TREES IN BOTANY | NASIRKS

 TREES CONCEPT🌳

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. In wider definitions, the taller palms, tree ferns, bananas, and bamboos are also trees. Trees are not a taxonomic group but include a variety of plant species that have independently evolved a 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. Trees have been in existence for 370 million years. It is estimated that there are some three trillion mature trees in the world.[1]

A tree typically has many secondary branches supported clear of the ground by the trunk. This trunk typically contains woody tissue for strength, and vascular tissue to carry materials from one part of the tree to another. Most trees, it is surrounded by a layer of bark which serves as a protective barrier. Below the ground, the roots branch and spread out widely; they serve to anchor the tree and extract moisture and nutrients from the soil. Above ground, the branches divide into smaller branches and shoots. The shoots typically bear leaves, which capture light energy and convert it into sugars by photosynthesis, providing the food for the tree's growth and development.

Trees usually reproduce using seeds. Flowers and fruit may be present, but some trees, such as conifers, instead have pollen cones and seed cones. Palms, bananas, and bamboos also produce seeds, but tree ferns produce spores instead.

Trees play a significant role in reducing erosion and 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 animals and 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, and 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, and they play a role in many of the world's mythologies. Definition

Diagram of secondary growth in a eudicot or coniferous tree showing idealized vertical and horizontal sections. A new layer of wood is added in each growing season, thickening the stem, existing branches, and roots.

Although "tree" is a term of common parlance, there is no universally recognized precise definition of what a tree is, either botanically or in common language.[2][3] 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.[4] Trees are also typically defined by height,[5] with smaller plants from 0.5 to 10 m (1.6 to 32.8 ft) being called shrubs,[6] so the minimum height of a tree is only loosely defined.[5] Large herbaceous plants such as papaya and bananas are trees in this broad sense.[3][7]

A commonly 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.[5][8] Under such a definition, herbaceous plants such as palms, bananas, and papayas are not considered trees regardless of their height, growth form, or stem girth. Certain monocots may be considered trees under a slightly looser definition;[9] while the Joshua tree, bamboos, and palms do not have secondary growth and never produce true wood with growth rings,[10][11] they may produce "pseudo-wood" by lignifying cells formed by primary growth.[12] Tree species in the genus Dracaena, despite also being monocots, do have secondary growth caused by meristem in their trunk, but it is different from the thickening meristem found in dicotyledonous trees.[13]

Aside from structural definitions, trees are commonly defined by use; for instance, as those plants which yield lumber.[14]

Overview

The tree growth habit is an evolutionary adaptation found in different groups of plants: by growing taller, trees can compete better for sunlight.[15] Trees tend to be tall and long-lived,[16] some reaching several thousand years old.[17] Several trees are among the oldest organisms now living.[18] Trees have modified structures such as thicker stems composed of specialized 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 usually growing larger and having a single main stem;[6] but there is no consistent distinction between a tree and a shrub,[19] made more confusing by the fact that trees may be reduced in size under harsher environmental conditions such as on mountains and subarctic areas. 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 percent of all living plant species.[20][21] The greatest number of these grow in tropical regions and many of these areas have not yet been fully surveyed by botanists, making tree diversity and ranges poorly known.[22]

Tall herbaceous monocotyledonous plants such as bananas lack secondary growth but are trees under the broadest definition.

The majority of tree species are angiosperms. There are about 1000 species of gymnosperm trees,[23] including conifers, cycads, ginkgophytes, and Gnetales; they produce seeds that are not enclosed in fruits, but in open structures such as pine cones, and many have tough waxy leaves, such as pine needles.[24] Most angiosperm trees are eudicots, the "true dicotyledons", so named because the seeds contain two cotyledons or seed leaves. There are also some trees among the old lineages of flowering plants called basal angiosperms or paleodicots; these include Amborella, Magnolia, nutmeg, and avocado,[25] while trees such as bamboo, palms, and bananas are monocots.

