Gore-Tex is a waterproof, breathable fabric membrane and registered trademark of W. L. Gore and Associates. Invented in 1969, Gore-Tex can repel liquid water while allowing water vapor to pass through and is designed to be a lightweight, waterproof fabric for all-weather use, it is composed of stretched polytetrafluoroethylene, more known by the generic trademark Teflon. Gore-Tex was co-invented by Gore's son, Robert W. Gore. In 1969, Bob Gore stretched heated rods of polytetrafluoroethylene and created expanded polytetrafluoroethylene, his discovery of the right conditions for stretching PTFE was a happy accident, born of frustration. Instead of stretching the heated material, he applied a sudden, accelerating yank; the solid PTFE unexpectedly stretched about 800%, forming a microporous structure, about 70% air. It was introduced to the public under the trademark Gore-Tex. Gore promptly applied for and obtained the following patents: U. S. Patent 3,953,566, issued April 27, 1976, for a porous form of polytetrafluoroethylene with a micro-structure characterized by nodes interconnected by fibrils U.
S. Patent 4,187,390, issued February 5, 1980 U. S. Patent 4,194,041 on March 18, 1980 for a "waterproof laminate", together with Samuel AllenAnother form of stretched PTFE tape was produced prior to Gore-Tex in 1966, by John W. Cropper of New Zealand. Cropper had constructed a machine for this use. However, Cropper chose to keep the process of creating expanded PTFE as a held trade secret and as such, it had remained unpublished. In the 1970s Garlock, Inc. infringed Gore's patents by using Cropper's machine and was sued by Gore in the Federal District Court of Ohio. The District Court held Gore's product and process patents to be invalid after a "bitterly contested case" that "involved over two years of discovery, five weeks of trial, the testimony of 35 witnesses, over 300 exhibits". On appeal, the Federal Circuit disagreed in the famous case of Gore v. Garlock, reversing the lower court's decision on the ground, as well as others, that Cropper forfeited any superior claim to the invention by virtue of having concealed the process for making ePTFE from the public.
As a public patent had not been filed, the new form of the material could not be recognised. Gore was thereby established as the legal inventor of ePTFE. Following the Gore v. Garlock decision, Gore sued Bard, Inc. for infringing its patent by making ePTFE vascular grafts. Bard promptly agreed to exit the market. Gore next sued IMPRA, Inc. a smaller maker of ePTFE vascular grafts, in the federal district court in Arizona. IMPRA had a competing patent application for the ePTFE vascular graft. In a nearly decade-long patent/antitrust battle, IMPRA proved that Gore-Tex was identical to prior art disclosed in a Japanese process patent by duplicating the prior art process and through statistical analysis, proved that Gore had withheld the best mode for using its patent, the main claim of Gore's product patent was declared invalid in 1990.. In 1996, IMPRA was purchased by Bard and Bard was thereby able to reenter the market. After IMPRA's vascular graft patent issued, Bard sued Gore for infringing it.
Gore-Tex is used in products manufactured by Patagonia, L. L. Bean, Inc. Galvin Green, Vasque, Outdoor Research, Arc'teryx, Haglöfs and The North Face among others. Other products have come to market exploiting similar technologies following the expiry of the main Gore-Tex patent. For his invention, Robert W. Gore was inducted into the U. S. National Inventors Hall of Fame in 2006. In 2015, Gore was ordered by the Federal Circuit Court of Appeals to pay Bard $1 billion in damages; the U. S. Supreme Court declined to review the Federal Circuit's decision. PTFE is made using an emulsion polymerization process that utilizes the fluorosurfactant PFOA, a persistent environmental contaminant. In 2013, Gore eliminated the use of PFOAs in the manufacture of its weatherproof functional fabrics. Gore-Tex materials are based on thermo-mechanically expanded PTFE and other fluoropolymer products, they are used in a wide variety of applications such as high-performance fabrics, medical implants, filter media, insulation for wires and cables and sealants.
