135-531: See text The Juglandaceae are a plant family known as the walnut family . They are trees , or sometimes shrubs, in the order Fagales . Members of this family are native to the Americas , Eurasia , and Southeast Asia . The nine or ten genera in the family have a total of around 50 species, and include the commercially important nut -producing trees walnut ( Juglans ), pecan ( Carya illinoinensis ), and hickory ( Carya ). The Persian walnut, Juglans regia ,
270-489: A coleoptile that forms the first leaf while the radicle is covered with a coleorhiza that connects to the primary root and adventitious roots form the sides. Here the hypocotyl is a rudimentary axis between radicle and plumule. The seeds of corn are constructed with these structures; pericarp, scutellum (single large cotyledon) that absorbs nutrients from the endosperm, plumule, radicle, coleoptile, and coleorhiza – these last two structures are sheath-like and enclose
405-401: A seed is a plant embryo and nutrient reserve enclosed in a seed coat, a protective outer covering called a testa. More generally, the term "seed" means anything that can be sown , which may include seed and husk or tuber . Seeds are the product of the ripened ovule , after the embryo sac is fertilized by sperm from pollen , forming a zygote . The embryo within a seed develops from
540-409: A symbiotic relationship with Frankia species, a filamentous bacterium that can fix nitrogen from the air, converting it into ammonia . They have actinorhizal root nodules on their roots in which the bacteria live. This process enables the tree to live in low nitrogen habitats where they would otherwise be unable to thrive. The plant hormones called cytokinins initiate root nodule formation, in
675-416: 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; many of these areas have not yet been fully surveyed by botanists , making tree diversity and ranges poorly known. The majority of tree species are angiosperms or hardwoods. Of
810-422: A curved megagametophyte often giving the seed a tight "C" shape. The last ovule shape is called amphitropous , where the ovule is partly inverted and turned back 90 degrees on its stalk (the funicle or funiculus ). In the majority of flowering plants, the zygote's first division is transversely oriented in regards to the long axis, and this establishes the polarity of the embryo. The upper or chalazal pole becomes
945-503: A drier savanna climate and insufficient rainfall to support dense forests, the canopy is not closed, and plenty of sunshine reaches the ground which is covered with grass and scrub. Acacia and baobab are well adapted to living in such areas. 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, defence, survival, energy storage and many other purposes. The radicle or embryonic root
1080-411: A few will end in a favorable place for growth. Herbaceous perennials and woody plants often have larger seeds; they can produce seeds over many years, and larger seeds have more energy reserves for germination and seedling growth and produce larger, more established seedlings after germination. Seeds serve several functions for the plants that produce them. Key among these functions are nourishment of
1215-418: A hundred litres are required to make one litre of birch syrup. Various parts of trees are used as spices. These include cinnamon , made from the bark of the cinnamon tree ( Cinnamomum zeylanicum ) and allspice , the dried small fruits of the pimento tree ( Pimenta dioica ). Nutmeg is a seed found in the fleshy fruit of the nutmeg tree ( Myristica fragrans ) and cloves are the unopened flower buds of
1350-415: A location and be there at a time favorable for germination and growth. When the fruits open and release their seeds in a regular way, it is called dehiscent , which is often distinctive for related groups of plants; these fruits include capsules , follicles , legumes , silicles and siliques . When fruits do not open and release their seeds in a regular fashion, they are called indehiscent, which include
1485-509: A process closely related to mycorrhizal association. It has been demonstrated that some trees are interconnected through their root system, forming a colony. The interconnections are made by the inosculation process, a kind of natural grafting or welding of vegetal tissues. The tests to demonstrate this networking are performed by injecting chemicals, sometimes radioactive , into a tree, and then checking for its presence in neighbouring trees. The roots are, generally, an underground part of
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#17328561209041620-404: A process known as cladoptosis . The crown is the spreading top of a tree including the branches and leaves, while the uppermost layer in a forest, formed by the crowns of the trees, is known as the canopy . A sapling is a young tree. Many tall palms are herbaceous monocots, which do not undergo secondary growth and never produce wood. In many tall palms, the terminal bud on the main stem
1755-422: A second spurt of growth, a Lammas growth may occur which is believed to be a strategy to compensate for loss of early foliage to insect predators. Primary growth is the elongation of the stems and roots. Secondary growth consists of a progressive thickening and strengthening of the tissues as the outer layer of the epidermis is converted into bark and the cambium layer creates new phloem and xylem cells. The bark
1890-489: 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 much of
2025-424: A similar growth form, by usually growing larger and having a single main stem; but there is no consistent distinction between a tree and a shrub, 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
2160-467: 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. The hyphae of the fungus can link different trees and a network is formed, transferring nutrients and signals from one place to another. The fungus promotes growth of
2295-636: A slightly looser definition; while the Joshua tree , bamboos and palms do not have secondary growth and never produce true wood with growth rings, they may produce "pseudo-wood" by lignifying cells formed by primary growth. 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. Aside from structural definitions, trees are commonly defined by use; for instance, as those plants which yield lumber. The tree growth habit
