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88-447: The dune and heath landscape of Thy was officially selected on 29 June 2007 to be the first national park in Denmark proper ( Northeast Greenland National Park was established in 1974). Other national parks have been established later. The governmental Forest and Nature Agency states: "A Danish national park contains the most unique and characteristic Danish nature. ... The idea

176-500: A dune slack . Dunes are most common in desert environments, where the lack of moisture hinders the growth of vegetation that would otherwise interfere with the development of dunes. However, sand deposits are not restricted to deserts, and dunes are also found along sea shores, along streams in semiarid climates, in areas of glacial outwash , and in other areas where poorly cemented sandstone bedrock disintegrates to produce an ample supply of loose sand. Subaqueous dunes can form from

264-555: A dune forms, plant succession occurs. The conditions on an embryo dune are harsh, with salt spray from the sea carried on strong winds. The dune is well drained and often dry, and composed of calcium carbonate from seashells. Rotting seaweed , brought in by storm waves adds nutrients to allow pioneer species to colonize the dune. For example, in the United Kingdom these pioneer species are often marram grass , sea wort grass and other sea grasses. These plants are well adapted to

352-406: A few (e.g. I. tegetiformans ) form spreading mats. This swollen base also contains male and female sporangia, protected by a thin, transparent covering ( velum ), which is used diagnostically to help identify quillwort species. They are heterosporous . Quillwort species are very difficult to distinguish by general appearance. The best way to identify them is by examining their megaspores under

440-590: A few tens of metres except at their nose, where vegetation stops or slows the advance of accumulating sand. Simple parabolic dunes have only one set of arms that trail upwind, behind the leading nose. Compound parabolic dunes are coalesced features with several sets of trailing arms. Complex parabolic dunes include subsidiary superposed or coalesced forms, usually of barchanoid or linear shapes. Parabolic dunes, like crescent dunes, occur in areas where very strong winds are mostly unidirectional. Although these dunes are found in areas now characterized by variable wind speeds,

528-422: A few unknown spots along the westcoast. Some of the lakes and ponds in or near the preserve are hard water habitats for rare and endangered plants like the water lobelia , quillwort , many species of chara and very rare plants like the slender naiad . Since 2009 several sightings of wolves have been reported from Thy and Hanstholm Vildtreservat, although wolves have been exterpidated from Denmark since 1813. In

616-535: A great number of conifers . Low-nutrient wet hollows (a kind of bog ) also occur, and between this varied coastal landscape, small ponds and lakes can be found. Remains from the Stone Age Littorina Sea are visible here, in the form of occasional limestone cliffs along the coast or further inland. The drifting sands in Thy have plagued the locals for centuries, encroaching on their lands and buildings. But it

704-413: A habitat for a variety of smaller animals like the natterjack toad and many insects. The wet hollows in between support quite different plants such as bog bilberry , bayberry , marsh gentian , cranberry and a variety of sundew . In the northern parts of the park is the game preserve of 'Hanstholm Vildtreservat', initiated in the 1930s. It is an area of about 40 km2, dominated by sandy heath and it

792-407: A low-lying pan within the basin where the water evaporates, depositing the gypsum and forming crystals known as selenite . The crystals left behind by this process are eroded by the wind and deposited as vast white dune fields that resemble snow-covered landscapes. These types of dune are rare, and only form in closed arid basins that retain the highly soluble gypsum that would otherwise be washed into

880-405: A microscope. Moreover, habitat, texture, spore size, and velum provide features that distinguish Isoëtes taxa. They also possess a vestigial form of secondary growth in the basal portions of its cormlike stem, an indication that they evolved from larger ancestors. Quillworts use crassulacean acid metabolism (CAM) for carbon fixation. Some aquatic species do not have stomata and the leaves have

968-524: A result of ingestion ( endozoochory ). These are among the reasons suggested for the ornamentations of the spore, with some authors demonstrating that certain patterns seem well-adapted for sticking to relevant animals like waterfowl. Another critical element of dispersal is the observation that in some species of Isoetes , the outer coat of megaspores have pockets that trap microspores, a condition known as synaptospory. Typically, heterospory means that colonization and long-dispersal are more difficult due to

