The Pinophyta known as Coniferophyta or Coniferae, or as conifers, are a division of vascular land plants containing a single extant class, Pinopsida. They are gymnosperms, cone-bearing seed plants. All extant conifers are perennial woody plants with secondary growth; the great majority are trees. Examples include cedars, Douglas firs, firs, kauri, pines, redwoods and yews; as of 1998, the division Pinophyta was estimated to contain eight families, 68 genera, 629 living species. Although the total number of species is small, conifers are ecologically important, they are the dominant plants over large areas of land, most notably the taiga of the Northern Hemisphere, but in similar cool climates in mountains further south. Boreal conifers have many wintertime adaptations; the narrow conical shape of northern conifers, their downward-drooping limbs, help them shed snow. Many of them seasonally alter their biochemistry to make them more resistant to freezing. While tropical rainforests have more biodiversity and turnover, the immense conifer forests of the world represent the largest terrestrial carbon sink.
Conifers are of great economic value for softwood paper production. The earliest conifers in the fossil record date to the late Carboniferous period arising from Cordaites, a genus of seed-bearing Gondwanan plants with cone-like fertile structures. Pinophytes and Ginkgophytes all developed at this time. An important adaptation of these gymnosperms was allowing plants to live without being so dependent on water. Other adaptations are pollen and the seed, which allows the embryo to be transported and developed elsewhere. Conifers appear to be one of the taxa that benefited from the Permian–Triassic extinction event, were the dominant land plants of the Mesozoic, they were overtaken by the flowering plants, which first appeared in the Cretaceous, became dominant in the Cenozoic era. They were the main food of herbivorous dinosaurs, their resins and poisons would have given protection against herbivores. Reproductive features of modern conifers had evolved by the end of the Mesozoic era. Conifer is a Latin word, a compound of conus and ferre, meaning "the one that bears cone".
The division name Pinophyta conforms to the rules of the International Code of Nomenclature for algae and plants, which state that the names of higher taxa in plants are either formed from the name of an included family, in this case Pinaceae, or are descriptive. A descriptive name in widespread use for the conifers is Coniferae. According to the ICN, it is possible to use a name formed by replacing the termination -aceae in the name of an included family, in this case preferably Pinaceae, by the appropriate termination, in the case of this division ‑ophyta. Alternatively, "descriptive botanical names" may be used at any rank above family. Both are allowed; this means that if conifers are considered a division, they may be called Coniferae. As a class they may be called Coniferae; as an order they may be called Coniferae or Coniferales. Conifers are the largest and economically most important component group of the gymnosperms, but they comprise only one of the four groups; the division Pinophyta consists of just one class, which includes both living and fossil taxa.
Subdivision of the living conifers into two or more orders has been proposed from time to time. The most seen in the past was a split into two orders and Pinales, but recent research into DNA sequences suggests that this interpretation leaves the Pinales without Taxales as paraphyletic, the latter order is no longer considered distinct. A more accurate subdivision would be to split the class into three orders, Pinales containing only Pinaceae, Araucariales containing Araucariaceae and Podocarpaceae, Cupressales containing the remaining families, but there has not been any significant support for such a split, with the majority of opinion preferring retention of all the families within a single order Pinales, despite their antiquity and diverse morphology; the conifers are now accepted as comprising seven families, with a total of 65–70 genera and 600–630 species. The seven most distinct families are linked in the box above right and phylogenetic diagram left. In other interpretations, the Cephalotaxaceae may be better included within the Taxaceae, some authors additionally recognize Phyllocladaceae as distinct from Podocarpaceae.
The family Taxodiaceae is here included in family Cupressaceae, but was recognized in the past and can still be found in many field guides. A new classification and linear sequence based on molecular data can be found in an article by Christenhusz et al; the conifers are an ancient group, with a fossil record extending back about 300 million years to the Paleozoic in the late Carboniferous period. Other classes and orders, now long extinct occur as fossils from the late Paleozoic and Mesozoic eras. Fossil conifers included many diverse forms, the most distinct from modern conifers being some herbaceous conifers with no woody stems. Major fossil orders of conifers or conifer-like plants include the Cordaitales, Vojnovskyales and also the Czekanowskiales (possibly
Plants are multicellular, predominantly photosynthetic eukaryotes of the kingdom Plantae. Plants were treated as one of two kingdoms including all living things that were not animals, all algae and fungi were treated as plants. However, all current definitions of Plantae exclude the fungi and some algae, as well as the prokaryotes. By one definition, plants form the clade Viridiplantae, a group that includes the flowering plants and other gymnosperms and their allies, liverworts and the green algae, but excludes the red and brown algae. Green plants obtain most of their energy from sunlight via photosynthesis by primary chloroplasts that are derived from endosymbiosis with cyanobacteria, their chloroplasts contain b, which gives them their green color. Some plants are parasitic or mycotrophic and have lost the ability to produce normal amounts of chlorophyll or to photosynthesize. Plants are characterized by sexual reproduction and alternation of generations, although asexual reproduction is common.