2430-563: A specified height. In wider definitions, the taller palms , tree ferns , bananas , and bamboos are also trees. Trees are not a monophyletic taxonomic group but consist of a wide variety of plant species that have independently evolved a trunk and branches as a way to tower above other plants to compete for sunlight. The majority of tree species are angiosperms or hardwoods; of the rest, many are gymnosperms or softwoods. Trees tend to be long-lived, some reaching several thousand years old. Trees evolved around 370 million years ago, and it
2565-435: A supply of nutrients for the embryo in most monocotyledons and the endospermic dicotyledons. Seeds have been considered to occur in many structurally different types (Martin 1946). These are based on a number of criteria, of which the dominant one is the embryo-to-seed size ratio. This reflects the degree to which the developing cotyledons absorb the nutrients of the endosperm, and thus obliterate it. Six types occur amongst
2700-550: A tegmen from the inner integument while unitegmic seeds have only one integument. Usually, parts of the testa or tegmen form a hard protective mechanical layer. The mechanical layer may prevent water penetration and germination. Amongst the barriers may be the presence of lignified sclereids . The outer integument has a number of layers, generally between four and eight organised into three layers: (a) outer epidermis, (b) outer pigmented zone of two to five layers containing tannin and starch, and (c) inner epidermis. The endotegmen
2835-484: 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 also typically defined by height, with smaller plants from 0.5 to 10 m (1.6 to 32.8 ft) 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 commonly applied narrower definition
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#17328561209042970-405: A way for the species to survive dry or cold seasons. Ephemeral plants are usually annuals that can go from seed to seed in as few as six weeks. Seed germination is a process by which a seed embryo develops into a seedling. It involves the reactivation of the metabolic pathways that lead to growth and the emergence of the radicle or seed root and plumule or shoot. The emergence of the seedling above
3105-436: A wide range of edible fruits are found in the tropics. Other commercially important fruit 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
3240-416: 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
3375-542: A widespread diverse group 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 fossilised specimens found in Triassic deposits. During the Mesozoic (245 to 66 million years ago) the conifers flourished and became adapted to live in all the major terrestrial habitats. Subsequently, the tree forms of flowering plants evolved during
3510-549: Is a flavouring obtained from distilling bark from the roots of the sassafras tree ( Sassafras albidum ). The leaves of trees are widely gathered as fodder for livestock and some can be eaten by humans but they tend to be high in tannins which makes them bitter. Leaves of the curry tree ( Murraya koenigii ) are eaten, those of kaffir lime ( Citrus × hystrix ) (in Thai food ) and Ailanthus (in Korean dishes such as bugak ) and those of
3645-512: 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 the Lawson's cypress , have no buds but instead have little pockets of meristem concealed among the scale-like leaves. When growing conditions improve, such as
3780-416: Is a sudden movement of sap at the end of the winter as trees prepare to burst into growth. In North America, the sap of the sugar maple ( Acer saccharum ) is used in the production of maple syrup . About 90% of the sap is water, the remaining 10% being a mixture of various sugars and certain minerals. The sap is harvested by drilling holes in the trunks of the trees and collecting the liquid that flows out of
3915-545: Is also found in the southern hemisphere, as for example in the Eastern Australia temperate forest, characterised by Eucalyptus forest and open acacia woodland. In tropical regions with a monsoon or monsoon-like climate, where a drier part of the year alternates with a wet period as in the Amazon rainforest , different species of broad-leaved trees dominate the forest, some of them being deciduous. In tropical regions with
4050-528: Is an evolutionary adaptation found in different groups of plants: by growing taller, trees are able to compete better for sunlight. Trees tend to be tall and long-lived, 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
4185-509: Is at a suitable temperature with proper soil moisture. This true dormancy or innate dormancy is therefore caused by conditions within the seed that prevent germination. Thus dormancy is a state of the seed, not of the environment. Induced dormancy, enforced dormancy or seed quiescence occurs when a seed fails to germinate because the external environmental conditions are inappropriate for germination, mostly in response to conditions being too dark or light, too cold or hot, or too dry. Seed dormancy
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4320-481: Is caused by conditions outside the embryo, including: Endogenous dormancy is caused by conditions within the embryo itself, including: The following types of seed dormancy do not involve seed dormancy, strictly speaking, as lack of germination is prevented by the environment, not by characteristics of the seed itself (see Germination ): Not all seeds undergo a period of dormancy. Seeds of some mangroves are viviparous; they begin to germinate while still attached to
4455-418: Is continually replaced by a living layer of cells called the cork cambium or phellogen. The London plane ( Platanus × hispanica ) 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
4590-402: Is derived from the inner epidermis of the inner integument, the exotegmen from the outer surface of the inner integument. The endotesta is derived from the inner epidermis of the outer integument, and the outer layer of the testa from the outer surface of the outer integument is referred to as the exotesta . If the exotesta is also the mechanical layer, this is called an exotestal seed, but if
4725-499: Is developed by the Indian banyan . Many large trees have buttress roots which flare out from the lower part of the trunk. These brace the tree rather like angle brackets and provide stability, reducing sway in high winds. They are particularly prevalent in tropical rainforests where the soil is poor and the roots are close to the surface. Some tree species have developed root extensions that pop out of soil, in order to get oxygen, when it
