Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic material, a natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin that resists compression. Wood is sometimes defined as only the secondary xylem in the stems of trees, or it is defined more broadly to include the same type of tissue elsewhere such as in the roots of trees or shrubs. In a living tree it performs a support function, enabling woody plants to grow large or to stand up by themselves, it conveys water and nutrients between the leaves, other growing tissues, the roots. Wood may refer to other plant materials with comparable properties, to material engineered from wood, or wood chips or fiber. Wood has been used for thousands of years for fuel, as a construction material, for making tools and weapons and paper. More it emerged as a feedstock for the production of purified cellulose and its derivatives, such as cellophane and cellulose acetate.
As of 2005, the growing stock of forests worldwide was about 434 billion cubic meters, 47% of, commercial. As an abundant, carbon-neutral renewable resource, woody materials have been of intense interest as a source of renewable energy. In 1991 3.5 billion cubic meters of wood were harvested. Dominant uses were for building construction. A 2011 discovery in the Canadian province of New Brunswick yielded the earliest known plants to have grown wood 395 to 400 million years ago. Wood can be dated by carbon dating and in some species by dendrochronology to determine when a wooden object was created. People have used wood for thousands of years for many purposes, including as a fuel or as a construction material for making houses, weapons, packaging and paper. Known constructions using wood date back ten thousand years. Buildings like the European Neolithic long house were made of wood. Recent use of wood has been enhanced by the addition of bronze into construction; the year-to-year variation in tree-ring widths and isotopic abundances gives clues to the prevailing climate at the time a tree was cut.
Wood, in the strict sense, is yielded by trees, which increase in diameter by the formation, between the existing wood and the inner bark, of new woody layers which envelop the entire stem, living branches, roots. This process is known as secondary growth; these cells go on to form thickened secondary cell walls, composed of cellulose and lignin. Where the differences between the four seasons are distinct, e.g. New Zealand, growth can occur in a discrete annual or seasonal pattern, leading to growth rings. If the distinctiveness between seasons is annual, these growth rings are referred to as annual rings. Where there is little seasonal difference growth rings are to be indistinct or absent. If the bark of the tree has been removed in a particular area, the rings will be deformed as the plant overgrows the scar. If there are differences within a growth ring the part of a growth ring nearest the center of the tree, formed early in the growing season when growth is rapid, is composed of wider elements.
It is lighter in color than that near the outer portion of the ring, is known as earlywood or springwood. The outer portion formed in the season is known as the latewood or summerwood. However, there are major differences, depending on the kind of wood; as a tree grows, lower branches die, their bases may become overgrown and enclosed by subsequent layers of trunk wood, forming a type of imperfection known as a knot. The dead branch may not be attached to the trunk wood except at its base, can drop out after the tree has been sawn into boards. Knots affect the technical properties of the wood reducing the local strength and increasing the tendency for splitting along the wood grain, but may be exploited for visual effect. In a longitudinally sawn plank, a knot will appear as a circular "solid" piece of wood around which the grain of the rest of the wood "flows". Within a knot, the direction of the wood is up to 90 degrees different from the grain direction of the regular wood. In the tree a knot is either the base of a dormant bud.
A knot is conical in shape with the inner tip at the point in stem diameter at which the plant's vascular cambium was located when the branch formed as a bud. In grading lumber and structural timber, knots are classified according to their form, size and the firmness with which they are held in place; this firmness is affected by, among other factors, the length of time for which the branch was dead while the attaching stem continued to grow. Knots materially affect cracking and warping, ease in working, cleavability of timber, they are defects which weaken timber and lower its value for structural purposes where strength is an important consideration. The weakening effect is much more serious when timber is subjected to forces perpendicular to the grain and/or tension than when under load along the grain and/or compression; the extent to which knots affect the strength of a beam depends upon their position, size and condition. A knot on the upper side is compressed. If there is a season check
Arabic is a Central Semitic language that first emerged in Iron Age northwestern Arabia and is now the lingua franca of the Arab world. It is named after the Arabs, a term used to describe peoples living in the area bounded by Mesopotamia in the east and the Anti-Lebanon mountains in the west, in northwestern Arabia, in the Sinai Peninsula. Arabic is classified as a macrolanguage comprising 30 modern varieties, including its standard form, Modern Standard Arabic, derived from Classical Arabic; as the modern written language, Modern Standard Arabic is taught in schools and universities, is used to varying degrees in workplaces and the media. The two formal varieties are grouped together as Literary Arabic, the official language of 26 states, the liturgical language of the religion of Islam, since the Quran and Hadith were written in Arabic. Modern Standard Arabic follows the grammatical standards of Classical Arabic, uses much of the same vocabulary. However, it has discarded some grammatical constructions and vocabulary that no longer have any counterpart in the spoken varieties, has adopted certain new constructions and vocabulary from the spoken varieties.