However, Gore-Tex fabric is best known for its use in protective, yet breathable, rainwear. The simplest sort of rain wear is a two layer sandwich; the outer layer is nylon or polyester and provides strength. The inner one is polyurethane, provides water resistance, at the cost of breathability. Early Gore-Tex fabric replaced the inner layer of PU with a thin, porous fluoropolymer membrane coating, bonded to a fabric; this membrane had about 9 billion pores per square inch. Each pore is 1/20,000 the size of a water droplet, making it impenetrable to liquid water while still allowing the more volatile water vapour molecules to pass through; the outer layer of Gore-Tex fabric is coated on the outside with a Durable Water Repellent treatment. The DWR prevents the main outer layer from becoming wet, which would reduce the breathability of the whole fabric. However, the DWR is not responsible for the jacket being waterproof - this is a common misconception. Without the DWR, the outer layer would become soaked, there would be no breathability, the wearer's sweat being produced on the inside would fail to evaporate, leading to dampness there.
This might give the appearance. Wear and cleaning will reduce the
Litterfall, plant litter, leaf litter, tree litter, soil litter, or duff, is dead plant material that have fallen to the ground. This detritus or dead organic material and its constituent nutrients are added to the top layer of soil known as the litter layer or O horizon. Litter has occupied the attention of ecologists at length for the reasons that it is an instrumental factor in ecosystem dynamics, it is indicative of ecological productivity, may be useful in predicting regional nutrient cycling and soil fertility. Litterfall is characterized as fresh and recognizable plant debris; this can be anything from leaves, needles, bark, seeds/nuts, logs, or reproductive organs. Items larger than 2 cm diameter are referred to as coarse litter, while anything smaller is referred to as fine litter or litter; the type of litterfall is most directly affected by ecosystem type. For example, leaf tissues account for about 70 percent of litterfall in forests, but woody litter tends to increase with forest age.
In grasslands, there is little aboveground perennial tissue so the annual litterfall is low and quite nearly equal to the net primary production. In soil science, soil litter is classified in three layers, which form on the surface of the O Horizon; these are the L, F, H layers: The litter layer is quite variable in its thickness, decomposition rate and nutrient content and is affected in part by seasonality, plant species, soil fertility and latitude. The most extreme variability of litterfall is seen as a function of seasonality. In tropical environments, the largest amount of debris falls in the latter part of dry seasons and early during wet season; as a result of this variability due to seasons, the decomposition rate for any given area will be variable. Latitude has a strong effect on litterfall rates and thickness. Litterfall declines with increasing latitude. In tropical rainforests, there is a thin litter layer due to the rapid decomposition, while in boreal forests, the rate of decomposition is slower and leads to the accumulation of a thick litter layer known as a mor.
Net primary production works inversely to this trend, suggesting that the accumulation of organic matter is a result of decomposition rate. Surface detritus facilitates the infiltration of rainwater into lower soil layers. Soil litter protects soil aggregates from raindrop impact, preventing the release of clay and silt particles from plugging soil pores. Releasing clay and silt particles reduces the capacity for soil to absorb water and increases cross surface flow, accelerating soil erosion. In addition soil litter reduces wind erosion by preventing soil from losing moisture and providing cover preventing soil transportation. Organic matter accumulation helps protect soils from wildfire damage. Soil litter can be removed depending on intensity and severity of wildfires and season. Regions with high frequency wildfires have reduced vegetation density and reduced soil litter accumulation. Climate influences the depth of plant litter. Humid tropical and sub-tropical climates have reduced organic matter layers and horizons due to year round decomposition and high vegetation density and growth.
In temperate and cold climates, litter tends to accuculate and decompose slower due to a shorter growing season. Net primary production and litterfall are intimately connected. In every terrestrial ecosystem, the largest fraction of all net primary production is lost to herbivores and litterfall. Due to their interconnectedness, global patterns of litterfall are similar to global patterns of net primary productivity. Litter provides habitat for a variety of organisms. Certain plants are specially adapted for thriving in the litter layers. For example, bluebell shoots puncture the layer to emerge in spring; some plants with rhizomes, such as common wood sorrel do well in this habitat. Many organisms that live on the forest floor are decomposers, such as fungi. Organisms whose diet consists of plant detritus, such as earthworms, are termed detritivores; the community of decomposers in the litter layer includes bacteria, nematodes, tardigrades, cryptostigmata, insect larvae, oribatid mites and millipedes.