4860-414: Is estimated that there are around three trillion mature trees in the world currently. A tree typically has many secondary branches supported clear of the ground by the trunk, which typically contains woody tissue for strength, and vascular tissue to carry materials from one part of the tree to another. For most trees the trunk is surrounded by a layer of bark which serves as a protective barrier. Below
4995-486: Is inelastic. Eventually the growth of a tree slows down and stops and it gets no taller. If damage occurs the tree may in time become hollow. Leaves are structures specialised for photosynthesis and are arranged on the tree in such a way as to maximise their exposure to light without shading each other. They are an important investment by the tree and may be thorny or contain phytoliths , lignins , tannins or poisons to discourage herbivory. Trees have evolved leaves in
5130-424: Is not available in the soil because of excess water. These root extensions are called pneumatophores , and are present, among others, in black mangrove and pond cypress. 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. It also transports water and nutrients from the roots to the aerial parts of the tree, and distributes
5265-444: Is not the same as seed persistence in the soil or on the plant, though even in scientific publications dormancy and persistence are often confused or used as synonyms. Often, seed dormancy is divided into four major categories: exogenous; endogenous; combinational; and secondary. A more recent system distinguishes five classes: morphological, physiological, morphophysiological, physical, and combinational dormancy. Exogenous dormancy
5400-454: Is one of the major nut crops of the world. Walnut, hickory, and gaulin are also valuable timber trees while pecan wood is also valued as cooking fuel. Members of the walnut family have large, aromatic leaves that are usually alternate, but opposite in Alfaroa and Oreomunnea . The leaves are pinnately compound or ternate, and usually 20–100 cm long. The trees are wind-pollinated , and
5535-535: Is protection against disease. Seeds protect and nourish the embryo or young plant. They usually give a seedling a faster start than a sporeling from a spore, because of the larger food reserves in the seed and the multicellularity of the enclosed embryo. Unlike animals, plants are limited in their ability to seek out favorable conditions for life and growth. As a result, plants have evolved many ways to disperse their offspring by dispersing their seeds (see also vegetative reproduction ). A seed must somehow "arrive" at
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5670-458: Is ready to eat when the process is complete, having been tenderised and flavoured by the smoke it has absorbed. In the cold process, the temperature is not allowed to rise above 100 °F (38 °C). The flavour of the food is enhanced but raw food requires further cooking. If it is to be preserved, meat should be cured before cold smoking. Wood has traditionally been used for fuel, especially in rural areas. In less developed nations it may be
5805-428: 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
5940-407: 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
6075-402: 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 , bananas and papayas are not considered trees regardless of their height, growth form or stem girth. Certain monocots may be considered trees under
6210-446: Is the ability of the embryo to germinate and is affected by a number of different conditions. Some plants do not produce seeds that have functional complete embryos, or the seed may have no embryo at all, often called empty seeds. Predators and pathogens can damage or kill the seed while it is still in the fruit or after it is dispersed. Environmental conditions like flooding or heat can kill the seed before or during germination. The age of
6345-471: Is the first part of a seedling to emerge from the seed during the process of germination . This develops into a taproot which 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
6480-452: Is the only one to develop, so they have unbranched trunks with large spirally arranged leaves. Some of the tree ferns, order Cyatheales , have tall straight trunks, growing up to 20 metres (66 ft), but these are composed not of wood but of rhizomes which grow vertically and are covered by numerous adventitious roots . The number of trees in the world, according to a 2015 estimate, is 3.04 trillion, of which 1.39 trillion (46%) are in
6615-411: Is then aborted or absorbed during early development. The seed is composed of the embryo (the result of fertilization) and tissue from the mother plant, which also form a cone around the seed in coniferous plants such as pine and spruce . Seeds are very diverse, and as such there are many terms are used to describe them. A typical seed includes two basic parts: In addition, the endosperm forms
6750-419: Is usually triploid , and is rich in oil or starch , and protein . In gymnosperms, such as conifers , the food storage tissue (also called endosperm) is part of the female gametophyte , a haploid tissue. The endosperm is surrounded by the aleurone layer (peripheral endosperm), filled with proteinaceous aleurone grains. Originally, by analogy with the animal ovum , the outer nucellus layer ( perisperm )
6885-504: The Cretaceous period. These began to displace the conifers during the Tertiary era (66 to 2 million years ago) when forests covered the globe. When the climate cooled 1.5 million years ago and the first of four glacial periods occurred, the forests retreated as the ice advanced. In the interglacials , trees recolonised the land that had been covered by ice, only to be driven back again in
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#17328561209047020-414: The annual growth rings There may also be rays running at right angles to growth rings. These are vascular rays which are thin sheets of living tissue permeating the wood. Many older trees may become hollow but may still stand upright for many years. 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
7155-417: The cone scales as they develop in some species of conifer . Angiosperm (flowering plants) seeds consist of three genetically distinct constituents: (1) the embryo formed from the zygote, (2) the endosperm, which is normally triploid, (3) the seed coat from tissue derived from the maternal tissue of the ovule. In angiosperms, the process of seed development begins with double fertilization , which involves