Much of the new vocabulary is used to denote concepts that have arisen in the post-classical era in modern times. Due to its grounding in Classical Arabic, Modern Standard Arabic is removed over a millennium from everyday speech, construed as a multitude of dialects of this language; these dialects and Modern Standard Arabic are described by some scholars as not mutually comprehensible. The former are acquired in families, while the latter is taught in formal education settings. However, there have been studies reporting some degree of comprehension of stories told in the standard variety among preschool-aged children; the relation between Modern Standard Arabic and these dialects is sometimes compared to that of Latin and vernaculars in medieval and early modern Europe. This view though does not take into account the widespread use of Modern Standard Arabic as a medium of audiovisual communication in today's mass media—a function Latin has never performed. During the Middle Ages, Literary Arabic was a major vehicle of culture in Europe in science and philosophy.
As a result, many European languages have borrowed many words from it. Arabic influence in vocabulary, is seen in European languages Spanish and to a lesser extent Portuguese, Catalan, owing to both the proximity of Christian European and Muslim Arab civilizations and 800 years of Arabic culture and language in the Iberian Peninsula, referred to in Arabic as al-Andalus. Sicilian has about 500 Arabic words as result of Sicily being progressively conquered by Arabs from North Africa, from the mid-9th to mid-10th centuries. Many of these words relate to related activities; the Balkan languages, including Greek and Bulgarian, have acquired a significant number of Arabic words through contact with Ottoman Turkish. Arabic has influenced many languages around the globe throughout its history; some of the most influenced languages are Persian, Spanish, Kashmiri, Bosnian, Bengali, Malay, Indonesian, Punjabi, Assamese, Sindhi and Hausa, some languages in parts of Africa. Conversely, Arabic has borrowed words from other languages, including Greek and Persian in medieval times, contemporary European languages such as English and French in modern times.
Classical Arabic is the liturgical language of 1.8 billion Muslims, Modern Standard Arabic is one of six official languages of the United Nations. All varieties of Arabic combined are spoken by as many as 422 million speakers in the Arab world, making it the fifth most spoken language in the world. Arabic is written with the Arabic alphabet, an abjad script and is written from right to left, although the spoken varieties are sometimes written in ASCII Latin from left to right with no standardized orthography. Arabic is a Central Semitic language related to the Northwest Semitic languages, the Ancient South Arabian languages, various other Semitic languages of Arabia such as Dadanitic; the Semitic languages changed a great deal between Proto-Semitic and the establishment of the Central Semitic languages in grammar. Innovations of the Central Semitic languages—all maintained in Arabic—include: The conversion of the suffix-conjugated stative formation into a past tense; the conversion of the prefix-conjugated preterite-tense formation into a present tense.
The elimination of other prefix-conjugated mood/aspect forms in favor of new moods formed by endings attached to the prefix-conjugation forms. The development of an internal passive. There are several features which Classical Arabic, the modern Arabic varieties, as well as the Safaitic and Hismaic inscriptions share which are unattested in any other Central Semitic language variety, including the Dadanitic and Taymanitic languages of the northern Hejaz; these features are evidence of common descent from Proto-Arabic. The following features can be reconstructed with confidence for Proto-Arabic: negative particles m *mā.