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1056-412: A significant role in minimizing wave energy as it moves onshore. As a result, coastal dunes, especially those in the foredune area affected by a storm surge , will retreat or erode. To counteract the damage from tropical activity on coastal dunes, short term post-storm efforts can be made by individual agencies through fencing to help with sand accumulation. How much a dune erodes during any storm surge

1144-462: A star dune superimposed on its crest is the most common complex dune. Simple dunes represent a wind regime that has not changed in intensity or direction since the formation of the dune, while compound and complex dunes suggest that the intensity and direction of the wind has changed. The sand mass of dunes can move either windward or leeward, depending on if the wind is making contact with the dune from below or above its apogee. If wind hits from above,

1232-423: A strong tie to the harsh North Sea. It was at some point during these times that the formerly lush environment around Thy collapsed. For many years the land had been stressed and over-exploited by excessive grazing and tree cutting. Finally drifting sands got the upper hand and have formed the environment and culture here for several centuries. It was not until around the year 1800 that larger organized efforts to stop

1320-551: A thick cuticle which prevents CO 2 uptake, a task that is performed by their hollow roots instead, which absorb CO 2 from the sediment. This has been studied extensively in Isoetes andicola . CAM is normally considered an adaptation to life in arid environments to prevent water loss with the plants opening their stomata at night rather than in the heat of the day. This allows CO 2 to enter and minimises water loss. As mostly submerged aquatic plants, quillworts do not lack water and

1408-804: A unidirectional wind. In the sheltered troughs between highly developed seif dunes, barchans may be formed, because the wind is constrained to be unidirectional by the dunes. Seif dunes are common in the Sahara. They range up to 300 m (980 ft) in height and 300 km (190 mi) in length. In the southern third of the Arabian Peninsula, a vast erg , called the Rub' al Khali or Empty Quarter, contains seif dunes that stretch for almost 200 km (120 mi) and reach heights of over 300 m (980 ft). Linear loess hills known as pahas are superficially similar. These hills appear to have been formed during

1496-599: A whole new fauna could establish in Thy, with red deer and roe deer as the largest animal. Trees and grasses are not the only vegetation in the dunes though. The sandy dune heaths in Thy National Park are a rare habitat in a European context, and they are to be protected by law, so they have been a major contributory factor in establishing the national park. It is a continuous job trying to preserve them from invading flora. The larger animals are grazing excess vegetation, trees are sometimes uprooted and controlled burning

1584-477: Is a type of sandstone that is formed when a marine or aeolian sand dune becomes compacted and hardened. Once in this form, water passing through the rock can carry and deposit minerals, which can alter the colour of the rock. Cross-bedded layers of stacks of lithified dunes can produce the cross-hatching patterns, such as those seen in Zion National Park in the western United States. A slang term, used in

1672-442: Is a very large aeolian landform, with a length of several kilometers and a height of tens to hundreds of meters, and which may have superimposed dunes. Dunes are made of sand-sized particles, and may consist of quartz, calcium carbonate, snow, gypsum, or other materials. The upwind/upstream/upcurrent side of the dune is called the stoss side; the downflow side is called the lee side. Sand is pushed (creep) or bounces ( saltation ) up

1760-459: Is about improving and strengthening the Danish nature, and giving both local and foreign visitors better possibilities to experience, use and get knowledge about nature, the landscape and the history of civilization." Thy National Park is thus not just a simple tourist attraction. The landscape comprise windy coasts and dune systems, either bare, covered by heaths , meadows or plantations , with

1848-410: Is blown along the beach. Dunes form where the beach is wide enough to allow for the accumulation of wind-blown sand, and where prevailing onshore winds tend to blow sand inland. The three key ingredients for coastal dune formation are a large sand supply, winds to move said sand supply, and a place for the sand supply to accumulate. Obstacles—for example, vegetation, pebbles and so on—tend to slow down

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1936-445: Is by saltation , where sand particles skip along the ground like a bouncing ball . When these skipping particles land, they may knock into other particles and cause them to move as well, in a process known as creep . With slightly stronger winds, particles collide in mid-air, causing sheet flows . In a major dust storm , dunes may move tens of metres through such sheet flows. Also as in the case of snow, sand avalanches , falling down