7290-412: The embryo , dispersal to a new location, and dormancy during unfavorable conditions. Seeds fundamentally are means of reproduction, and most seeds are the product of sexual reproduction which produces a remixing of genetic material and phenotype variability on which natural selection acts. Plant seeds hold endophytic microorganisms that can perform various functions, the most important of which
7425-421: The flowering plants , the ovary ripens into a fruit which contains the seed and serves to disseminate it. Many structures commonly referred to as "seeds" are actually dry fruits. Sunflower seeds are sometimes sold commercially while still enclosed within the hard wall of the fruit, which must be split open to reach the seed. Different groups of plants have other modifications, the so-called stone fruits (such as
7560-424: The legumes (such as beans and peas ), trees such as the oak and walnut , vegetables such as squash and radish , and sunflowers . According to Bewley and Black (1978), Brazil nut storage is in hypocotyl and this place of storage is uncommon among seeds. All gymnosperm seeds are albuminous. The seed coat develops from the maternal tissue, the integuments , originally surrounding the ovule. The seed coat in
7695-416: The peach ) have a hardened fruit layer (the endocarp ) fused to and surrounding the actual seed. Nuts are the one-seeded, hard-shelled fruit of some plants with an indehiscent seed, such as an acorn or hazelnut . The first land plants evolved around 468 million years ago, and reproduced using spores. The earliest seed bearing plants to appear were the gymnosperms , which have no ovaries to contain
7830-546: 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. Large tree-like plants with lignified trunks in the Pteridophyta , Arecales , Cycadophyta and Poales such as the tree ferns, palms, cycads and bamboos have different structures and outer coverings. Although
7965-525: The 12,000 years since the start of human agriculture, the number of trees worldwide has decreased by 46%. There are approximately 64,100 known tree species in the world. With 43% of all tree species, South America has the highest biodiversity, followed by Eurasia (22%), Africa (16%), North America (15%), and Oceania (11%). In suitable environments, such as the Daintree Rainforest in Queensland , or
8100-603: The English oak ( Quercus robur ) and 306 species of invertebrate on the Tasmanian oak ( Eucalyptus obliqua ). Non-native tree species provide a less biodiverse community, for example in the United Kingdom the sycamore ( Acer pseudoplatanus ), which originates from southern Europe, has few associated invertebrate species, though its bark supports a wide range of lichens, bryophytes and other epiphytes. Trees differ ecologically in
8235-535: The European bay tree ( Laurus nobilis ) and the California bay tree ( Umbellularia californica ) are used for flavouring food. Camellia sinensis , the source of tea, is a small tree but seldom reaches its full height, being heavily pruned to make picking the leaves easier. Wood smoke can be used to preserve food. In the hot smoking process the food is exposed to smoke and heat in a controlled environment. The food
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#17328561209048370-496: The arrival of warmer weather and the longer days associated with spring in temperate regions, growth starts again. The expanding shoot pushes its way out, shedding the scales in the process. These leave behind scars on the surface of the twig. The whole year's growth may take place in just a few weeks. The new stem is unlignified at first and may be green and downy. The Arecaceae (palms) have their leaves spirally arranged on an unbranched trunk. In some tree species in temperate climates,
8505-437: The bark functions as a protective barrier, it is itself attacked by boring insects such as beetles. These lay their eggs in crevices and the larvae chew their way through the cellulose tissues leaving a gallery of tunnels. This may allow fungal spores to gain admittance and attack the tree. Dutch elm disease is caused by a fungus ( Ophiostoma species) carried from one elm tree to another by various beetles. The tree reacts to
8640-631: The cells are filled with starch , as for instance cereal grains , or not (non-farinaceous). The endosperm may also be referred to as "fleshy" or "cartilaginous" with thicker soft cells such as coconut , but may also be oily as in Ricinus (castor oil), Croton and Poppy . The endosperm is called "horny" when the cell walls are thicker such as date and coffee , or "ruminated" if mottled, as in nutmeg , palms and Annonaceae . In most monocotyledons (such as grasses and palms ) and some ( endospermic or albuminous ) dicotyledons (such as castor beans )
8775-405: The cells of the outer epidermis enlarge radially and their walls thicken, with nucleus and cytoplasm compressed into the outer layer. these cells which are broader on their inner surface are called palisade cells. In the inner epidermis, the cells also enlarge radially with plate like thickening of the walls. The mature inner integument has a palisade layer, a pigmented zone with 15–20 layers, while
8910-440: The clove tree ( Syzygium aromaticum ). Many trees have flowers rich in nectar which are attractive to bees. The production of forest honey is an important industry in rural areas of the developing world where it is undertaken by small-scale beekeepers using traditional methods. The flowers of the elder ( Sambucus ) are used to make elderflower cordial and petals of the plum ( Prunus spp. ) can be candied. Sassafras oil
9045-465: The cone for years waiting for a trigger event to liberate it. Fire stimulates release and germination of seeds of the jack pine , and also enriches the forest floor with wood ash and removes competing vegetation. Similarly, a number of angiosperms including Acacia cyclops and Acacia mangium have seeds that germinate better after exposure to high temperatures. The single extant species of Ginkgophyta ( Ginkgo biloba ) has fleshy seeds produced at
9180-407: The early 19th century, influential works being: Angiosperm seeds are "enclosed seeds", produced in a hard or fleshy structure called a fruit that encloses them for protection. Some fruits have layers of both hard and fleshy material. In gymnosperms, no special structure develops to enclose the seeds, which begin their development "naked" on the bracts of cones. However, the seeds do become covered by
9315-424: The ease with which they can be found by herbivores. Tree apparency varies with a tree's size and semiochemical content, and with the extent to which it is concealed by nonhost neighbours from its insect pests . In ecosystems such as mangrove swamps, trees play a role in developing the habitat, since the roots of the mangrove trees reduce the speed of flow of tidal currents and trap water-borne sediment, reducing