Eritrea the State of Eritrea, is a country in the Horn of Africa, with its capital at Asmara. It is bordered by Sudan in the west, Ethiopia in the south, Djibouti in the southeast; the northeastern and eastern parts of Eritrea have an extensive coastline along the Red Sea. The nation has a total area of 117,600 km2, includes the Dahlak Archipelago and several of the Hanish Islands, its toponym Eritrea is based on the Greek name for the Red Sea, first adopted for Italian Eritrea in 1890. Eritrea is a multi-ethnic country, with nine recognized ethnic groups in its population of around 5 million. Most residents speak languages from the Afroasiatic family, either of the Ethiopian Semitic languages or Cushitic branches. Among these communities, the Tigrinyas make up about 55% of the population, with the Tigre people constituting around 30% of inhabitants. In addition, there are a number of Nilo-Saharan-speaking Nilotic ethnic minorities. Most people in the territory adhere to Islam; the Kingdom of Aksum, covering much of modern-day Eritrea and northern Ethiopia, was established during the first or second centuries AD.
It adopted Christianity around the middle of the fourth century. In medieval times much of Eritrea fell under the Medri Bahri kingdom, with a smaller region being part of Hamasien; the creation of modern-day Eritrea is a result of the incorporation of independent, distinct kingdoms and sultanates resulting in the formation of Italian Eritrea. After the defeat of the Italian colonial army in 1942, Eritrea was administered by the British Military Administration until 1952. Following the UN General Assembly decision, in 1952, Eritrea would govern itself with a local Eritrean parliament but for foreign affairs and defense it would enter into a federal status with Ethiopia for a period of 10 years. However, in 1962 the government of Ethiopia annulled the Eritrean parliament and formally annexed Eritrea, but the Eritreans that argued for complete Eritrean independence since the ouster of the Italians in 1941, anticipated what was coming and in 1960 organized the Eritrean Liberation Front in opposition.
In 1991, after 30 years of continuous armed struggle for independence, the Eritrean liberation fighters entered the capital city, Asmara, in victory. Eritrea is a one-party state in which national legislative elections have never been held since independence. According to Human Rights Watch, the Eritrean government's human rights record is among the worst in the world; the Eritrean government has dismissed these allegations as politically motivated. The compulsory military service requires long, indefinite conscription periods, which some Eritreans leave the country to avoid; because all local media is state-owned, Eritrea was ranked as having the second-least press freedom in the global Press Freedom Index, behind only North Korea. The sovereign state of Eritrea is a member of the African Union, the United Nations, the Intergovernmental Authority on Development, is an observer in the Arab League alongside Brazil, Venezuela and Turkey; the name Eritrea is derived from the ancient Greek name for the Red Sea.
It was first formally adopted with the formation of Italian Eritrea. The name persisted over the course of subsequent British and Ethiopian occupation, was reaffirmed by the 1993 independence referendum and 1997 constitution. At Buya in Eritrea, one of the oldest hominids representing a possible link between Homo erectus and an archaic Homo sapiens was found by Italian scientists. Dated to over 1 million years old, it is the oldest skeletal find of its kind and provides a link between hominids and the earliest anatomically modern humans, it is believed that the section of the Danakil Depression in Eritrea was a major player in terms of human evolution, may contain other traces of evolution from Homo erectus hominids to anatomically modern humans. During the last interglacial period, the Red Sea coast of Eritrea was occupied by early anatomically modern humans, it is believed that the area was on the route out of Africa that some scholars suggest was used by early humans to colonize the rest of the Old World.
In 1999, the Eritrean Research Project Team composed of Eritrean, American and French scientists discovered a Paleolithic site with stone and obsidian tools dated to over 125,000 years old near the Bay of Zula south of Massawa, along the Red Sea littoral. The tools are believed to have been used by early humans to harvest marine resources such as clams and oysters. According to linguists, the first Afroasiatic-speaking populations arrived in the region during the ensuing Neolithic era from the family's proposed urheimat in the Nile Valley. Other scholars propose that the Afroasiatic family developed in situ in the Horn, with its speakers subsequently dispersing from there. Together with Djibouti, northern Somalia, the Red Sea coast of Sudan, Eritrea is considered the most location of the land which the ancient Egyptians called Punt, first mentioned in the 25th century BC; the ancient Puntites had close relations with Ancient Egypt during the rule of Pharaoh Sahure and Queen Hatshepsut. This is confirmed by genetic studies of mummified baboons.