2024-400: Is called a dune field , while broad, flat regions covered with wind-swept sand or dunes, with little or no vegetation, are called ergs or sand seas . Dunes occur in different shapes and sizes, but most kinds of dunes are longer on the stoss (upflow) side, where the sand is pushed up the dune, and have a shorter slip face in the lee side. The valley or trough between dunes is called

2112-467: Is carried out occasionally, to give the sandy heath a chance. The heath vegetation might look simple to the untrained eye, but is in fact very varied, dominated by common heather , sand sedge , marram grass , the edible black crowberry , bell heather and various lichens . The sandy heaths of Thy attract many birds with some very rare breeders (in Denmark), such as crane and wood sandpiper and they form

2200-428: Is due to a number of pressures related to their proximity to the ocean and confinement to growth on sandy substrates. These include: Plants have evolved many adaptations to cope with these pressures: In deserts where large amounts of limestone mountains surround a closed basin , such as at White Sands National Park in south-central New Mexico , occasional storm runoff transports dissolved limestone and gypsum into

2288-553: Is in fact the largest connected sandy heath in Denmark. About 60–70% of the preserve is open to the public, except from 1 April to 15 July, when the birds are breeding. Hanstholm game preserve is home to many species of birds, some rare or endangered in Denmark, including the European golden plover , which breeds nowhere else in the country. In the coastal dunes, beach morning glory and the herb of scots loveage can be found. They are both very rare in Denmark, only to be found here and maybe

2376-479: Is narrow, 2–20 centimetres (0.8–8 in) long (exceptionally up to 100 cm or 40 in) and 0.5–3.0 mm (0.02–0.12 in) wide; they can be either evergreen , winter deciduous , or dry-season deciduous. Only 4% of total biomass, the tips of the leaves, is chlorophyllous. The roots broaden to a swollen base up to 5 mm (0.2 in) wide where they attach in clusters to a bulb-like, underground rhizome characteristic of most quillwort species, though

2464-404: Is related to its location on the coastal shoreline and the profile of the beach during a particular season. In those areas with harsher winter weather, during the summer a beach tends to take on more of a convex appearance due to gentler waves, while the same beach in the winter may take on more of a concave appearance. As a result, coastal dunes can get eroded much more quickly in the winter than in

2552-549: The Hanstholm Fortress , as a modern museum of the events that took place here in World War II. In total, five restored bunkers and batteries are spread along the coastline in the national park. Dune A dune is a landform composed of wind- or water-driven sand . It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex . A large dune complex

2640-642: The Jurassic epoch, though the timing of the origin of modern Isoetes is subject to considerable uncertainty. The name of the genus may also be spelled Isoëtes . The diaeresis (two dots over the e) indicates that the o and the e are to be pronounced in two distinct syllables. Including this in print is optional; either spelling ( Isoetes or Isoëtes ) is correct. Quillworts are mostly aquatic or semi-aquatic in clear ponds and slow-moving streams, though several (e.g. I. butleri , I. histrix and I. nuttallii ) grow on wet ground that dries out in

2728-532: The Lycopodiopsida class, are part of the oldest extant lineage that reflects this shift to a sporophyte dominant lifecycle. In closely related lineages, such as the extinct Lepidodendron , spores were dispersed by the sporophyte through large collections of sporangia called strobili for wind-based spore dispersal. However, Isoetes are small heterosporous semi-aquatic plants, with different reproductive needs and challenges than large tree-like land plants. Like

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2816-512: The Western Desert of Egypt . The largest crescentic dunes on Earth, with mean crest-to-crest widths of more than three kilometres, are in China's Taklamakan Desert . Abundant barchan dunes may merge into barchanoid ridges, which then grade into linear (or slightly sinuous) transverse dunes, so called because they lie transverse, or across, the wind direction, with the wind blowing perpendicular to

2904-501: The slipface of the dunes—that face away from the winds—also move the dunes forward. Isoetes See text Isoetes , commonly known as the quillworts , is a genus of lycopod . It is the only living genus in the family Isoetaceae and order Isoetales . There are currently 192 recognized species, with a cosmopolitan distribution mostly in aquatic habitats but with the individual species often scarce to rare. Species virtually identical to modern quillworts have existed since