9450-440: The elaiosomes are eaten. The remainder of the seed, which is hard and inedible to the ants, then germinates either within the nest or at a removal site where the seed has been discarded by the ants. This dispersal relationship is an example of mutualism , since the plants depend upon the ants to disperse seeds, while the ants depend upon the plants seeds for food. As a result, a drop in numbers of one partner can reduce success of
9585-429: The embryo is embedded in the endosperm (and nucellus), which the seedling will use upon germination . In the non-endospermic dicotyledons the endosperm is absorbed by the embryo as the latter grows within the developing seed, and the cotyledons of the embryo become filled with stored food. At maturity, seeds of these species have no endosperm and are also referred to as exalbuminous seeds. The exalbuminous seeds include
9720-645: The ends of short branches on female trees, and Gnetum , a tropical and subtropical group of gymnosperms produce seeds at the tip of a shoot axis. 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 were found in New York state in 2007 dating back to the Middle Devonian (about 385 million years ago). Prior to this discovery, Archaeopteris
9855-461: The far north of the northern hemisphere is moist taiga or northern coniferous forest (also called boreal forest). Taiga is the world's largest land biome , forming 29% of the world's forest cover. The long cold winter of the far north is unsuitable for plant growth and trees must grow rapidly in the short summer season when the temperature rises and the days are long. Light is very limited under their dense cover and there may be little plant life on
9990-452: The fertilised ovule, an immature plant from which a new plant will grow under proper conditions. The embryo has one cotyledon or seed leaf in monocotyledons , two cotyledons in almost all dicotyledons and two or more in gymnosperms. In the fruit of grains (caryopses) the single monocotyledon is shield shaped and hence called a scutellum . The scutellum is pressed closely against the endosperm from which it absorbs food and passes it to
10125-645: The first few years of their lives deriving energy from the fungi and do not produce green leaves. At up to 55 pounds (25 kilograms) the largest seed is the coco de mer (Lodoicea maldivica). This indicates a 25 Billion fold difference in seed weight. Plants that produce smaller seeds can generate many more seeds per flower, while plants with larger seeds invest more resources into those seeds and normally produce fewer seeds. Small seeds are quicker to ripen and can be dispersed sooner, so autumn all blooming plants often have small seeds. Many annual plants produce great quantities of smaller seeds; this helps to ensure at least
10260-510: The flowers are usually arranged in catkins . The fruits of the Juglandaceae are often confused with drupes but are accessory fruit because the outer covering of the fruit is technically an involucre and thus not morphologically part of the carpel; this means it cannot be a drupe but is instead a drupe-like nut. The known living genera are grouped into subfamilies, tribes, and subtribes as follows: Modern molecular phylogenetics suggest
10395-524: The following relationships: Myricaceae ( outgroup ) Rhoiptelea Engelhardia Alfaroa Oreomunnea Carya Annamocarya Platycarya Cyclocarya Pterocarya Juglans Tree In botany , a tree is a perennial plant with an elongated stem , or trunk , usually supporting branches and leaves. 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
10530-480: 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 , mostly composed of dead cells of phellem (cork). It provides a thick, waterproof covering to the living inner tissue. It protects the trunk against the elements, disease, animal attack and fire. It is perforated by a large number of fine breathing pores called lenticels , through which oxygen diffuses. Bark
10665-401: The forest floor, although fungi may abound. Similar woodland is found on mountains where the altitude causes the average temperature to be lower thus reducing the length of the growing season. Where rainfall is relatively evenly spread across the seasons in temperate regions, temperate broadleaf and mixed forest typified by species like oak, beech, birch and maple is found. Temperate forest
10800-507: The fruits achenes , caryopses , nuts , samaras , and utricles . Other seeds are enclosed in fruit structures that aid wind dispersal in similar ways: Myrmecochory is the dispersal of seeds by ants . Foraging ants disperse seeds which have appendages called elaiosomes (e.g. bloodroot , trilliums , acacias , and many species of Proteaceae ). Elaiosomes are soft, fleshy structures that contain nutrients for animals that eat them. The ants carry such seeds back to their nest, where
10935-425: The fruits and either discard the seeds, or swallow them so they pass through the gut to be deposited in the animal's droppings well away from the parent tree. The germination of some seeds is improved when they are processed in this way. Nuts may be gathered by animals such as squirrels that cache any not immediately consumed. Many of these caches are never revisited; the nut-casing softens with rain and frost, and
11070-408: The fusion of two male gametes with the egg cell and the central cell to form the primary endosperm and the zygote. Right after fertilization, the zygote is mostly inactive, but the primary endosperm divides rapidly to form the endosperm tissue. This tissue becomes the food the young plant will consume until the roots have developed after germination . After fertilization, the ovules develop into
11205-446: The grasses, are not distinct structures, but are fused with the fruit wall to form a pericarp .) The testae of both monocots and dicots are often marked with patterns and textured markings, or have wings or tufts of hair. When the seed coat forms from only one layer, it is also called the testa, though not all such testae are homologous from one species to the next. The funiculus abscisses (detaches at fixed point – abscission zone),
11340-480: The ground underneath trees there is shade, and often there is undergrowth, leaf litter , and decaying wood that provide other habitat. Trees stabilise the soil, prevent rapid run-off of rain water, help prevent desertification, have a role in climate control and help in the maintenance of biodiversity and ecosystem balance. Many species of tree support their own specialised invertebrates . In their natural habitats, 284 different species of insect have been found on