In 2010, a study was conducted on baboon mummies that were brought from Punt to Egypt as gifts by the ancient Egyptians. The scientists from the Egyptian Museum and the University of California used oxygen isotope analysis to examine hairs from two baboon mummies, preserved in the British Museum. One of the baboons had distorted isotopic data, so t
Leaf vegetables called leafy greens, salad greens, pot herbs, vegetable greens, or greens, are plant leaves eaten as a vegetable, sometimes accompanied by tender petioles and shoots. Although they come from a wide variety of plants, most share a great deal with other leaf vegetables in nutrition and cooking methods. Nearly one thousand species of plants with edible leaves are known. Leaf vegetables most come from short-lived herbaceous plants, such as lettuce and spinach. Woody plants of various species provide edible leaves; the leaves of many fodder crops are edible for humans, but only eaten under famine conditions. Examples include; these plants are much more prolific than traditional leaf vegetables, but exploitation of their rich nutrition is difficult, due to their high fiber content. This can be overcome by further processing such as drying and grinding into powder or pulping and pressing for juice. Leaf vegetables contain many typical plant nutrients, but since they are photosynthetic tissues, their vitamin K levels are notable.
Phylloquinone, the most common form of the vitamin, is directly involved in photosynthesis. This causes leaf vegetables to be the primary food class that interacts with the anticoagulant warfarin. Leaf vegetables are low in calories and fat, high in protein per calorie, dietary fiber, vitamin C, pro-vitamin A carotenoids, folate and vitamin K; the vitamin K content of leaf vegetables is high, since these are photosynthetic tissues and phylloquinone is involved in photosynthesis. Accordingly, users of vitamin K antagonist medications, such as warfarin, must take special care to limit consumption of leaf vegetables. If leaves are cooked for food, they may be referred to as boiled greens. Leaf vegetables may be stir-fried, steamed, or consumed raw. Leaf vegetables stewed with pork is a traditional dish in soul food and Southern U. S. cuisine. They are commonly eaten in a variety of South Asian dishes such as saag. Leafy greens can be used to wrap other ingredients into an edible package in a manner similar to a tortilla.
Many green leafy vegetables, such as lettuce or spinach, can be eaten raw, for example in sandwiches or salads. A green smoothie enables large quantities of raw leafy greens to be consumed by blending the leaves with fruit and water. In certain countries of Africa, various species of nutritious amaranth are widely eaten boiled. Celosia argentea var. argentea or "Lagos spinach" is one of the main boiled greens in West African cuisine. In Greek cuisine, khorta are a common side dish, eaten hot or cold and seasoned with olive oil and lemon. At least 80 different kinds of greens are used, depending on the area and season, including black mustard, wild sorrel, fennel, kale, black nightshade, lamb's quarters, wild leeks, hoary mustard, smooth sow thistle and the fresh leaves of the caper plant. Preboggion, a mixture of different wild boiled greens, is used in Ligurian cuisine to stuff ravioli and pansoti. One of the main ingredients of preboggion are borage leaves. Preboggion is sometimes added to minestrone soup and frittata.
In the cuisine of the Southern United States and traditional African-American cuisine, collard, garden cress, dandelion and pokeweed greens are cooked, served with pieces of ham or bacon. The boiling water, called potlikker, is used as broth. Water in which pokeweed has been prepared contains toxins removed by the boiling, should be discarded. List of leaf vegetables Leaf protein concentrate Mesclun
The Septuagint is the earliest extant Greek translation of the Hebrew scriptures from the original Hebrew. It is estimated that the first five books of the Old Testament, known as the Torah or Pentateuch, were translated in the mid-3rd century BCE and the remaining texts were translated in the 2nd century BCE. Considered the primary Greek translation of the Old Testament, it is quoted a number of times in the New Testament,particularly in the Pauline epistles,by the Apostolic Fathers, by the Greek Church Fathers; the full title in Ancient Greek: Ἡ τῶν Ἑβδομήκοντα μετάφρασις "The Translation of the Seventy", derives from the story recorded in the Letter of Aristeas that the Septuagint was translated at the request of Ptolemy II Philadelphus by 70 Jewish scholars who independently produced identical translations. The miraculous character of the Aristeas legend is indicative of the esteem in which the translation was held in the ancient Jewish diaspora and early Christian circles, it is clear that a Greek translation was in circulation among the Alexandrian Jews who were not fluent in Hebrew, but in Greek.