2992-454: The southwest US , for consolidated and hardened sand dunes is "slickrock", a name that was introduced by pioneers of the Old West because their steel-rimmed wagon wheels could not gain traction on the rock. Sand dunes can have a negative impact on humans when they encroach on human habitats. Sand dunes move via a few different means, all of them helped along by wind. One way that dunes can move

3080-510: The Late Jurassic of North America has been described as the "earliest clear example of a isoetalean lycopsid containing all the major features uniting modern Isoetes", including the loss of the elongated stem and vegetative leaves. Based on this, it has been stated that "the overall morphology of Isoetes appears to have persisted virtually unchanged since at least the Jurassic". The timing of

3168-551: The action of water flow ( fluvial processes) on sand or gravel beds of rivers , estuaries , and the sea-bed. Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the beach . In most cases, the dunes are important in protecting the land against potential ravages by storm waves from the sea . Artificial dunes are sometimes constructed to protect coastal areas. The dynamic action of wind and water can sometimes cause dunes to drift, which can have serious consequences. For example,

3256-433: The air. The genome sequence also provided two insights into its structure. First, genes and repeated non-coding regions were fairly evenly distributed across all the chromosomes . This is similar to genomes of other non-seed plants, but different from the seed plants ( angiosperms ) where there are distinctly more genes at the ends of chromosomes. Secondly, there was also evidence that the whole genome had been duplicated in

3344-438: The ancient past. Like all land plants, Isoetes undergoes an alternation of generations between a diploid sporophyte stage and a sexual haploid gametophyte stage. However, the dominance of one stage over the other has shifted over time. The development of vascular tissue and subsequent diversification of land plants coincides with the increased dominance of the sporophyte and reduction of the gametophyte. Isoetes , as members of

3432-500: The autumn of 2012 a dead wolf was found, presumably immigrated from the Lausitz -region in Sachsen , Germany. A few wolves might be living here today. Many of the plants in the park are edible or have edible fruits, including blackberries, cranberries, sea-buckthorns or the seakale often found on the beaches, but a few of them are rare and protected, like the scots loveage herb. Parts of

3520-409: The coastal environment of the sand dune vital to their species' survival. Over the course of time coastal dunes may be impacted by tropical cyclones or other intense storm activity, dependent on their location. Recent work has suggested that coastal dunes tend to evolve toward a high or low morphology depending on the growth rate of dunes relative to storm frequency. During a storm event, dunes play

3608-604: The concave sides of the dunes. These dunes form under winds that blow consistently from one direction (unimodal winds). They form separate crescents when the sand supply is comparatively small. When the sand supply is greater, they may merge into barchanoid ridges, and then transverse dunes (see below). Some types of crescentic dunes move more quickly over desert surfaces than any other type of dune. A group of dunes moved more than 100 metres per year between 1954 and 1959 in China 's Ningxia Province , and similar speeds have been recorded in

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3696-405: The corridors between individual dunes. Because all dune arms are oriented in the same direction, and, the inter-dune corridors are generally swept clear of loose sand, the corridors can usually be traversed in between the trailing arms of the dune. However to cross straight over the dune by going over the trailing arms, can be very difficult. Also, traversing the nose is very difficult as well because

3784-505: The destructive sand drifts was initiated and it took more than a hundred years for them to succeed. From modern times, perhaps the most dramatic evidence of human activities are the German bunkers built here during World War II , when Denmark was occupied . The German Wehrmacht found it important to fortify themselves in Thy and all along the west coast. As part of the Atlantic Wall , Thy

3872-491: The direction (s) of prevailing winds, are known as lunettes, source-bordering dunes, bourrelets and clay dunes. They may be composed of clay, silt, sand, or gypsum, eroded from the basin floor or shore, transported up the concave side of the dune, and deposited on the convex side. Examples in Australia are up to 6.5 km long, 1 km wide, and up to 50 metres high. They also occur in southern and West Africa , and in parts of

3960-657: The distance between the water line and where vegetation can grow. Coastal dunes can be classified by where they develop, or begin to take shape. Dunes are commonly grouped into either the Primary Dune Group or the Secondary Dune Group. Primary dunes gain most of their sand from the beach itself, while secondary dunes gain their sand from the primary dune. Along the Florida Panhandle, most dunes are considered to be foredunes or hummocks. Different locations around