Their consumption of the litterfall results in the breakdown of simple carbon compounds into carbon dioxide and water, releases inorganic ions into the soil where the surrounding plants can reabsorb the nutrients that were shed as litterfall. In this way, litterfall becomes an important part of the nutrient cycle that sustains forest environments; as litter decomposes, nutrients are released into the environment. The portion of the litter, not decomposable is known as humus. Litter aids in soil moisture retention by cooling the ground surface and holding moisture in decaying organic matter; the flora and fauna working to decompose soil litter aid in soil respiration. A litter layer of decomposing biomass provides a continuous energy source for macro- and micro-organisms. Numerous reptiles, amphibians and some mammals rely on litter for shelter and forage. Amphibians such as salamanders and caecilians inhabit the damp microclimate underneath fallen leaves for part or all of their life cycle. Thi
A fern is a member of a group of vascular plants that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e. having specialized tissues that conduct water and nutrients and in having life cycles in which the sporophyte is the dominant phase. Like other vascular plants, ferns have complex leaves called megaphylls, that are more complex than the microphylls of clubmosses. Most ferns are leptosporangiate ferns, sometimes referred to as true ferns, they produce coiled fiddleheads that expand into fronds. The group includes about 10,560 known extant species. Ferns are defined here in the broad sense, being all of the Polypodiopsida, comprising both the leptosporangiate and eusporangiate ferns, the latter itself comprising ferns other than those denominated true ferns, including horsetails or scouring rushes, whisk ferns, marattioid ferns, ophioglossoid ferns. Ferns first appear in the fossil record about 360 million years ago in the late Devonian period, but many of the current families and species did not appear until 145 million years ago in the early Cretaceous, after flowering plants came to dominate many environments.
The fern Osmunda claytoniana is a paramount example of evolutionary stasis. Ferns are not of major economic importance, but some are used for food, medicine, as biofertilizer, as ornamental plants and for remediating contaminated soil, they have been the subject of research for their ability to remove some chemical pollutants from the atmosphere. Some fern species, such as bracken and water fern are significant weeds world wide; some fern genera, such as Azolla can fix nitrogen and make a significant input to the nitrogen nutrition of rice paddies. They play certain roles in mythology and art. Like the sporophytes of seed plants, those of ferns consist of stems and roots. Stems: Fern stems are referred to as rhizomes though they grow underground only in some of the species. Epiphytic species and many of the terrestrial ones have above-ground creeping stolons, many groups have above-ground erect semi-woody trunks; these can reach up to 20 meters tall in a few species. Leaf: The green, photosynthetic part of the plant is technically a megaphyll and in ferns, it is referred to as a frond.
New leaves expand by the unrolling of a tight spiral called a crozier or fiddlehead fern. This uncurling of the leaf is termed circinate vernation. Leaves are divided into a sporophyll. A trophophyll frond is a vegetative leaf analogous to the typical green leaves of seed plants that does not produce spores, instead only producing sugars by photosynthesis. A sporophyll frond is a fertile leaf that produces spores borne in sporangia that are clustered to form sori. In most ferns, fertile leaves are morphologically similar to the sterile ones, they photosynthesize in the same way. In some groups, the fertile leaves are much narrower than the sterile leaves, may have no green tissue at all; the anatomy of fern leaves can either be simple or divided. In tree ferns, the main stalk that connects the leaf to the stem has multiple leaflets; the leafy structures that grow from the stipe are known as pinnae and are again divided into smaller pinnules. Roots: The underground non-photosynthetic structures that take up water and nutrients from soil.
They are always fibrous and structurally are similar to the roots of seed plants. Like all other vascular plants, the diploid sporophyte is the dominant phase or generation in the life cycle; the gametophytes of ferns, are different from those of seed plants. They are free-living and resemble liverworts, whereas those of seed plants develop within the spore wall and are dependent on the parent sporophyte for their nutrition. A fern gametophyte consists of: Prothallus: A green, photosynthetic structure, one cell thick heart or kidney shaped, 3–10 mm long and 2–8 mm broad; the prothallus produces gametes by means of: Antheridia: Small spherical structures that produce flagellate sperm. Archegonia: A flask-shaped structure that produces a single egg at the bottom, reached by the sperm by swimming down the neck. Rhizoids: root-like structures that consist of single elongated cells, that absorb water and mineral salts over the whole structure. Rhizoids anchor the prothallus to the soil. Ferns first appear in the fossil record in the early Carboniferous period.