Wood gives structural strength to the trunk of most types of trees; this supports the plant as it grows larger. The vascular system of trees allows water, nutrients, and other chemicals to be distributed around the plant, and without it, trees would not be able to grow as large as they do. Trees, as relatively tall plants, need to draw water up the stem

Distribution

Further information: Forest

The Daintree Rainforest

The number of trees in the world, according to a 2015 estimate, is 3.04 trillion, of which 1.39 trillion (46%) are in the tropics or sub-tropics, 0.61 trillion (20%) in the temperate zones, and 0.74 trillion (24%) in the coniferous boreal forests. The estimate is about eight times higher than previous estimates and is based on tree densities measured on over 400,000 plots. It remains subject to a wide margin of error, not least because the samples are mainly from Europe and North America. The estimate suggests that about 15 billion trees are cut down annually and about 5 billion are planted. In the 12,000 years since the start of human agriculture, the number of trees worldwide has decreased by 46%.[1][35][36][37]

In suitable environments, such as the Daintree Rainforest in Queensland, or the mixed podocarp and broadleaf forest of Ulva Island, New Zealand, the forest is the more-or-less stable climatic climax community at the end of a plant succession, where open areas such as grassland are colonized by taller plants, which in turn give way to trees that eventually form a forest canopy.[38][39]

Parts and function

Roots

A young red pine (Pinus resinosa) with the spread of roots visible, as a result of soil erosion

The roots of a tree serve to anchor it to the ground and gather water and nutrients to transfer to all parts of the tree. They are also used for reproduction, defense, survival, energy storage, and many other purposes. The radicle or embryonic root is the first part of a seedling to emerge from the seed during the process of germination. This develops into a taproot that goes straight downwards. Within a few weeks, lateral roots branch out of the side of this and grow horizontally through the upper layers of the soil. In most trees, the taproot eventually withers away and the wide-spreading laterals remain. Near the tip of the finer roots are single-cell root hairs. These are in immediate contact with the soil particles and can absorb water and nutrients such as potassium in the solution. The roots require oxygen to respire and only a few species such as mangroves and the pond cypress (Taxodium ascendens) can live in permanently waterlogged soil.[49]

In the soil, the roots encounter the hyphae of fungi. Many of these are known as mycorrhiza and form a mutualistic relationship with the tree roots. Some are specific to a single tree species, which will not flourish in the absence of its mycorrhizal associate. Others are generalists and associate with many species. The tree acquires minerals such as phosphorus from the fungus, while the fungus obtains the carbohydrate products of photosynthesis from the tree.[50] The hyphae of the fungus can link different trees and a network is formed, transferring nutrients and signals from one place to another.[51] The fungus promotes the growth of the roots and helps protect the trees against predators and pathogens. It can also limit the damage done to a tree by pollution as the fungus accumulate heavy metals within its tissues.[52] Fossil evidence shows that roots have been associated with mycorrhizal fungi since the early Paleozoic, four hundred million years ago, when the first vascular plants colonized dry land.[53]

Trunk

Northern beech (Fagus sylvatica) trunk in autumn

The main purpose of the trunk is to raise the leaves above the ground, enabling the tree to overtop other plants and outcompete them for light.[60] It also transports water and nutrients from the roots to the aerial parts of the tree and distributes the food produced by the leaves to all other parts, including the roots

In the case of angiosperms and gymnosperms, the outermost layer of the trunk is the bark, ii mostly composed of dead cells of phellem (cork).[62] It provides a thick, waterproof covering to the living inner tissue. It protects the trunk against elements, disease, animal attack, and fire. It is perforated by a large number of fine breathing pores called lenticels, through which oxygen diffuses. The bark is continually replaced by a living layer of cells called the cork cambium or phellogen.[62] The London plane periodically sheds its bark in large flakes. Similarly, the bark of the silver birch (Betula pendula) peels off in strips. As the tree's girth expands, newer layers of bark are larger in circumference, and the older layers develop fissures in many species. In some trees such as the pine (Pinus species), the bark exudes sticky resin which deters attackers whereas in rubber trees (Hevea brasiliensis) it is a milky latex that oozes out. The quinine bark tree (Cinchona officinalis) contains bitter substances to make the bark unpalatable.[61] Large tree-like plants with lignified trunks in the Pteridophyta, 

Buds and growth

Buds, leaves, flowers, and fruit of oak (Quercus robur)

Buds, leaves, and reproductive structures of white fir (Abies alba)

Form, leaves, and reproductive structures of queen sago (Cycas circinalis)

