Misplaced Pages

#531468

84-434: Psilotum is a genus of fern -like vascular plants . It is one of two genera in the family Psilotaceae commonly known as whisk ferns , the other being Tmesipteris . Plants in these two genera were once thought to be descended from the earliest surviving vascular plants , but more recent phylogenies place them as basal ferns, as a sister group to Ophioglossales . They lack true roots and leaves are very reduced,

168-734: A class Equisetopsida ( Embryophyta ) encompassing all land plants. This is referred to as Equisetopsida sensu lato to distinguish it from the narrower use to refer to horsetails alone, Equisetopsida sensu stricto . They placed the lycopods into subclass Lycopodiidae and the ferns, keeping the term monilophytes, into five subclasses, Equisetidae, Ophioglossidae, Psilotidae, Marattiidae and Polypodiidae, by dividing Smith's Psilotopsida into its two orders and elevating them to subclass (Ophioglossidae and Psilotidae). Christenhusz et al. (2011) followed this use of subclasses but recombined Smith's Psilotopsida as Ophioglossidae, giving four subclasses of ferns again. Christenhusz and Chase (2014) developed

252-488: A fabric with small spaces. In small passages, such as that between the plant cell walls (or in tracheids), a column of water behaves like rubber – when molecules evaporate from one end, they pull the molecules behind them along the channels. Therefore, transpiration alone provided the driving force for water transport in early plants. However, without dedicated transport vessels, the cohesion-tension mechanism cannot transport water more than about 2 cm, severely limiting

336-634: A few species (e.g., Cyathea brownii on Norfolk Island and Cyathea medullaris in New Zealand ). Roots are underground non-photosynthetic structures that take up water and nutrients from soil . They are always fibrous and are structurally very similar to the roots of seed plants. As in all vascular plants , the sporophyte is the dominant phase or generation in the life cycle . The gametophytes of ferns, however, are very different from those of seed plants. They are free-living and resemble liverworts , whereas those of seed plants develop within

420-556: A film of surface moisture, enabling them to grow to much greater size. As a result of their independence from their surroundings, they lost their ability to survive desiccation – a costly trait to retain. During the Devonian, maximum xylem diameter increased with time, with the minimum diameter remaining pretty constant. By the middle Devonian, the tracheid diameter of some plant lineages ( Zosterophyllophytes ) had plateaued. Wider tracheids allow water to be transported faster, but

504-423: A great deal of resistance on flow; vessel members have perforated end walls, and are arranged in series to operate as if they were one continuous vessel. The function of end walls, which were the default state in the Devonian, was probably to avoid embolisms . An embolism is where an air bubble is created in a tracheid. This may happen as a result of freezing, or by gases dissolving out of solution. Once an embolism

588-400: A hundred meters from ground level to a tree 's highest branches. Transpirational pull requires that the vessels transporting the water be very small in diameter; otherwise, cavitation would break the water column. And as water evaporates from leaves, more is drawn up through the plant to replace it. When the water pressure within the xylem reaches extreme levels due to low water input from

672-455: A million times more conductive than the inter-cell method, giving the potential for transport over longer distances, and higher CO 2 diffusion rates. The earliest macrofossils to bear water-transport tubes are Silurian plants placed in the genus Cooksonia . The early Devonian pretracheophytes Aglaophyton and Horneophyton have structures very similar to the hydroids of modern mosses. Plants continued to innovate new ways of reducing

756-487: A new classification of ferns and lycopods. They used the term Polypodiophyta for the ferns, subdivided like Smith et al. into four groups (shown with equivalents in the Smith system), with 21 families, approximately 212 genera and 10,535 species; This was a considerable reduction in the number of families from the 37 in the system of Smith et al., since the approach was more that of lumping rather than splitting. For instance

840-908: A number of families were reduced to subfamilies. Subsequently, a consensus group was formed, the Pteridophyte Phylogeny Group (PPG), analogous to the Angiosperm Phylogeny Group , publishing their first complete classification in November 2016. They recognise ferns as a class, the Polypodiopsida, with four subclasses as described by Christenhusz and Chase, and which are phylogenetically related as in this cladogram: Equisetales Ophioglossales Psilotales Marattiales Osmundales Hymenophyllales Gleicheniales Schizaeales Xylem#Development The most distinctive xylem cells are

