the post-fire flora.
These species, in turn, support an increase in raptors. Bird species such as the Black-backed Woodpecker, Olive-sided Flycatcher, Mountain Bluebird, Chipping Sparrow, Mountain Quail achieve highest abundances in complex early seral forests. Bats use complex early seral forests because of greater insect prey as well as suitable roosts. Stand-replacing fires stimulate an increased flow of aquatic prey to terrestrial habitats, driving increases in riparian consumers; the trees killed by fire are beneficial to the ecological integrity of stream communities because they are a main source of large woody debris inputs. There is reproduction by some forest fungi species that are restricted to burns and the dead wood provides substrate for fungal growth that supports many arthropod species, including unique fire-following native beetles. Beetles, in general, colonize fire-killed trees in complex early seral forests and their abundant larvae support species like Black-backed Woodpeckers Forest and Spotted Owl management documents state that severe wildfire is a cause of recent declines in populations of spotted owls poses a primary threat to Spotted Owl population viability, but a systematic review and meta-analysis found fires created more benefits than costs for spotted owls
In botany, a tree is a perennial plant with an elongated stem, or trunk, supporting branches and leaves in most species. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are usable as lumber or plants above a specified height. Trees are not a taxonomic group but include a variety of plant species that have independently evolved a woody trunk and branches as a way to tower above other plants to compete for sunlight. Trees tend to be long-lived, some reaching several thousand years old. In wider definitions, the taller palms, tree ferns and bamboos are trees. Trees have been in existence for 370 million years, it is estimated. A tree has many secondary branches supported clear of the ground by the trunk; this trunk contains woody tissue for strength, vascular tissue to carry materials from one part of the tree to another. For most trees it is surrounded by a layer of bark. Below the ground, the roots spread out widely. Above ground, the branches divide into smaller shoots.
The shoots bear leaves, which capture light energy and convert it into sugars by photosynthesis, providing the food for the tree's growth and development. Trees reproduce using seeds. Flowers and fruit may be present, but some trees, such as conifers, instead have pollen cones and seed cones. Palms and bamboos produce seeds, but tree ferns produce spores instead. Trees play a significant role in moderating the climate, they remove carbon dioxide from the atmosphere and store large quantities of carbon in their tissues. Trees and forests provide a habitat for many species of plants. Tropical rainforests are among the most biodiverse habitats in the world. Trees provide shade and shelter, timber for construction, fuel for cooking and heating, fruit for food as well as having many other uses. In parts of the world, forests are shrinking as trees are cleared to increase the amount of land available for agriculture; because of their longevity and usefulness, trees have always been revered, with sacred groves in various cultures, they play a role in many of the world's mythologies.
Although "tree" is a term of common parlance, there is no universally recognised precise definition of what a tree is, either botanically or in common language. In its broadest sense, a tree is any plant with the general form of an elongated stem, or trunk, which supports the photosynthetic leaves or branches at some distance above the ground. Trees are typically defined by height, with smaller plants from 0.5 to 10 m being called shrubs, so the minimum height of a tree is only loosely defined. Large herbaceous plants such as papaya and bananas are trees in this broad sense. A applied narrower definition is that a tree has a woody trunk formed by secondary growth, meaning that the trunk thickens each year by growing outwards, in addition to the primary upwards growth from the growing tip. Under such a definition, herbaceous plants such as palms and papayas are not considered trees regardless of their height, growth form or stem girth. Certain monocots may be considered trees under a looser definition.
Aside from structural definitions, trees are defined by use. The tree growth habit is an evolutionary adaptation found in different groups of plants: by growing taller, trees are able to compete better for sunlight. Trees tend some reaching several thousand years old. Several trees are among the oldest organisms now living. Trees have modified structures such as thicker stems composed of specialised cells that add structural strength and durability, allowing them to grow taller than many other plants and to spread out their foliage, they differ from shrubs, which have a similar growth form, by growing larger and having a single main stem. The tree form has evolved separately in unrelated classes of plants in response to similar environmental challenges, making it a classic example of parallel evolution. With an estimated 60,000-100,000 species, the number of trees worldwide might total twenty-five per cent of all living plant species; the greatest number of these grow in tropical regions and many of these areas have not yet been surveyed by botanists, making tree diversity and ranges poorly known.
The majority of tree species are angiosperms. There are about 1000 species of gymnosperm trees, including conifers, cycads and gnetales. Most angiosperm trees are eudicots, the "true dicotyledons", so named because the seeds contain two cotyledons or seed leaves. There are some trees among the old lineages of flowering plants called basal angiosperms or paleodicots. Wood gives structural strength to the trunk of most types of tree; the vascular system of trees allows water and other chemicals to be di