By the Triassic, the first evidence of ferns related to several modern families appeared. The great fern radiation occurred in the late Cretaceous, when many modern families of ferns first appeared. Ferns were traditionally classified in the class Filices, in a Division of the Plant Kingdom named Pteridophyta or Filicophyta. Pteridophyta is no longer recognised as a valid taxon; the ferns are referred to as Polypodiophyta or, when treated as a subdivision of Tracheophyta, although this name sometimes only refers to leptosporangiate ferns. Traditionally, all of the spore producing vascular plants were informally denominated the pteridophytes, rendering the term synonymous with ferns and fern allies; this can be confusing because members of the division Pteridophyta were denominated pteridophytes. Traditionally, three discrete groups have be
Nanga Parbat, locally known as Diamer, is the ninth highest mountain in the world at 8,126 metres above sea level. Located in the Diamer District of Pakistan’s Gilgit Baltistan region, Nanga Parbat is the western anchor of the Himalayas; the name Nanga Parbat is derived from the Sanskrit words nagna and parvata which together mean "Naked Mountain". The mountain is locally known by its Tibetan name Diamer or Deo Mir, meaning "huge mountain". Nanga Parbat is one of the eight-thousanders. An immense, dramatic peak rising far above its surrounding terrain, Nanga Parbat is a notoriously difficult climb. Numerous mountaineering deaths in the mid and early-20th century lent it the nickname "Killer Mountain.” Nanga Parbat forms the western anchor of the Himalayan Range and is the westernmost eight-thousander. It lies just south of the Indus River in the Diamer District of Gilgit–Baltistan in Pakistan. Not far to the north is the western end of the Karakoram range. Nanga Parbat has tremendous vertical relief over local terrain in all directions.
To the south, Nanga Parbat has what is referred to as the highest mountain face in the world: the Rupal Face rises 4,600 m above its base. To the north, the complex, somewhat more sloped Rakhiot Flank rises 7,000 m from the Indus River valley to the summit in just 25 km, one of the 10 greatest elevation gains in so short a distance on earth. Nanga Parbat is one of only two peaks on earth that rank in the top twenty of both the highest mountains in the world, the most prominent peaks in the world, ranking ninth and fourteenth respectively; the other is Mount Everest, first on both lists. It is the second most prominent peak of the Himalayas, after Mount Everest; the key col for Nanga Parbat is Zoji La in Kashmir, which connects it to higher peaks in the remaining Himalaya-Karakoram range. On the Tibetan Plateau Nanga Parbat is the western most peak of the Himalayas where as Namcha Barwa marks the east end; the core of Nanga Parbat is a long ridge trending southwest–northeast. The ridge is an enormous bulk of rock.
It has three faces, Diamir face and Rupal. The southwestern portion of this main ridge is known as the Mazeno Wall, has a number of subsidiary peaks. In the other direction, the main ridge arcs northeast at Rakhiot Peak; the south/southeast side of the mountain is dominated by the Rupal Face. The north/northwest side of the mountain, leading to the Indus, is more complex, it is split into the Rakhiot face by a long ridge. There are a number of subsidiary summits, including North Peak some three kilometres north of the main summit. Near the base of the Rupal Face is a glacial lake called Latbo, above a seasonal shepherds' village of the same name; because of its accessibility, attempts to summit Nanga Parbat began soon after it was discovered by Europeans. In 1895 Albert F. Mummery led an expedition to the peak, reached 6,100 m on the Diamir Face, but Mummery and two Gurkha companions died reconnoitering the Rakhiot Face. In the 1930s, Nanga Parbat became the focus of German interest in the Himalayas.