There are about 320 thousand species of plants, of which the great majority, some 260–290 thousand, are seed plants. Green plants provide a substantial proportion of the world's molecular oxygen and are the basis of most of Earth's ecosystems on land. Plants that produce grain and vegetables form humankind's basic foods, have been domesticated for millennia. Plants have many cultural and other uses, as ornaments, building materials, writing material and, in great variety, they have been the source of medicines and psychoactive drugs; the scientific study of plants is known as a branch of biology. All living things were traditionally placed into one of two groups and animals; this classification may date from Aristotle, who made the distincton between plants, which do not move, animals, which are mobile to catch their food. Much when Linnaeus created the basis of the modern system of scientific classification, these two groups became the kingdoms Vegetabilia and Animalia. Since it has become clear that the plant kingdom as defined included several unrelated groups, the fungi and several groups of algae were removed to new kingdoms.
However, these organisms are still considered plants in popular contexts. The term "plant" implies the possession of the following traits multicellularity, possession of cell walls containing cellulose and the ability to carry out photosynthesis with primary chloroplasts; when the name Plantae or plant is applied to a specific group of organisms or taxon, it refers to one of four concepts. From least to most inclusive, these four groupings are: Another way of looking at the relationships between the different groups that have been called "plants" is through a cladogram, which shows their evolutionary relationships; these are not yet settled, but one accepted relationship between the three groups described above is shown below. Those which have been called "plants" are in bold; the way in which the groups of green algae are combined and named varies between authors. Algae comprise several different groups of organisms which produce food by photosynthesis and thus have traditionally been included in the plant kingdom.
The seaweeds range from large multicellular algae to single-celled organisms and are classified into three groups, the green algae, red algae and brown algae. There is good evidence that the brown algae evolved independently from the others, from non-photosynthetic ancestors that formed endosymbiotic relationships with red algae rather than from cyanobacteria, they are no longer classified as plants as defined here; the Viridiplantae, the green plants – green algae and land plants – form a clade, a group consisting of all the descendants of a common ancestor. With a few exceptions, the green plants have the following features in common, they undergo closed mitosis without centrioles, have mitochondria with flat cristae. The chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic cyanobacteria. Two additional groups, the Rhodophyta and Glaucophyta have primary chloroplasts that appear to be derived directly from endosymbiotic cyanobacteria, although they differ from Viridiplantae in the pigments which are used in photosynthesis and so are different in colour.
These groups differ from green plants in that the storage polysaccharide is floridean starch and is stored in the cytoplasm rather than in the plastids. They appear to have had a common origin with Viridiplantae and the three groups form the clade Archaeplastida, whose name implies that their chloroplasts were derived from a single ancient endosymbiotic event; this is the broadest modern definition of the term'plant'. In contrast, most other algae not only have different pigments but have chloroplasts with three or four surrounding membranes, they are not close relatives of the Archaeplastida having acquired chloroplasts separately from ingested or symbiotic green and red algae. They are thus not included in the broadest modern definition of the plant kingdom, although they were in the past; the green plants or Viridiplantae were traditionally divided into the green algae (including
A seed is an embryonic plant enclosed in a protective outer covering. The formation of the seed is part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiosperm plants. Seeds are the product of the ripened ovule, after fertilization by pollen and some growth within the mother plant; the embryo is developed from the seed coat from the integuments of the ovule. Seeds have been an important development in the reproduction and success of gymnosperm and angiosperm plants, relative to more primitive plants such as ferns and liverworts, which 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; the term "seed" has a general meaning that antedates the above – anything that can be sown, e.g. "seed" potatoes, "seeds" of corn or sunflower "seeds". In the case of sunflower and corn "seeds", what is sown is the seed enclosed in a shell or husk, whereas the potato is a tuber.