4048-604: The dune and underlying soils . The stability of the dunes was once attributed to the vegetative cover but recent research has pointed to water as the main source of parabolic dune stability. The vegetation that covers them—grasses, shrubs, and trees—help anchor the trailing arms. In inland deserts, parabolic dunes commonly originate and extend downwind from blowouts in sand sheets only partly anchored by vegetation. They can also originate from beach sands and extend inland into vegetated areas in coastal zones and on shores of large lakes. Most parabolic dunes do not reach heights higher than

4136-513: The dunes and provide horticultural benefits, but instead spread taking land away from native species. Ammophila arenaria , known as European beachgrass, has a similar story, though it has no horticulture benefits. It has great ground coverage and, as intended, stabilized the dunes but as an unintended side effect prevented native species from thriving in those dunes. One such example is the dune field at Point Reyes, California . There are now efforts to get rid of both of these invasive species. As

4224-593: The dunes, washing humus into the slacks, and the slacks may be much more developed than the exposed tops of the dunes. It is usually in the slacks that more rare species are developed and there is a tendency for the dune slacks' soil to be waterlogged where only marsh plants can survive. In Europe these plants include: creeping willow, cotton grass, yellow iris , reeds, and rushes. As for vertebrates in European dunes, natterjack toads sometimes breed here. Dune ecosystems are extremely difficult places for plants to survive. This

4312-592: The dunes. Typically these are heather , heaths and gorses . These too are adapted to the low soil water content and have small, prickly leaves which reduce transpiration. Heather adds humus to the soil and is usually replaced by coniferous trees, which can tolerate low soil pH , caused by the accumulation and decomposition of organic matter with nitrate leaching. Coniferous forests and heathland are common climax communities for sand dune systems. Young dunes are called yellow dunes and dunes which have high humus content are called grey dunes . Leaching occurs on

4400-401: The effective winds associated with the growth and migration of both the parabolic and crescent dunes probably are the most consistent in wind direction. The grain size for these well-sorted, very fine to medium sands is about 0.06 to 0.5 mm. Parabolic dunes have loose sand and steep slopes only on their outer flanks. The inner slopes are mostly well packed and anchored by vegetation, as are

4488-452: The fact that a single spore cannot grow a bisexual gametophyte and thus cannot establish a new population from a single spore as can happen in homosporous ferns. Isoetes may mitigate this issue via microspores stuck to megaspores, greatly increasing the possibility of successful fertilization upon dispersal. Compared to other genera, Isoetes is poorly known. The first critical monograph on their taxonomy, written by Norma Etta Pfeiffer ,

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4576-509: The genome sequence, it appears that in quillworts, both forms are involved in photosynthesis. In addition, circadian expression of key CAM pathway genes peaked at different times of day than in angiosperms. These fundamental differences in biochemistry suggest that CAM in quillworts is probably another example of convergent evolution of CAM during the more than 300 million years since the genus diverged from other plants. However, they may also be because of differences between life in water and in

4664-540: The genus, separating two South American species into the genus Stylites , although molecular data place these species among other species of Isoetes , so that Stylites does not warrant taxonomic recognition. The earliest fossil that has been assigned to the genus is † Isoetes beestonii from the latest Permian of New South Wales, Australia, around 252 million years ago. However, the relationships of pre-Jurassic isoetaleans to modern Isotetes have been regarded as unclear by other authors. Isoetites rolandii from

4752-519: The geological record . All these dune shapes may occur in three forms: simple (isolated dunes of basic type), compound (larger dunes on which smaller dunes of same type form), and complex (combinations of different types). Simple dunes are basic forms with the minimum number of slipfaces that define the geometric type. Compound dunes are large dunes on which smaller dunes of similar type and slipface orientation are superimposed. Complex dunes are combinations of two or more dune types. A crescentic dune with

4840-595: The globe have dune formations unique to their given coastal profile. Coastal sand dunes can provide privacy and/or habitats to support local flora and fauna. Animals such as sand snakes, lizards, and rodents can live in coastal sand dunes, along with insects of all types. Often the vegetation of sand dunes is discussed without acknowledging the importance that coastal dunes have for animals. Further, some animals, such as foxes and feral pigs can use coastal dunes as hunting grounds to find food. Birds are also known to utilize coastal dunes as nesting grounds. All these species find