Dormant Magnolia bud

Trees do not usually grow continuously throughout the year but mostly have spurts of active expansion followed by periods of rest. This pattern of growth is related to climatic conditions; growth normally ceases when conditions are either too cold or too dry. In readiness for the inactive period, trees form buds to protect the meristem, the zone of active growth. Before the period of dormancy, the last few leaves produced at the tip of a twig form scales. These are thick, small, and closely wrapped and enclose the growing point in a waterproof sheath. Inside this bud, there is a rudimentary stalk and neatly folded miniature leaves, ready to expand when the next growing season arrives. Buds also form in the axils of the leaves ready to produce new side shoots. A few trees, such as the eucalyptus, have "naked buds" with no protective scales and some conifers, such as Lawson's cypress, have no buds but instead have little pockets of meristem concealed among the scale-like leaves.[71]

Leaves

Leaves are structures specialized for photosynthesis and are arranged on the tree in such a way as to maximize their exposure to light without shading each other.[75] They are an important investment by the tree and maybe thorny or contain phytoliths, lignins, tannins, or poisons to discourage herbivory. Trees have evolved leaves in a wide range of shapes and sizes, in response to environmental pressures including climate and predation. They can be broad or needle-like, simple or compound, lobed or entire, smooth or hairy, delicate or tough, deciduous or evergreen. The needles of coniferous trees are compact but are structurally similar to those of broad-leaved trees. They are adapted for life in environments where resources are low or water is scarce. Frozen ground may limit water availability and conifers are often found in colder places at higher altitudes and higher latitudes than broad-leaved trees. In conifers such as fir trees, the branches hang down at an angle to the trunk, enabling them to shed snow. In contrast, broad-leaved trees in temperate regions deal with winter weather by shedding their leaves. When the days get shorter and the temperature begins to decrease, the leaves no longer make new chlorophyll, and the red and yellow pigments already present in the blades become apparent.[75] Synthesis in the leaf of a plant hormone called auxin also ceases. 

Reproduction

Further information: Plant reproduction, Pollination, and Seed dispersal

Trees can be pollinated either by wind or by animals, mostly insects. Many angiosperm trees are insect-pollinated. Wind pollination may take advantage of increased wind speeds high above the ground.[80] Trees use a variety of methods of seed dispersal. Some rely on wind, with winged or plumed seeds. Others rely on animals, for example with edible fruits. Others again eject their seeds (ballistic dispersal) or use gravity so that seeds fall and sometimes roll.[81]

Seeds

Seeds are the primary way that trees reproduce and their seeds vary greatly in size and shape. Some of the largest seeds come from trees, but the largest tree, Sequoiadendron giganteum, produces one of the smallest tree seeds.[82] The great diversity in tree fruits and seeds reflects the many different ways that tree species have evolved to disperse their offspring.

Wind dispersed seed of elm (Ulmus), ash (Fraxinus), and maple (Acer)

For a tree seedling to grow into an adult tree it needs light. If seeds only fell straight to the ground, competition among the concentrated saplings and the shade of the parent would likely prevent it from flourishing. Many seeds such as birch are small and have papery wings to aid dispersal by the wind. Ash trees and maples have larger seeds with blade-shaped wings that spiral down to the ground when released. The kapok tree has cottony threads to catch the breeze.[83]

The seeds of conifers, the largest group of gymnosperms, are enclosed in a cone and most species have seeds that are light and papery that can be blown considerable distances once free from the cone.[84] Sometimes the seed remains in the cone for years waiting for a trigger event to liberate it. Fire stimulates the release and germination of seeds of the jack pine, and also enriches the forest floor with wood ash, and removes competing vegetation.[85] Similarly, several angiosperms including Acacia cyclops and Acacia magnesium have seeds that germinate better after exposure to high temperatures.[86]