Many structures referred to as "seeds" are dry fruits. Plants producing berries are called baccate. 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 have a hardened fruit layer 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. Seeds are produced in several related groups of plants, their manner of production distinguishes the angiosperms from the gymnosperms. Angiosperm seeds are produced in a hard or fleshy structure called a fruit that encloses the seeds for protection in order to secure healthy growth; some fruits have layers of both 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 the cone scales.
Seed production in natural plant populations varies from year to year in response to weather variables and diseases, internal cycles within the plants themselves. Over a 20-year period, for example, forests composed of loblolly pine and shortleaf pine produced from 0 to nearly 5 million sound pine seeds per hectare. Over this period, there were six bumper, five poor, nine good seed crops, when evaluated for production of adequate seedlings for natural forest reproduction. Angiosperm seeds consist of three genetically distinct constituents: the embryo formed from the zygote, the endosperm, triploid, 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 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 inactive, but the primary endosperm divides 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 the seeds; the ovule consists of a number of components: The funicle or seed stalk which attaches the ovule to the placenta and hence ovary or fruit wall, at the pericarp. The nucellus, the remnant of the megasporangium and main region of the ovule where the megagametophyte develops; the micropyle, a small pore or opening in the apex of the integument of the ovule where the pollen tube enters during the process of fertilization. The chalaza, the base of the ovule opposite the micropyle, where integument and nucellus are joined together; the shape of the ovules as they develop affects the final shape of the seeds. Plants 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 a curved megagametophyte giving the seed a tight "C" shape.
The last ovule shape is called amphitropous, where the ovule is inverted and turned back 90 degrees on its stalk. In the majority of flowering plants, the zygote's first division is transversely oriented in regards to the long axis, this establishes the polarity of the embryo; the upper or chalazal pole becomes 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: The cotyledons, the seed leaves, attached to the embryonic axis. There may be two; the cotyledons are the source of nutrients in the non-endospermic dicotyledons, in which case they replace the endosperm, are thick and leathery. In endospermic seeds the cotyledons are papery. Dicotyledons have the point of attachment opposite one another on the axis; the epicotyl, the embryonic axis above the point of attachment of the cotyledon.
The plumule, the tip of the epicotyl, has a feathery appearance due to the presence of young leaf primordia at the apex, will become the shoot upon germination. The hypocotyl, the embryonic axis below the point of attachment of the cotyledon, connecting the epicotyl and the radicle, being the stem-root transition zone; the radicle, the basal tip of the hy
A cone is an organ on plants in the division Pinophyta that contains the reproductive structures. The familiar woody cone is the female cone; the male cones, which produce pollen, are herbaceous and much less conspicuous at full maturity. The name "cone" derives from the fact; the individual plates of a cone are known as scales. The male cone is structurally similar across all conifers, differing only in small ways from species to species. Extending out from a central axis are microsporophylls. Under each microsporophyll is several microsporangia; the female cone contains ovules. The female cone structure varies more markedly between the different conifer families, is crucial for the identification of many species of conifers; the members of the pine family have cones. These pine cones the woody female cones, are considered the "archetypal" tree cones; the female cone has two types of scale: the bract scales, the seed scales, one subtended by each bract scale, derived from a modified branchlet. On the upper-side base of each seed scale are two ovules that develop into seeds after fertilization by pollen grains.
The bract scales develop first, are conspicuous at the time of pollination. The scales open temporarily to receive gametophytes close during fertilization and maturation, re-open again at maturity to allow the seed to escape. Maturation takes 6–8 months from pollination in most Pinaceae genera, but 12 months in cedars and 18–24 months in most pines; the cones open either by the seed scales flexing back when they dry out, or by the cones disintegrating with the seed scales falling off. The cones are conic, cylindrical or ovoid, small to large, from 2–60 cm long and 1–20 cm broad. After ripening, the opening of non-serotinous pine cones is associated with their moisture content—cones are open when dry and closed when wet; this assures that the small, wind disseminated seeds will be dispersed during dry weather, thus, the distance traveled from the parent tree will be enhanced. A pine cone will go through many cycles of opening and closing during its life span after seed dispersal is complete; this process occurs with older cones while attached to branches and after the older cones have fallen to the forest floor.