The German mountaineers were unable to attempt Mount Everest, as only the British had access to Tibet. German efforts focused on Kanchenjunga, to which Paul Bauer led two expeditions in 1930 and 1931, but with its long ridges and steep faces Kanchenjunga was more difficult than Everest and neither expedition made much progress. K2 was known to be harder still, its remoteness meant that reaching its base would be a major undertaking. Nanga Parbat was therefore the highest mountain accessible to Germans and deemed reasonably possible by climbers at the time; the first German expedition to Nanga Parbat was led by Willy Merkl in 1932. It is sometimes referred to as a German-American expedition, as the eight climbers included Rand Herron, an American, Fritz Wiessner, who would become an American citizen the following year. While the team were all strong climbers, none had Himalayan experience, poor planning, coupled with bad weather, prevented the team progressing far beyond the Rakhiot Peak northeast of the Nanga Parbat summit, reached by Peter Aschenbrenner and Herbert Kunigk, but they did establish the feasibility of a route via Rakhiot Peak and the main ridge.
Merkl led another expedition in 1934, better prepared and financed with the full backing of the new Nazi government. Early in the expedition Alfred Drexel died of high altitude pulmonary edema; the Tyrolean climbers Peter Aschenbrenner and Erwin Schneider reached an estimated height of 7,900 m on July 6, but were forced to return because of worsening weather. On July 7 they and 14 others were trapped by a storm at 7,480 m. During the desperate retreat that followed, three famous German mountaineers, Uli Wieland, Willo Welzenbach and Merkl himself, six Sherpas died of exhaustion and altitude sickness, several more suffered severe frostbite; the last survivor to reach safety, Ang Tsering, did so having spent seven days battling through the storm. It has been said that the disaster, "for sheer protracted agony, has no parallel in climbing annals."In 1937, Karl Wien led another expedition to the mountain, following the same route as Merkl's expeditions had done. Progress was made, but more than before due to heavy snowfall.
About 14 June seven Germans and nine Sherpas the entire team, were at Camp IV below Rakhiot Peak when it was overrun by an avalanche. All sixteen men died; the Germans returned in 1938 led by Paul Bauer, but the expedition was plagued by bad weather, Baue
El Capitan is a vertical rock formation in Yosemite National Park, located on the north side of Yosemite Valley, near its western end. The granite monolith is about 3,000 feet from base to summit along its tallest face, is a popular objective for rock climbers; the formation was named "El Capitan" by the Mariposa Battalion when they explored the valley in 1851. El Capitan was taken to be a loose Spanish translation of the local Native American name for the cliff, variously transcribed as "To-to-kon oo-lah" or "To-tock-ah-noo-lah", it is unclear if the Native American name referred to a specific tribal chief or meant "the chief" or "rock chief". The top of El Capitan can be reached by hiking out of Yosemite Valley on the trail next to Yosemite Falls proceeding west. For climbers, the challenge is to climb up the sheer granite face. There are many named climbing routes, all of them arduous, including Iron Hawk and Sea of Dreams. El Capitan is composed entirely of granite, a pale, coarse-grained granite emplaced 100 mya.
In addition to El Capitan, this granite forms most of the rock features of the western portions of Yosemite Valley. A separate intrusion of igneous rock, the Taft Granite, forms the uppermost portions of the cliff face. A third igneous rock, diorite, is present as dark-veined intrusions through both kinds of granite prominent in the area known as the North America Wall. Along with most of the other rock formations of Yosemite Valley, El Capitan was carved by glacial action. Several periods of glaciation have occurred in the Sierra Nevada, but the Sherwin Glaciation, which lasted from 1.3 million years ago to 1 mya, is considered to be responsible for the majority of the sculpting. The El Capitan Granite is free of joints, as a result the glacial ice did not erode the rock face as much as other, more jointed, rocks nearby. Nonetheless, as with most of the rock forming Yosemite's features, El Capitan's granite is under enormous internal tension brought on by the compression experienced prior to the erosion that brought it to the surface.