924-426: A plant's vascular system based on the classical research of Dixon-Joly (1894), Eugen Askenasy (1845–1903) (1895), and Dixon (1914,1924). Water is a polar molecule . When two water molecules approach one another, the slightly negatively charged oxygen atom of one forms a hydrogen bond with a slightly positively charged hydrogen atom in the other. This attractive force, along with other intermolecular forces ,

SECTION 10

#1732801990532

1008-462: A protective coating called an indusium . The arrangement of the sporangia is important in classification. In monomorphic ferns, the fertile and sterile leaves look morphologically the same, and both are able to photosynthesize. In hemidimorphic ferns, just a portion of the fertile leaf is different from the sterile leaves. In dimorphic (holomorphic) ferns, the two types of leaves are morphologically distinct . The fertile leaves are much narrower than

1092-422: A range of mechanisms to contain the damage. Small pits link adjacent conduits to allow fluid to flow between them, but not air – although these pits, which prevent the spread of embolism, are also a major cause of them. These pitted surfaces further reduce the flow of water through the xylem by as much as 30%. The diversification of xylem strand shapes with tracheid network topologies increasingly resistant to

1176-549: A robust internal structure that held long narrow channels for transporting water from the soil to all the different parts of the above-soil plant, especially to the parts where photosynthesis occurred. During the Silurian, CO 2 was readily available, so little water needed expending to acquire it. By the end of the Carboniferous, when CO 2 levels had lowered to something approaching today's, around 17 times more water

1260-430: A short-lived structure anchored to the ground by rhizoids called gametophyte which produce gametes. When a mature fertile frond bears sori, and spores are released, the spores will settle on the soil and send out rhizoids , while it develops into a prothallus . The prothallus bears spherical antheridia ( s.g. antheridium ) which produce antherozoids (male gametophytes) and archegonia ( s.g. archegonium ) which release

1344-812: A significant input to the nitrogen nutrition of rice paddies . They also play certain roles in folklore. Extant ferns are herbaceous perennials and most lack woody growth. When woody growth is present, it is found in the stem. Their foliage may be deciduous or evergreen , and some are semi-evergreen depending on the climate. Like the sporophytes of seed plants, those of ferns consist of stems, leaves and roots. Ferns differ from spermatophytes in that they reproduce by spores rather than having flowers and producing seeds. However, they also differ from spore-producing bryophytes in that, like seed plants, they are polysporangiophytes , their sporophytes branching and producing many sporangia. Also unlike bryophytes, fern sporophytes are free-living and only briefly dependent on

1428-431: A simple vascular cylinder, homosporous and terminal eusporangia and a lack of roots. Unfortunately, no fossils of psilophytes are known to exist. A careful study of the morphology and anatomy suggests that whisk ferns are not closely related to rhyniophytes, and that the ancestral features present in living psilophytes represent a reduction from a more typical modern fern plant. Significant differences between Psilotum and

1512-568: A single oosphere . The antherozoid swims up the archegonium and fertilize the oosphere, resulting in a zygote, which will grow into a separate sporophyte, while the gametophyte shortly persists as a free-living plant. Carl Linnaeus (1753) originally recognized 15 genera of ferns and fern allies, classifying them in class Cryptogamia in two groups, Filices (e.g. Polypodium ) and Musci (mosses). By 1806 this had increased to 38 genera, and has progressively increased since ( see Schuettpelz et al (2018) ). Ferns were traditionally classified in

1596-446: A stem or root is elongating. Later, 'metaxylem' develops in the strands of xylem. Metaxylem vessels and cells are usually larger; the cells have thickenings typically either in the form of ladderlike transverse bars (scalariform) or continuous sheets except for holes or pits (pitted). Functionally, metaxylem completes its development after elongation ceases when the cells no longer need to grow in size. There are four primary patterns to