The condition of fallen pine cones is a crude indication of the forest floor's moisture content, an important indication of wildfire risk. Closed cones indicate damp conditions; as a result of this, pine cones have been used by people in temperate climates to predict dry and wet weather hanging a harvested pine cone from some string outside to measure the humidity of the air. Members of the Araucariaceae have the bract and seed scales fused, have only one ovule on each scale; the cones are spherical or nearly so, large to large, 5–30 cm diameter, mature in 18 months. In Agathis, the seeds are winged and separate from the seed scale, but in the other two genera, the seed is wingless and fused to the scale; the cones of the Podocarpaceae are similar in function, though not in development, to those of the Taxaceae, being berry-like with the scales modified, evolved to attract birds into dispersing the seeds. In most of the genera, two to ten or more scales are fused together into a swollen, brightly coloured, edible fleshy aril.
Only one or two scales at the apex of the cone are fertile, each bearing a single wingless seed, but in Saxegothaea several scales may be fertile. The fleshy scale complex is 0.5–3 cm long, the seeds 4–10 mm long. In some genera, the scales are minute and not fleshy, but the seed coat develops a fleshy layer instead, the cone having the appearance of one to three small plums on a central stem; the seeds have a hard coat evolved to resist digestion in the bird's stomach. Members of the cypress family differ in that the bract and seed scales are fused, with the bract visible as no more than a small lump or spine on the scale; the botanical term galbulus is sometimes used instead of strobilus for members of this family. The female cones have one to 20 ovules on each scale, they have peltate scales, as opposed to the imbricate cones described above, though some have imbricate scales. The cones are small, 0.3–6 cm or 1⁄8–2 3⁄8 inches long, spherical or nearly so, like those of Nootka cypress, while others, such as western redcedar and California incense-cedar, are narrow.
The scales are arranged either spirally, or in decussate whorls of two or three four. The genera with spiral scale arrangement were treated in a separate family in the past. In most of the genera, the cones are woody and the seeds have two narrow wings, but in three genera, the seeds are wingless, in Juniperus, the cones are fleshy and
Montane ecosystems refers to any ecosystem found in mountains. These ecosystems are affected by climate, which gets colder as elevation increases, they are stratified according to elevation. Dense forests are common at moderate elevations. However, as the elevation increases, the climate becomes harsher, the plant community transitions to grasslands or tundra; as elevation increases, the climate becomes cooler, due to a decrease in atmospheric pressure and the adiabatic cooling of airmasses. The change in climate by moving up 100 meters on a mountain is equivalent to moving 80 kilometers towards the nearest pole; the characteristic flora and fauna in the mountains tend to depend on elevation, because of the change in climate. This dependency causes life zones to form: bands of similar ecosystems at similar altitude. One of the typical life zones on mountains is the montane forest: at moderate elevations, the rainfall and temperate climate encourages dense forests to grow. Holdridge defines the climate of montane forest as having a biotemperature of between 6 and 12 °C, where biotemperature is the mean temperature considering temperatures below 0 °C to be 0 °C.
Above the elevation of the montane forest, the trees thin out in the subalpine zone, become twisted krummholz, fail to grow. Therefore, montane forests contain trees with twisted trunks; this phenomenon is observed due to the increase in the wind strength with the elevation. The elevation where trees fail to grow is called the tree line; the biotemperature of the subalpine zone is between 3 and 6 °C. Above the tree line the ecosystem is called the alpine zone or alpine tundra, dominated by grasses and low-growing shrubs; the biotemperature of the alpine zone is between 1.5 and 3 °C. Many different plant species live in the alpine environment, including perennial grasses, forbs, cushion plants and lichens. Alpine plants must adapt to the harsh conditions of the alpine environment, which include low temperatures, ultraviolet radiation, a short growing season. Alpine plants display adaptations such as rosette structures, waxy surfaces, hairy leaves; because of the common characteristics of these zones, the World Wildlife Fund groups a set of related ecoregions into the "montane grassland and shrubland" biome.
Climates with biotemperatures below 1.5 °C tend to consist purely of ice. Montane forests occur between the subalpine zone; the elevation at which one habitat changes to another varies across the globe by latitude. The upper limit of montane forests, the forest line or timberline, is marked by a change to hardier species that occur in less dense stands. For example, in the Sierra Nevada of California, the montane forest has dense stands of lodgepole pine and red fir, while the Sierra Nevada subalpine zone contains sparse stands of whitebark pine; the lower bound of the montane zone may be a "lower timberline" that separates the montane forest from drier steppe or desert region. Montane forests differ from lowland forests in the same area; the climate of montane forests is colder than lowland climate at the same latitude, so the montane forests have species typical of higher-latitude lowland forests. Humans can disturb montane forests through agriculture. On isolated mountains, montane forests surrounded by treeless dry regions are typical "sky island" ecosystems.