4928-408: The harsh conditions of the foredune, typically having deep roots which reach the water table , root nodules that produce nitrogen compounds, and protected stoma , reducing transpiration . Also, the deep roots bind the sand together, and the dune grows into a foredune as more sand is blown over the grasses. The grasses add nitrogen to the soil, meaning other, less hardy plants can then colonize

5016-456: The landscape in the shape of dolmens , burial mounds , kitchen middens and organized flint productions. As the land continued to rise from the sea, human activity increased as well and Thy was a very active spot, with a thriving culture in the Bronze Ages . We have archaeological evidence from their settlements in the area and many barrows from that period can still be seen, especially in

5104-478: The last ice age under permafrost conditions dominated by sparse tundra vegetation. Star dunes are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound. They tend to accumulate in areas with multidirectional wind regimes. Star dunes grow upward rather than laterally. They dominate the Grand Erg Oriental of the Sahara. In other deserts, they occur around

5192-515: The margins of the sand seas , particularly near topographic barriers. In the southeast Badain Jaran Desert of China, the star dunes are up to 500 metres tall and may be the tallest dunes on Earth. Oval or circular mounds that generally lack a slipface. Dome dunes are rare and occur at the far upwind margins of sand seas. Fixed crescentic dunes that form on the leeward margins of playas and river valleys in arid and semiarid regions in response to

5280-665: The molecular clock as providing no firm evidence for the origin time of the genus, which could date to the Mesozoic or even the late Paleozoic , depending on the calibration method used. As of November 2019 , Plants of the World Online accepted the following extant species: Many species, such as the Louisiana quillwort and the mat-forming quillwort, are endangered species . Several species of Isoetes are commonly called Merlin's grass , especially I. lacustris , but also

5368-430: The motile sperm from a microgametophyte locates the archegonia of a megagametophyte and swims inside to fertilize the egg. Outside of heterospory, a distinguishing feature of Isoetes (and Selaginella ) from other pteridophytes , is that their gametophytes grow inside the spores. This means that the gametophytes never leave the protection of the spore that disperses them, cracking the perispore (the outer layer of

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5456-520: The northern reaches near Hanstholm. An unknown number of the pre-historic remains have been covered by the drifting sands and dunes over the aeons though, so the fact that so many mounds are still visible is an indication of just how active the area was. The human activity continued into the iron Age , the Viking Age and the Middle Ages , with their own individual traces in the national park, often with

5544-654: The nose is usually made up of loose sand without much if any vegetation. A type of extensive parabolic dune that lacks discernible slipfaces and has mostly coarse grained sand is known as a zibar . The term zibar comes from the Arabic word to describe "rolling transverse ridges ... with a hard surface". The dunes are small, have low relief, and can be found in many places across the planet from Wyoming (United States) to Saudi Arabia to Australia. Spacing between zibars ranges from 50 to 400 metres and they do not become more than 10 metres high. The dunes form at about ninety degrees to

5632-427: The origin of the crown group is uncertain. Wood et al (2020) asserted there to be no morphological features that define the major clades within Isoetes, and no fossils are known that can be definitively assigned to the crown group. While Wood et al. suggested a young origin dating to the early Cenozoic based on molecular clock estimates , the results were questioned by Wikström et al. (2023) who regarded

5720-471: The park have been designated as a bird protection and international Ramsar area, as well as Natura 2000 and various other protections. The cultural history found in Thy National Park is as old as the land itself. At the end of the last Ice Age , when the ice melted and receded, the land began to rise by the process of post-glacial rebound and the virgin grounds were soon settled by Stone Age cultures. Their early presence and activities are still visible in

5808-440: The prevailing wind which blows away the small, fine-grained sand leaving behind the coarser grained sand to form the crest. Occurring wherever winds periodically reverse direction, reversing dunes are varieties of any of the above shapes. These dunes typically have major and minor slipfaces oriented in opposite directions. The minor slipfaces are usually temporary, as they appear after a reverse wind and are generally destroyed when