11475-557: 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 . Flowering plants have their seeds inside fruits, while conifers carry their seeds in cones, and tree ferns produce spores instead. Trees play
11610-421: The ground. 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. 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
11745-431: The growing parts. Embryo descriptors include small, straight, bent, curved, and curled. Within the seed, there usually is a store of nutrients for the seedling that will grow from the embryo. The form of the stored nutrition varies depending on the kind of plant. In angiosperms, the stored food begins as a tissue called the endosperm , which is derived from the mother plant and the pollen via double fertilization . It
11880-413: The growth of the fungus by blocking off the xylem tissue carrying sap upwards and the branch above, and eventually the whole tree, is deprived of nourishment and dies. In Britain in the 1990s, 25 million elm trees were killed by this disease. The innermost layer of bark is known as the phloem and this is involved in the transport of the sap containing the sugars made by photosynthesis to other parts of
12015-447: The heartwood are blocked in some species. Heartwood is usually darker in colour than the sapwood. It is the dense central core of the trunk giving it rigidity. Three quarters of the dry mass of the xylem is cellulose , a polysaccharide , and most of the remainder is lignin, a complex polymer . A transverse section through a tree trunk or a horizontal core will show concentric circles of lighter or darker wood – tree rings. These rings are
12150-434: The inner epidermis may remain a single layer, it may also divide to produce two to three layers and accumulates starch, and is referred to as the colourless layer. By contrast, the outer epidermis becomes tanniferous . The inner integument may consist of eight to fifteen layers. As the cells enlarge, and starch is deposited in the outer layers of the pigmented zone below the outer epidermis, this zone begins to lignify, while
12285-444: The innermost layer is known as the fringe layer. In gymnosperms, which do not form ovaries, the ovules and hence the seeds are exposed. This is the basis for their nomenclature – naked seeded plants. Two sperm cells transferred from the pollen do not develop the seed by double fertilization, but one sperm nucleus unites with the egg nucleus and the other sperm is not used. Sometimes each sperm fertilizes an egg cell and one zygote
12420-411: The inserted spigots; the sap is then heated to concentrate the flavour. Similarly in northern Europe the spring rise in the sap of the silver birch ( Betula pendula ) is tapped and collected, either to be drunk fresh or fermented into an alcoholic drink. In Alaska, the sap of the sweet birch ( Betula lenta ) is made into a syrup with a sugar content of 67%. Sweet birch sap is more dilute than maple sap;
12555-400: The inside. The newly created xylem is the sapwood . It is composed of water-conducting cells and associated cells which are often living, and is usually pale in colour. It transports water and minerals from the roots to the upper parts of the tree. The oldest, inner part of the sapwood is progressively converted into heartwood as new sapwood is formed at the cambium. The conductive cells of
12690-528: The largest tree, Sequoiadendron giganteum , produces one of the smallest tree seeds. The great diversity in tree fruits and seeds reflects the many different ways that tree species have evolved to disperse their offspring. 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
12825-424: The latter example these hairs are the source of the textile crop cotton . Other seed appendages include the raphe (a ridge), wings, caruncles (a soft spongy outgrowth from the outer integument in the vicinity of the micropyle), spines, or tubercles. A scar also may remain on the seed coat, called the hilum , where the seed was attached to the ovary wall by the funicle. Just below it is a small pore, representing
12960-402: The main area of growth of the embryo, while the lower or micropylar pole produces the stalk-like suspensor that attaches to the micropyle. The suspensor absorbs and manufactures nutrients from the endosperm that are used during the embryo's growth. The main components of the embryo are: Monocotyledonous plants have two additional structures in the form of sheaths. The plumule is covered with
13095-422: The mature seed can be a paper-thin layer (e.g. peanut ) or something more substantial (e.g. thick and hard in honey locust and coconut ), or fleshy as in the sarcotesta of pomegranate . The seed coat helps protect the embryo from mechanical injury, predators, and drying out. Depending on its development, the seed coat is either bitegmic or unitegmic . Bitegmic seeds form a testa from the outer integument and
13230-509: The mechanical layer is the endotegmen, then the seed is endotestal. The exotesta may consist of one or more rows of cells that are elongated and pallisade like (e.g. Fabaceae ), hence 'palisade exotesta'. In addition to the three basic seed parts, some seeds have an appendage, an aril , a fleshy outgrowth of the funicle ( funiculus ), (as in yew and nutmeg ) or an oily appendage, an elaiosome (as in Corydalis ), or hairs (trichomes). In
13365-456: The micropyle of the ovule. Seeds are very diverse in size. The dust-like orchid seeds are the smallest, with about one million seeds per gram; they are often embryonic seeds with immature embryos and no significant energy reserves. Orchids and a few other groups of plants are mycoheterotrophs which depend on mycorrhizal fungi for nutrition during germination and the early growth of the seedling. Some terrestrial orchid seedlings, in fact, spend
13500-409: The mixed podocarp and broadleaf forest of Ulva Island, New Zealand , forest is the more-or-less stable climatic climax community at the end of a plant succession, where open areas such as grassland are colonised by taller plants, which in turn give way to trees that eventually form a forest canopy. In cool temperate regions, conifers often predominate; a widely distributed climax community in
13635-399: The monocotyledons, ten in the dicotyledons, and two in the gymnosperms (linear and spatulate). This classification is based on three characteristics: embryo morphology, amount of endosperm and the position of the embryo relative to the endosperm. In endospermic seeds, there are two distinct regions inside the seed coat, an upper and larger endosperm and a lower smaller embryo. The embryo is