Montane forests in temperate climate are one of temperate coniferous forest or temperate broadleaf and mixed forest, forest types that are well known from northern Europe, northern United States, southern Canada. The trees are, however not identical to those found further north: geology and climate causes different related species to occur in montane forests. Montane forests around the world tend to be more species-rich than those in Europe, because major mountain chains in Europe are oriented east-west. Montane forests in temperate climate occur in Europe, in North America, south-western South America, New Zealand and Himalaya. Montane forests in Mediterranean climate are warm and dry except in winter, when they are wet and mild; these forests are mixed conifer and broadleaf forests, with only a few conifer species. Pine and Juniper are typical trees found in Mediterranean montane forests; the broadleaf trees show more variety and are evergreen, e.g. evergreen Oak. This type of forest is found in the Mediterranean Basin, North Africa and the southwestern US, Iran and Afghanistan.
In the tropics, montane forests can consist of broadleaf forest in addition to coniferous forest. One example of a tropical montane forest is a cloud forest, which gains its moisture from clouds and fog. Cloud forests exhibit an abundance of mosses covering the ground and vegetation, in which case they are referred to as mossy forests. Mossy forests develop on the saddles of mountains, where moisture introduced by settling clouds is more retained. Depending on latitude, the lower limit of montane rainforests on large mountains is between 1,500 and 2,500 metres while the upper limit is from 2,400 to 3,300 metres; the subalpine zone is the biotic zone below the tree line around the world. In tropical regions of Southeast Asia the tree line may be above 4,000 m, whereas in Scotland it may be as low as 450 m. Species that occur in this zone depend on the location of the zone on the Earth, for example, snow gum in Australia, or subalpine larch, mountain h
The Kenai Peninsula is a large peninsula jutting from the coast of Southcentral Alaska. The name Kenai is derived from the word "Kenaitze" or "Kenaitze Indian Tribe", the name of the Native Athabascan Alaskan tribe, the Kahtnuht’ana Dena’ina, that inhabited the area, they called the Kenai Peninsula Yaghanen. The peninsula extends 150 miles southwest from the Chugach Mountains, south of Anchorage, it is separated from the mainland on the west on the east by Prince William Sound. Most of the peninsula is part of the Kenai Peninsula Borough. Gerasim Izmailov was the first European man to explore and map the peninsula in 1789, though Athabaskan and Alutiiq Native groups have lived on the peninsula for thousands of years; the glacier-covered Kenai Mountains, rising 7,000 feet, run along the southeast spine of the peninsula along the coast of the Gulf of Alaska. Much of the range is within Kenai Fjords National Park; the northwest coast along the Cook Inlet is marshy, dotted with numerous small lakes.
Several larger lakes extend through the interior of the peninsula, including Skilak Lake and Tustumena Lake. Rivers include the Kenai River, famous for its salmon population, as well as its tributary, the Russian River, the Kasilof River, the Anchor River. Kachemak Bay, a small inlet off the larger Cook Inlet, extends into the peninsula's southwest end, much of, part of Kachemak Bay State Park; the Kenai Peninsula has many glaciers in southern areas. It is home to both the Sargent Icefield and Harding Icefields and numerous glaciers that spawn off them; the peninsula includes several of the most populous towns in south central Alaska, including Seward on the Gulf of Alaska Coast, Kenai and Cooper Landing along the Cook Inlet and Kenai River, Homer, along Kachemak Bay, along with numerous smaller villages and settlements. Homer famously marks the terminus of the paved highway system of North America and is a popular destination for travelers who have driven to Alaska from the lower 48 states.