These forces contribute to the creation of features such as the Texas Flake, a large block of granite detaching from the main rock face about halfway up the side of the cliff. Between the two main faces, the Southwest and the Southeast, is a prow. While today there are numerous established routes on both faces, the most popular and famous route is The Nose, which follows the south buttress; the Nose was first climbed in 1958 by Warren Harding, Wayne Merry and George Whitmore in 47 days using "siege" tactics: climbing in an expedition style using fixed ropes along the length of the route, linking established camps along the way. The fixed manila ropes allowed the climbers to ascend and descend from the ground up throughout the 18-month project, although they presented unique levels of danger as well, sometimes breaking due to the long exposure to cold temperatures; the climbing team relied on aid climbing, using rope and expansion bolts to make it to the summit. The second ascent of The Nose was in 1960 by Royal Robbins, Joe Fitschen, Chuck Pratt and Tom Frost, who took seven days in the first continuous climb of the route without siege tactics.
The first solo climb of The Nose was done by Tom Bauman in 1969. The first ascent of The Nose in one day was accomplished in 1975 by John Long, Jim Bridwell and Billy Westbay. Today, The Nose takes fit climbers 4–5 full days of climbing. Efforts during the 1960s and 1970s explored the other faces of El Capitan, many of the early routes are still popular today. Among the early classics are Salathé Wall on the southwest face, the North America Wall on the southeast face. Climbed in the 1960s are routes such as: Dihedral Wall. Ascents include: Wall of the Early Morning Light, now known as Dawn Wall, on the Southeast face, adjacent to the prow. Today there are over 70 routes on "El Cap" of various difficulties and danger levels. New routes continue to be established consisting of additions to, or links between, existing routes. After his successful solo ascent of the Leaning Tower, Royal Robbins turned his attention to the Yvon Chouinard-T. M. Herbert Muir Wall route, completing the first solo ascent of El Capitan during a 10-day push in 1968.
The first solo ascents of El Capitan's four classic "siege" routes were accomplished by Thomas Bauman on The Nose in 1969. Other noteworthy early solo ascents were the solo first ascent of Cosmos by Jim Dunn in 1972, Zodiac by Charlie Porter in 1972; these ascents were long 7- to 14-day ordeals that required the solo climber lead each pitch, rappel, clean the climbing gear, reascend the lead rope, haul equipment, food
Nomadic tents are a vital source of housing for nomads living in mountainous regions of Central Asia. They are made from yak wool, hand spun into yarn and takes about a year to make a mid-sized tent. Tibetan tents on the contrary are thin in comparison where the sky can be seen through the hand spun yarn inside the tent. Nomad tents are held up using hand spun 8 to 12 wooden poles; the top of the tent has a large opening, used to let smoke out and to let the warm sunshine in. Prayer flags can be found flying from the tent roofs; the inside of nomad tents are basic as the nomads very poor, own few belongings. Inside there will be some sleeping mats and blankets, a stove, a table or two, a few extra clothes and a little food. Nearly all tents will have a picture of a local lama and will have a picture of the 14th Dalai Lama. A thangka painting will be found hanging inside. Traditionally yaks are kept tied up outside of the tent using lines of rope with have 8 to 10 small loops around one of the yaks feet at night that are made secure by two wooden stakes driven into the ground.
A few dogs will be kept tied up outside the tent. Large piles of dried yak dung are stored close to the tent as an important source of fuel, it is common to see Tibetan buddhist sculptures made in the yak dung. Hand woven yak wool tents are declining rapidly. Many nomads now only live in these tents in the summer months, they live in mudbrick homes the rest of the year. Others are now moving into towns to live in traditional style Tibetan homes or are being relocated into cities where the government provides them with a modern style apartment. Although there the number of yak wool tents each year in Tibet and Inner China reduces every year, there are still several areas that have them in abundance; the northern regions of the Nagchu and Ngari prefectures in the Tibet Autonomous Region, Yushu prefecture in southern Qinghai and northern Ganzi prefecture in Sichuan province all have nomads still living in yak wool tents to this day. Transhumance Yurt Life on the Tibetan Plateau
A leaf is an organ of a vascular plant and is the principal lateral appendage of the stem. The leaves and stem together form the shoot. Leaves are collectively referred to as foliage, as in "autumn foliage". A leaf is a thin, dorsiventrally flattened organ borne above ground and specialized for photosynthesis. In most leaves, the primary photosynthetic tissue, the palisade mesophyll, is located on the upper side of the blade or lamina of the leaf but in some species, including the mature foliage of Eucalyptus, palisade mesophyll is present on both sides and the leaves are said to be isobilateral. Most leaves have distinct upper surface and lower surface that differ in colour, the number of stomata, the amount and structure of epicuticular wax and other features. Leaves can have many different shapes and textures; the broad, flat leaves with complex venation of flowering plants are known as megaphylls and the species that bear them, the majority, as broad-leaved or megaphyllous plants. In the clubmosses, with different evolutionary origins, the leaves are simple and are known as microphylls.