13770-519: The next glacial period. Trees are an important part of the terrestrial ecosystem , providing essential habitats including many kinds of forest for communities of organisms. Epiphytic plants such as ferns , some mosses, liverworts, orchids and some species of parasitic plants (e.g., mistletoe ) hang from branches; these along with arboreal lichens, algae, and fungi provide micro-habitats for themselves and for other organisms, including animals. Leaves, flowers and fruits are seasonally available. On
13905-420: The old lineages of flowering plants called basal angiosperms or paleodicots ; these include Amborella , Magnolia , nutmeg and avocado , while trees such as bamboo, palms and bananas are monocots . Wood gives structural strength to the trunk of most types of tree; this supports the plant as it grows larger. The vascular system of trees allows water, nutrients and other chemicals to be distributed around
14040-468: 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. 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. Seed In botany ,
14175-403: The optimal conditions for survival of the resulting seedling; the second is spreading germination of a batch of seeds over time so a catastrophe (e.g. late frosts, drought, herbivory ) does not result in the death of all offspring of a plant ( bet-hedging ). Seed dormancy is defined as a seed failing to germinate under environmental conditions optimal for germination, normally when the environment
14310-472: The other. In South Africa , the Argentine ant ( Linepithema humile ) has invaded and displaced native species of ants. Unlike the native ant species, Argentine ants do not collect the seeds of Mimetes cucullatus or eat the elaiosomes. In areas where these ants have invaded, the numbers of Mimetes seedlings have dropped. Seed dormancy has two main functions: the first is synchronizing germination with
14445-410: The parent. The large, heavy root allows the seed to penetrate into the ground when it falls. Many garden plant seeds will germinate readily as soon as they have water and are warm enough; though their wild ancestors may have had dormancy, these cultivated plants lack it. After many generations of selective pressure by plant breeders and gardeners, dormancy has been selected out. For annuals , seeds are
14580-585: The plant and reduce water loss. Both the production of wood and the production of cork are forms of secondary growth. Trees are either evergreen , having foliage that persists and remains green throughout the year, or deciduous , shedding their leaves at the end of the growing season and then having a dormant period without foliage. Most conifers are evergreens, but larches ( Larix and Pseudolarix ) are deciduous, dropping their needles each autumn, and some species of cypress ( Glyptostrobus , Metasequoia and Taxodium ) shed small leafy shoots annually in
14715-409: The plant, and without it trees would not be able to grow as large as they do. Trees need to draw water high up the stem through the xylem from the roots by capillary action , as water continually evaporates from the leaves in the process of transpiration . If insufficient water is available the leaves will die. The three main parts of trees include the root, stem, and leaves; they are integral parts of
14850-455: The plumule and radicle, acting as a protective covering. The maturing ovule undergoes marked changes in the integuments, generally a reduction and disorganization but occasionally a thickening. The seed coat forms from the two integuments or outer layers of cells of the ovule, which derive from tissue from the mother plant, the inner integument forms the tegmen and the outer forms the testa . (The seed coats of some monocotyledon plants, such as
14985-405: The rate of germination. This is given as a percent of germination over a certain amount of time, 90% germination in 20 days, for example. 'Dormancy' is covered above; many plants produce seeds with varying degrees of dormancy, and different seeds from the same fruit can have different degrees of dormancy. It's possible to have seeds with no dormancy if they are dispersed right away and do not dry (if
15120-437: The rest, many are gymnosperms or softwood trees; these include conifers , cycads , ginkgophytes and gnetales , which produce seeds which are not enclosed in fruits, but in open structures such as pine cones , and many have tough waxy leaves, such as pine needles. 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
15255-428: The roots and helps protect the trees against predators and pathogens. It can also limit damage done to a tree by pollution as the fungus accumulate heavy metals within its tissues. 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 colonised dry land. Some trees such as Alder ( Alnus species) have
15390-415: The scar forming an oval depression, the hilum . Anatropous ovules have a portion of the funiculus that is adnate (fused to the seed coat), and which forms a longitudinal ridge, or raphe , just above the hilum. In bitegmic ovules (e.g. Gossypium described here) both inner and outer integuments contribute to the seed coat formation. With continuing maturation the cells enlarge in the outer integument. While
15525-413: The seed affects its health and germination ability: since the seed has a living embryo, over time cells die and cannot be replaced. Some seeds can live for a long time before germination, while others can only survive for a short period after dispersal before they die. Seed vigor is a measure of the quality of seed, and involves the viability of the seed, the germination percentage, germination rate, and
15660-464: The seeds on the surface of water. Mangroves often grow in water and some species have buoyant fruits with seeds that start germinating before they detach from the parent tree. These float on the water and may become lodged on emerging mudbanks and successfully take root. Other seeds, such as apple pips and plum stones, have fleshy receptacles and smaller fruits like hawthorns have seeds enclosed in edible tissue; animals including mammals and birds eat
15795-405: The seeds. The ovule consists of a number of components: The shape of the ovules as they develop often affects the final shape of the seeds. Plants generally produce ovules of four shapes: the most common shape is called anatropous , with a curved shape. Orthotropous ovules are straight with all the parts of the ovule lined up in a long row producing an uncurved seed. Campylotropous ovules have