Seward is the southern terminus of the Alaska Railroad. There are airports with scheduled flights in Kenai and Homer as well as smaller general aviation airports in Soldotna and Seward; the Seward Highway connects Seward to Anchorage, the Sterling Highway is the backbone of Kenai Peninsula connecting the larger towns to Anchorage. The peninsula has a coastal climate, mild, with abundant rainfall, it is one of the few areas in Alaska that allow for agriculture, with a growing season adequate for producing hay and several other crops. The peninsula has natural gas and coal deposits, as well as abundant commercial and personal-use fisheries. Tourism is guiding services for hunters and fishers; the Kenai Peninsula is known as "Alaska's Playground"
Bark is the outermost layers of stems and roots of woody plants. Plants with bark include trees, woody vines, shrubs. Bark is a nontechnical term, it consists of the inner bark and the outer bark. The inner bark, which in older stems is living tissue, includes the innermost area of the periderm; the outer bark in older stems includes the dead tissue on the surface of the stems, along with parts of the innermost periderm and all the tissues on the outer side of the periderm. The outer bark on trees which lies external to the last formed periderm is called the rhytidome. Products derived from bark include: bark shingle siding and wall coverings and other flavorings, tanbark for tannin, latex, poisons, various hallucinogenic chemicals and cork. Bark has been used to make cloth and ropes and used as a surface for paintings and map making. A number of plants are grown for their attractive or interesting bark colorations and surface textures or their bark is used as landscape mulch. What is called bark includes a number of different tissues.
Cork is an external, secondary tissue, impermeable to water and gases, is called the phellem. The cork is produced by the cork cambium, a layer of meristematically active cells which serve as a lateral meristem for the periderm; the cork cambium, called the phellogen, is only one cell layer thick and it divides periclinally to the outside producing cork. The phelloderm, not always present in all barks, is a layer of cells formed by and interior to the cork cambium. Together, the phellem and phelloderm constitute the periderm. Cork cell walls contain suberin, a waxy substance which protects the stem against water loss, the invasion of insects into the stem, prevents infections by bacteria and fungal spores; the cambium tissues, i.e. the cork cambium and the vascular cambium, are the only parts of a woody stem where cell division occurs. Phloem is a nutrient-conducting tissue composed of sieve tubes or sieve cells mixed with parenchyma and fibers; the cortex is the primary tissue of roots. In stems the cortex is between the epidermis layer and the phloem, in roots the inner layer is not phloem but the pericycle.
From the outside to the inside of a mature woody stem, the layers include: Bark Periderm Cork, includes the rhytidome Cork cambium Phelloderm Cortex Phloem Vascular cambium Wood Sapwood Heartwood Pith In young stems, which lack what is called bark, the tissues are, from the outside to the inside: Epidermis, which may be replaced by periderm Cortex Primary and secondary phloem Vascular cambium Secondary and primary xylem. As the stem ages and grows, changes occur that transform the surface of the stem into the bark; the epidermis is a layer of cells that cover the plant body, including the stems, leaves and fruits, that protects the plant from the outside world. In old stems the epidermal layer and primary phloem become separated from the inner tissues by thicker formations of cork. Due to the thickening cork layer these cells die; this dead layer is the rough corky bark that forms around other stems. A secondary covering called the periderm forms on small woody stems and many non-woody plants, composed of cork, the cork cambium, the phelloderm.
The periderm forms from the phellogen. The periderm replaces the epidermis, acts as a protective covering like the epidermis. Mature phellem cells have suberin in their walls to protect the stem from desiccation and pathogen attack. Older phellem cells are dead; the skin on the potato tuber constitutes the cork of the periderm. In woody plants the epidermis of newly grown stems is replaced by the periderm in the year; as the stems grow a layer of cells form under the epidermis, called the cork cambium, these cells produce cork cells that turn into cork. A limited number of cell layers may form interior to the cork cambium, called the phelloderm; as the stem grows, the cork cambium produces new layers of cork which are impermeable to gases and water and the cells outside the periderm, namely the epidermis and older secondary phloem die. Within the periderm are lenticels, which form during the production of the first periderm layer. Since there are living cells within the cambium layers that need to exchange gases during metabolism, these lenticels, because they have numerous intercellular spaces, allow gaseous exchange with the outside atmosphere.
As the bark develops, new lenticels are formed within the cracks of the cork layers. The rhytidome is the most familiar part of bark, being the outer layer that covers the trunks of trees, it is composed of dead cells and is produced by the formation of multiple layers of suberized periderm and phloem tissue. The rhytidome is well developed in older stems and roots of trees. In shrubs, older bark is exfoliated and thick rhytidome accumulates, it is thickest and most distinctive at the trunk or bole of the tree. Bark tissues make up by weight between 10–20% of woody vascular plants and consists of various biopolymers, lignin, suberin and polysaccharides. Up to 40% of the bark tissue is made of lignin which forms an important part of a plant providing stru