Evolutionary history

Lepidodendron, an extinct lycophyte tree

Palms and cycads as they might have appeared in the middle Tertiary

Further information: Evolutionary history of plants

The earliest trees were tree ferns, horsetails, and lycophytes, which grew in forests in the Carboniferous period. The first tree may have been Wattieza, fossils of which have been found in New York State in 2007 dating back to the Middle Devonian (about 385 million years ago). Before this discovery, Archaeopteris was the earliest known tree.[95] Both of these reproduced by spores rather than seeds and are considered to be links between ferns and the gymnosperms which evolved in the Triassic period. The gymnosperms include conifers, cycads, Gnetales, and ginkgos and these may have appeared as a result of a whole-genome duplication event that took place about 319 million years ago.[96] Ginkgophyta was once a widespread diverse group[97] of which the only survivor is the maidenhair tree Ginkgo biloba. This is considered to be a living fossil because it is virtually unchanged from the fossilized specimens found in Triassic deposits.[98]

Food

Further information: nut (fruit) and fruit

Trees are the source of many of the world's best-known fleshy fruits. Apples, pears, plums, cherries, and citrus are all grown commercially in temperate climates and a wide range of edible fruits are found in the tropics. Other commercially important fruits include dates, figs, and olives. Palm oil is obtained from the fruits of the oil palm (Elaeis guineensis). The fruits of the cocoa tree (Theobroma cacao) are used to make cocoa and chocolate and the berries of coffee trees, Coffea arabica, and Coffea canephora are processed to extract the coffee beans. In many rural areas of the world, fruit is gathered from forest trees for consumption.[115] Many trees bear edible nuts which can loosely be described as being large, oily kernels found inside a hard shell. These include coconuts (Cocos nucifera), Brazil nuts (Bertholletia excelsa), pecans (Carya illinoinensis), hazelnuts (Corylus), almonds (Prunus dulcis), walnuts (Juglans regia), pistachios (Pistacia vera), and many others. They are high in nutritive value and contain high-quality protein, vitamins, and minerals as well as dietary fiber.[116] A variety of nut oils are extracted by pressing for culinary use; some such as walnut, pistachio, and hazelnut oils are prized for their distinctive flavors, but they tend to spoil quickly.[117]

Fuel

Selling firewood at a market

Wood has traditionally been used for fuel, especially in rural areas. In less-developed nations it may be the only fuel available and collecting firewood is often a time-consuming task as it becomes necessary to travel further and further afield in the search for fuel.[125] It is often burned inefficiently on an open fire. In more developed countries other fuels are available and burning wood is a choice rather than a necessity. Modern wood-burning stoves are very fuel-efficient and new products such as wood pellets are available to burn.[126]

Art

Besides inspiring artists down the centuries, trees have been used to create art. Living trees have been used in bonsai and in tree shaping, and both living and dead specimens have been sculpted into sometimes fantastic shapes.[132]

Bonsai

Informal upright style of bonsai on a juniper tree

Bonsai (盆栽, lit. "Tray planting")[133] is the practice of hòn nonbộ originated in China and spread to Japan more than a thousand years ago, there are similar practices in other cultures like the living miniature landscapes of Vietnam hòn non-bộ. The word bonsai is often used in English as an umbrella term for all miniature trees in containers or pots.[134]

Mythology

Main article: Tree worship

Yggdrasil, the World Ash of Norse mythology

Trees have been venerated since time immemorial. To the ancient Celts, certain trees, especially the oak, ash, and thorn, held special significance[161] as providing fuel, building materials, ornamental objects, and weaponry. Other cultures have similarly revered trees, often linking the lives and fortunes of individuals to them or using them as oracles. In Greek mythology, dryads were believed to be shy nymphs who inhabited trees.

The Oubangui people of west Africa plant a tree when a child is born. As the tree flourishes, so does the child but if the tree fails to thrive, the health of the child is considered at risk. When it flowers it is time for marriage. Gifts are left at the tree periodically and when the individual dies, their spirit is believed to live on in the tree.[162]

Superlative trees

The General Sherman Tree is thought to be the world's largest by volume

Main article: List of superlative trees

Trees have a theoretical maximum height of 130 m (430 ft),[169] but the tallest known specimen on earth is believed to be a coast redwood (Sequoia sempervirens) at Redwood National Park, California. It has been named Hyperion and is 115.85 m (380.1 ft) tall.[170] In 2006, it was reported to be 379.1 ft (115.5 m) tall.[171] The tallest known broad-leaved tree is mountain ash (Eucalyptus regnans) growing in Tasmania with a height of 99.8 m (327 ft).

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