5896-633: The primary way by which species are identified, although no one functional purpose of the intricate surface patterns is agreed upon. The megasporangia occur within the outermost microphylls (single-veined leaves) of the plant while the microsporangia are found in the innermost microphylls. This pattern of development is hypothesized to improve the dispersal of the heavier megaspore. These spores then germinate and divide into mega- and micro- gametophytes. The microgametophytes have antheridia, which in turn produce sperm. The megagametophytes have archegonia, which produce egg cells. Fertilization takes place when

5984-553: The rest of the Lycopodiopsida class, Isoetes reproduces with spores. Among the lycophytes, both Isoetes and the Selaginellaceae (spikemosses) are heterosporous , while the remaining lycophyte family Lycopodiaceae (clubmosses) is homosporous . As heterosporous plants, fertile Isoetes sporophytes produce megaspores and microspores, which develop in the megasporangia and microsporangia. These spores are highly ornate and are

6072-478: The resultant direction of sand movement (hence the name "longitudinal"). Some linear dunes merge to form Y-shaped compound dunes. Formation is debated. Ralph Bagnold , in The Physics of Blown Sand and Desert Dunes , suggested that some seif dunes form when a barchan dune moves into a bidirectional wind regime, and one arm or wing of the crescent elongates. Others suggest that seif dunes are formed by vortices in

6160-475: The ridge crest. Seif dunes are linear (or slightly sinuous) dunes with two slip faces. The two slip faces make them sharp-crested. They are called seif dunes after the Arabic word for "sword". They may be more than 160 kilometres (100 miles) long, and thus easily visible in satellite images (see illustrations). Seif dunes are associated with bidirectional winds. The long axes and ridges of these dunes extend along

6248-415: The sand in the dune migrates forward. In plan view, these are U-shaped or V-shaped mounds of well-sorted, very fine to medium sand with elongated arms that extend upwind behind the central part of the dune. There are slipfaces that often occur on the outer side of the nose and on the outer slopes of the arms. These dunes often occur in semiarid areas where the precipitation is retained in the lower parts of

6336-425: The sand particles move leeward; the leeward flux of sand is greater than the windward flux. Conversely, if sand hits from below, sand particles move windward. Further, if the wind is carrying sand particles when it hits the dune, the dune's sand particles will saltate more than if the wind had hit the dune without carrying sand particles. Coastal dunes form when wet sand is deposited along the coast and dries out and

6424-438: The sea. A nabkha , or coppice dune, is a small dune anchored by vegetation. They usually indicate desertification or soil erosion, and serve as nesting and burrow sites for animals. Sub-aqueous ( underwater ) dunes form on a bed of sand or gravel under the actions of water flow. They are ubiquitous in natural channels such as rivers and estuaries, and also form in engineered canals and pipelines. Dunes move downstream as

6512-416: The spore) just enough to allow the passage of gametes. This is fundamentally different from ferns, where the gametophyte is a photosynthetic plant exposed to the elements of its environment. However, containment creates a separate problem for Isoetes, which is that the gametophytes have no way to acquire energy on their own. Isoetes sporophytes solve this problem by provisioning starches and other nutrients to

6600-482: The spores as an energy reserve for the eventual gametophytes. Although not a homologous process, this provisioning is somewhat analogous to other modes of offspring resource investment in seed-plants, such as fruits and seeds. The extent to which resources provisioned to the megaspore also support the growth of the new sporophyte is unknown in Isoetes . Spore dispersal occurs primarily in water ( hydrochory ) but may also occur via adherence to animals ( zoochory ) and as

6688-402: The steeper slip face facing downstream. Ripple marks preserved in sedimentary strata in the geological record can be used to determine the direction of current flow, and thus an indication of the source of the sediments. Dunes on the bed of a channel significantly increase flow resistance, their presence and growth playing a major part in river flooding . A lithified (consolidated) sand dune

6776-604: The stoss side, and slides down the lee side. A side of a dune that the sand has slid down is called a slip face (or slipface). The Bagnold formula gives the speed at which particles can be transported. Five basic dune types are recognized: crescentic, linear, star, dome, and parabolic. Dune areas may occur in three forms: simple (isolated dunes of basic type), compound (larger dunes on which smaller dunes of same type form), and complex (combinations of different types). Barchan dunes are crescent-shaped mounds which are generally wider than they are long. The lee-side slipfaces are on