The evidence of Egyptian papyri from the period have led most scholars to view as probable Aristeas's dating of the translation of the Pentateuch to the third century BCE. Whatever share the Ptolemaic court may have had in the translation, it satisfied a need felt by the Jewish community, among whom a knowledge of Hebrew was waning before the demands of every-day life." While there are other contemporaneous Greek versions of the Old Testament, most did not survive except as fragments. Modern critical editions of the Septuagint are based on the Codices Vaticanus and Alexandrinus; the Septuagint derives its name from the Latin versio septuaginta interpretum, "translation of the seventy interpreters", Greek: ἡ μετάφρασις τῶν ἑβδομήκοντα, hē metáphrasis tōn hebdomḗkonta, "translation of the seventy". However, it was not until the time of Augustine of Hippo that the Greek translation of the Jewish scriptures came to be called by the Latin term Septuaginta; the Roman numeral LXX is used as an abbreviation G or G. Seventy-two Jewish scholars were asked by the Greek King of Egypt Ptolemy II Philadelphus to translate the Torah from Biblical Hebrew into Greek, for inclusion in the Library of Alexandria.
This narrative is found in the pseudepigraphic Letter of Aristeas to his brother Philocrates, is repeated by Philo of Alexandria, Josephus and by various sources, including St. Augustine; the story is found in the Tractate Megillah of the Babylonian Talmud: King Ptolemy once gathered 72 Elders. He placed them in 72 chambers, each of them in a separate one, without revealing to them why they were summoned, he entered each one's room and said: "Write for me the Torah of Moshe, your teacher". God put it in the heart of each one to translate identically. Philo of Alexandria, who relied extensively on the Septuagint, says that the number of scholars was chosen by selecting six scholars from each of the twelve tribes of Israel. According to rabbinic tradition, the Septuagint was handed in to Ptolemy on the date of an annual fast and mourning for the Jewish people; the date of the 3rd century BCE is supported by a number of factors, including the Greek being representative of early Koine, citations beginning as early as the 2nd century BCE, early manuscripts datable to the 2nd century.
After the Torah, other books were translated over the next two to three centuries. It is not altogether clear, translated when, or where; the quality and style of the different translators varied from book to book, from the literal to paraphrasing to interpretative. The translation process of the Septuagint itself and from the Septuagint into other versions can be broken down into several distinct stages, during which the social milieu of the translators shifted from Hellenistic Judaism to Early Christianity; the translation of the Septuagint itself began in the 3rd century BCE and was completed by 132 BCE in Alexandria, but in time elsewhere as well. The Septuagint is the basis for the Old Latin, Syriac, Old Armenian, Old Georgian and Coptic versions of the Christian Old Testament; the Septuagint is written in Koine Greek. Some sections of the Septuagint may show Semiticisms, or idioms and phrases based on Semitic languages like Hebrew and Aramaic. Other books, such as Daniel and Proverbs, show Greek influence more strongly.
The Septuagint may elucidate pronunciation of pre-Masoretic Hebrew: many proper nouns are spelled out with Greek vowels in the translation, while contemporary Hebrew texts lacked vowel pointing. However, it is unlikely; as the work of translation progressed, the canon of the Greek Bible expanded. The Hebrew bible called Tanakh, has three divisions: the Torah, the Neviʾim, the Ketuvim; the Septuagint has four: law, history and prophets, with the books of the Apocrypha inserted where appropriate. The Torah has held pre-eminence as the basis of the canon.