Some leaves, such as bulb scales, are not above ground. In many aquatic species the leaves are submerged in water. Succulent plants have thick juicy leaves, but some leaves are without major photosynthetic function and may be dead at maturity, as in some cataphylls and spines. Furthermore, several kinds of leaf-like structures found in vascular plants are not homologous with them. Examples include flattened plant stems called phylloclades and cladodes, flattened leaf stems called phyllodes which differ from leaves both in their structure and origin; some structures of non-vascular plants function much like leaves. Examples include the phyllids of liverworts. Leaves are the most important organs of most vascular plants. Green plants are autotrophic, meaning that they do not obtain food from other living things but instead create their own food by photosynthesis, they capture the energy in sunlight and use it to make simple sugars, such as glucose and sucrose, from carbon dioxide and water. The sugars are stored as starch, further processed by chemical synthesis into more complex organic molecules such as proteins or cellulose, the basic structural material in plant cell walls, or metabolised by cellular respiration to provide chemical energy to run cellular processes.
The leaves draw water from the ground in the transpiration stream through a vascular conducting system known as xylem and obtain carbon dioxide from the atmosphere by diffusion through openings called stomata in the outer covering layer of the leaf, while leaves are orientated to maximise their exposure to sunlight. Once sugar has been synthesized, it needs to be transported to areas of active growth such as the plant shoots and roots. Vascular plants transport sucrose in a special tissue called the phloem; the phloem and xylem are parallel to each other but the transport of materials is in opposite directions. Within the leaf these vascular systems branch to form veins which supply as much of the leaf as possible, ensuring that cells carrying out photosynthesis are close to the transportation system. Leaves are broad and thin, thereby maximising the surface area directly exposed to light and enabling the light to penetrate the tissues and reach the chloroplasts, thus promoting photosynthesis.
They are arranged on the plant so as to expose their surfaces to light as efficiently as possible without shading each other, but there are many exceptions and complications. For instance plants adapted to windy conditions may have pendent leaves, such as in many willows and eucalyptss; the flat, or laminar, shape maximises thermal contact with the surrounding air, promoting cooling. Functionally, in addition to carrying out photosynthesis, the leaf is the principal site of transpiration, providing the energy required to draw the transpiration stream up from the roots, guttation. Many gymnosperms have thin needle-like or scale-like leaves that can be advantageous in cold climates with frequent snow and frost; these are interpreted as reduced from megaphyllous leaves of their Devonian ancestors. Some leaf forms are adapted to modulate the amount of light they absorb to avoid or mitigate excessive heat, ultraviolet damage, or desiccation, or to sacrifice light-absorption efficiency in favour of protection from herbivory.
For xerophytes the major constraint drought. Some window plants such as Fenestraria species and some Haworthia species such as Haworthia tesselata and Haworthia truncata are examples of xerophytes. and Bulbine mesembryanthemoides. Leaves function to store chemical energy and water and may become specialised organs serving other functions, such as tendrils of peas and other legumes, the protective spines of cacti and the insect traps in carnivorous plants such as Nepenthes and Sarracenia. Leaves are the fundamental structural units from which cones are constructed in gymnosperms and from which flowers are constructed in flowering plants; the internal organisation of most kinds of leaves has evolved to maximise exposure of the photosynthetic organelles, the chloroplasts, to light and to increase the absorption of carbon dioxide while at the same time controlling water loss. Their surfaces are waterproofed by the plant cuticle and gas exchange between the mesophyll cells and the atmosphere is controlled by minute openings called stomata which open or close to regulate the rate exchange of carbon dioxide and water vapour into