6864-663: The summer. The converse is true in areas with harsher summer weather. There are many threats to these coastal communities. Some coastal dunes, for example ones in San Francisco, have been completely altered by urbanization; reshaping the dune for human use. This puts native species at risk. Another danger, in California and places in the UK specifically, is the introduction of invasive species. Plant species, such as Carpobrotus edulis , were introduced from South Africa in an attempt to stabilize

6952-481: The summer. The quillworts are spore-producing plants and highly reliant on water dispersion. Quillworts have different ways to spread their spores based on the environment. Quillwort leaves are hollow and quill-like, with a minute ligule at the base of the upper surface. arising from a central corm . The sporangia are sunk deeply in the leaf bases. Each leaf will either have many small spores or fewer large spores. Both types of leaf are found on each plant. Each leaf

7040-509: The town of Eucla, Western Australia , had to be relocated in the 1890s because of dune drift. The modern word "dune" came into English from French around 1790, which in turn came from Middle Dutch dūne . A universally precise distinction does not exist between ripples, dunes, and draas , which are all deposits of the same type of materials. Dunes are generally defined as greater than 7 cm tall and may have ripples, while ripples are deposits that are less than 3 cm tall. A draa

7128-471: The upstream slope is eroded and the sediment deposited on the downstream or lee slope in typical bedform construction. In the case of sub-aqueous barchan dunes, sediment is lost by their extremities, known as horns. These dunes most often form as a continuous 'train' of dunes, showing remarkable similarity in wavelength and height. The shape of a dune gives information about its formation environment. For instance, rivers produce asymmetrical ripples, with

7216-434: The use of CAM is considered to avoid competition with other aquatic plants for CO 2 during daytime. The first detailed quillwort genome sequence, of I. taiwanensis , showed that there were differences from CAM in terrestrial plants. CAM involves the enzyme phosphoenolpyruvate carboxylase (PEPC) and plants have two forms of the enzyme. One is normally involved in photosynthesis and the other in central metabolism. From

7304-534: The western United States, especially Texas. U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes. These dunes are formed from blowout dunes where the erosion of vegetated sand leads to a U-shaped depression. The elongated arms are held in place by vegetation; the largest arm known on Earth reaches 12 km. Sometimes these dunes are called U-shaped, blowout , or hairpin dunes, and they are well known in coastal deserts. Unlike crescent shaped dunes, their crests point upwind. The bulk of

7392-402: The wind and lead to the deposition of sand grains. These small "incipient dunes or "shadow dunes" tend to grow in the vertical direction if the obstacle slowing the wind can also grow vertically (i.e., vegetation). Coastal dunes expand laterally as a result of lateral growth of coastal plants via seed or rhizome . Models of coastal dunes suggest that their final equilibrium height is related to

7480-483: The wind next blows in the dominant direction. Draas are very large-scale dune bedforms; they may be tens or a few hundreds of metres in height, kilometres wide, and hundreds of kilometres in length. After a draa has reached a certain size, it generally develops superimposed dune forms. They are thought to be more ancient and slower-moving than smaller dunes, and to form by vertical growth of existing dunes. Draas are widespread in sand seas and are well-represented in

7568-419: Was not until around the year 1800 that something was done about it and plantings of various grasses and trees was organized. Marram grass and various conifers was the main solution and the dune plantations are here today as living witnesses to the hard struggles against the forces of Nature. The plantations provided not only protection against the unrelenting sand, but also jobs, timber and firewood and with them

7656-565: Was published in 1922 and remained a standard reference into the twenty-first century. Even after studies with cytology, scanning electron microscopy, and chromatography, species are difficult to identify and their phylogeny is disputed. Vegetative characteristics commonly used to distinguish other genera, such as leaf length, rigidity, color, or shape are variable and depend on the habitat. Most classification systems for Isoetes rely on spore characteristics, which make species identification nearly impossible without microscopy. Some botanists split

7744-549: Was strategically important in dominating the North Sea , limiting the Allied marine forces, as an effective defence against the British air force and for many other reasons. Many of the massive concrete bunkers and fortifications are still around today, some abandoned or ruined and some restored and now functioning as museums and testimonies of that bleak era. In the north part of the park lies

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