Perfume is a mixture of fragrant essential oils or aroma compounds and solvents, used to give the human body, food and living-spaces an agreeable scent. It is in liquid form and used to give a pleasant scent to a person's body. Ancient texts and archaeological excavations show the use of perfumes in some of the earliest human civilizations. Modern perfumery began in the late 19th century with the commercial synthesis of aroma compounds such as vanillin or coumarin, which allowed for the composition of perfumes with smells unattainable from natural aromatics alone; the word perfume derives from the Latin perfumare, meaning "to smoke through". Perfumery, as the art of making perfumes, began in ancient Mesopotamia and Egypt, or maybe Ancient China, was further refined by the Romans and the Arabs; the world's first-recorded chemist is considered a woman named Tapputi, a perfume maker mentioned in a cuneiform tablet from the 2nd millennium BC in Mesopotamia. She distilled flowers and calamus with other aromatics filtered and put them back in the still several times.
In India and perfumery existed in the Indus civilization. One of the earliest distillations of Ittar was mentioned in the Hindu Ayurvedic text Charaka Samhita and Sushruta Samhita. In 2003, archaeologists uncovered what are believed to be the world's oldest surviving perfumes in Pyrgos, Cyprus; the perfumes date back more than 4,000 years. They were discovered in an ancient perfumery, a 300-square-meter factory housing at least 60 stills, mixing bowls and perfume bottles. In ancient times people used herbs and spices, such as almond, myrtle, conifer resin, bergamot, as well as flowers. In May 2018, an ancient perfume “Rodo” was recreated for the Greek National Archaeological Museum's anniversary show “Countless Aspects of Beauty”, allowing visitors to approach antiquity through their olfaction receptors. In the 9th century the Arab chemist Al-Kindi wrote the Book of the Chemistry of Perfume and Distillations, which contained more than a hundred recipes for fragrant oils, aromatic waters, substitutes or imitations of costly drugs.
The book described 107 methods and recipes for perfume-making and perfume-making equipment, such as the alembic. The Persian chemist Ibn Sina introduced the process of extracting oils from flowers by means of distillation, the procedure most used today, he first experimented with the rose. Until his discovery, liquid perfumes consisted of mixtures of oil and crushed herbs or petals, which made a strong blend. Rose water was more delicate, became popular. Both the raw ingredients and the distillation technology influenced western perfumery and scientific developments chemistry; the art of perfumery was known in western Europe from 1221, taking into account the monks' recipes of Santa Maria delle Vigne or Santa Maria Novella of Florence, Italy. In the east, the Hungarians produced in 1370 a perfume made of scented oils blended in an alcohol solution – best known as Hungary Water – at the behest of Queen Elizabeth of Hungary; the art of perfumery prospered in Renaissance Italy, in the 16th century the personal perfumer to Catherine de' Medici, Rene the Florentine, took Italian refinements to France.
His laboratory was connected with her apartments by a secret passageway, so that no formulae could be stolen en route. Thanks to Rene, France became one of the European centers of perfume and cosmetics manufacture. Cultivation of flowers for their perfume essence, which had begun in the 14th century, grew into a major industry in the south of France. Between the 16th and 17th centuries, perfumes were used by the wealthy to mask body odors resulting from infrequent bathing. Due to this patronage, the perfume industry developed. In 1693, Italian barber Giovanni Paolo Feminis created a perfume water called Aqua Admirabilis, today best known as eau de cologne. By the 18th century the Grasse region of France and Calabria were growing aromatic plants to provide the growing perfume industry with raw materials. Today and France remain the center of European perfume design and trade. Perfume types reflect the concentration of aromatic compounds in a solvent, which in fine fragrance is ethanol or a mix of water and ethanol.
Various sources differ in the definitions of perfume types. The intensity and longevity of a perfume is based on the concentration and longevity of the aromatic compounds, or perfume oils, used; as the percentage of aromatic compounds increases, so does the intensity and longevity of the scent. Specific terms are used to describe a fragrance's approximate concentration by the percent of perfume oil in the volume of the final product; the most widespread terms are: parfum or extrait, in English known as perfume extract, pure perfume, or perfume: 15–40% aromatic compounds.
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