15930-445: The seeds. They arose during the late Devonian period (416 million to 358 million years ago). From these early gymnosperms, seed ferns evolved during the Carboniferous period (359 to 299 million years ago); they had ovules that were borne in a cupule, which consisted of groups of enclosing branches likely used to protect the developing seed. Published literature about seed storage, viability and its hygrometric dependence began in
16065-434: The soil particles and can absorb water and nutrients such as potassium in 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. 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
16200-420: The soil surface is the next phase of the plant's growth and is called seedling establishment. Three fundamental conditions must exist before germination can occur. (1) The embryo must be alive, called seed viability. (2) Any dormancy requirements that prevent germination must be overcome. (3) The proper environmental conditions must exist for germination. Far red light can prevent germination. Seed viability
16335-471: The start of the dry season. Many deciduous trees flower before the new leaves emerge. A few trees do not have true leaves but instead have structures with similar external appearance such as Phylloclades – modified stem structures – as seen in the genus Phyllocladus . 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
16470-507: The strength of the seedlings produced. The germination percentage is simply the proportion of seeds that germinate from all seeds subject to the right conditions for growth. The germination rate is the length of time it takes for the seeds to germinate. Germination percentages and rates are affected by seed viability, dormancy and environmental effects that impact on the seed and seedling. In agriculture and horticulture quality seeds have high viability, measured by germination percentage plus
16605-413: The surviving seeds germinate in the spring. Pine cones may similarly be hoarded by red squirrels , and grizzly bears may help to disperse the seed by raiding squirrel caches. 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. Sometimes the seed remains in
16740-661: 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. Synthesis in the leaf of a plant hormone called auxin also ceases. This causes the cells at the junction of the petiole and the twig to weaken until the joint breaks and the leaf floats to the ground. In tropical and subtropical regions, many trees keep their leaves all year round. Individual leaves may fall intermittently and be replaced by new growth but most leaves remain intact for some time. Other tropical species and those in arid regions may shed all their leaves annually, such as at
16875-408: The tree, but some tree species have evolved roots that are aerial . The common purposes for aerial roots may be of two kinds, to contribute to the mechanical stability of the tree, and to obtain oxygen from air. An instance of mechanical stability enhancement is the red mangrove that develops prop roots that loop out of the trunk and branches and descend vertically into the mud. A similar structure
17010-432: The tree. It is a soft spongy layer of living cells, some of which are arranged end to end to form tubes. These are supported by parenchyma cells which provide padding and include fibres for strengthening the tissue. Inside the phloem is a layer of undifferentiated cells one cell thick called the vascular cambium layer. The cells are continually dividing, creating phloem cells on the outside and wood cells known as xylem on
17145-492: 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
17280-403: The vascular system which interconnects all the living cells. In trees and other plants that develop wood, the vascular cambium allows the expansion of vascular tissue that produces woody growth. Because this growth ruptures the epidermis of the stem, woody plants also have a cork cambium that develops among the phloem. The cork cambium gives rise to thickened cork cells to protect the surface of
17415-429: The water depth and creating suitable conditions for further mangrove colonisation. Thus mangrove swamps tend to extend seawards in suitable locations. Mangrove swamps also provide an effective buffer against the more damaging effects of cyclones and tsunamis. 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
17550-437: The wind. Ash trees and maples have larger seeds with blade shaped wings which spiral down to the ground when released. The kapok tree has cottony threads to catch the breeze. The flame tree Delonix regia shoots its seeds through the air when the two sides of its long pods crack apart explosively on drying. The miniature cone-like catkins of alder trees produce seeds that contain small droplets of oil that help disperse
17685-465: 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 . Although "tree" is a common word, there is no universally recognised precise definition of what a tree is, either botanically or in common language. In its broadest sense,
17820-769: The world, fruit is gathered from forest trees for consumption. 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 ), hazel nuts ( 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 fibre. 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 flavours, but they tend to spoil quickly. In temperate climates there
17955-457: The zygote and grows within the mother plant to a certain size before growth is halted. The formation of the seed is the defining part of the process of reproduction in seed plants ( spermatophytes ). Other plants such as ferns , mosses and liverworts , do not have seeds and use water-dependent means to propagate themselves. Seed plants now dominate biological niches on land, from forests to grasslands both in hot and cold climates . In
18090-426: Was referred to as albumen , and the inner endosperm layer as vitellus. Although misleading, the term began to be applied to all the nutrient matter. This terminology persists in referring to endospermic seeds as "albuminous". The nature of this material is used in both describing and classifying seeds, in addition to the embryo to endosperm size ratio. The endosperm may be considered to be farinaceous (or mealy) in which
18225-450: Was the earliest known tree. 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 which took place about 319 million years ago. Ginkgophyta was once
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