1680-458: A vascular bundle will contain primary xylem only. The branching pattern exhibited by xylem follows Murray's law . Primary xylem is formed during primary growth from procambium . It includes protoxylem and metaxylem. Metaxylem develops after the protoxylem but before secondary xylem. Metaxylem has wider vessels and tracheids than protoxylem. Secondary xylem is formed during secondary growth from vascular cambium . Although secondary xylem

1764-415: A young vascular plant grows, one or more strands of primary xylem form in its stems and roots. The first xylem to develop is called 'protoxylem'. In appearance, protoxylem is usually distinguished by narrower vessels formed of smaller cells. Some of these cells have walls that contain thickenings in the form of rings or helices. Functionally, protoxylem can extend: the cells can grow in size and develop while

SECTION 20

#1732801990532

1848-500: Is polyphyletic , the term fern allies should be abandoned, except in a historical context. More recent genetic studies demonstrated that the Lycopodiophyta are more distantly related to other vascular plants , having radiated evolutionarily at the base of the vascular plant clade , while both the whisk ferns and horsetails are as closely related to leptosporangiate ferns as the ophioglossoid ferns and Marattiaceae . In fact,

1932-462: Is a fern (in the broad sense that includes horsetails) and that psilophytes are sister to ophioglossoid ferns. Fern The ferns ( Polypodiopsida or Polypodiophyta ) are a group of vascular plants (plants with xylem and phloem ) 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

2016-437: Is a lot lighter, thus cheaper to make, as vessels need to be much more reinforced to avoid cavitation. Xylem development can be described by four terms: centrarch, exarch, endarch and mesarch . As it develops in young plants, its nature changes from protoxylem to metaxylem (i.e. from first xylem to after xylem ). The patterns in which protoxylem and metaxylem are arranged are essential in studying plant morphology. As

2100-499: Is also found in members of the gymnosperm groups Gnetophyta and Ginkgophyta and to a lesser extent in members of the Cycadophyta , the two main groups in which secondary xylem can be found are: The xylem, vessels and tracheids of the roots, stems and leaves are interconnected to form a continuous system of water-conducting channels reaching all parts of the plants. The system transports water and soluble mineral nutrients from

2184-474: Is formed, it usually cannot be removed (but see later); the affected cell cannot pull water up, and is rendered useless. End walls excluded, the tracheids of prevascular plants were able to operate under the same hydraulic conductivity as those of the first vascular plant, Cooksonia . The size of tracheids is limited as they comprise a single cell; this limits their length, which in turn limits their maximum useful diameter to 80 μm. Conductivity grows with

2268-578: Is intermediate between the eusporangiate ferns and the leptosporangiate ferns. Rai and Graham (2010) broadly supported the primary groups, but queried their relationships, concluding that "at present perhaps the best that can be said about all relationships among the major lineages of monilophytes in current studies is that we do not understand them very well". Grewe et al. (2013) confirmed the inclusion of horsetails within ferns sensu lato , but also suggested that uncertainties remained in their precise placement. Other classifications have raised Ophioglossales to

2352-597: Is lost much faster than CO 2 is absorbed, so plants need to replace it, and have developed systems to transport water from the moist soil to the site of photosynthesis. Early plants sucked water between the walls of their cells, then evolved the ability to control water loss (and CO 2 acquisition) through the use of stomata. Specialized water transport tissues soon evolved in the form of hydroids, tracheids, then secondary xylem, followed by an endodermis and ultimately vessels. The high CO 2 levels of Silurian-Devonian times, when plants were first colonizing land, meant that

2436-435: Is one of the principal factors responsible for the occurrence of surface tension in liquid water. It also allows plants to draw water from the root through the xylem to the leaf. Water is constantly lost through transpiration from the leaf. When one water molecule is lost another is pulled along by the processes of cohesion and tension. Transpiration pull, utilizing capillary action and the inherent surface tension of water,

2520-497: Is the primary mechanism of water movement in plants. However, it is not the only mechanism involved. Any use of water in leaves forces water to move into them. Transpiration in leaves creates tension (differential pressure) in the cell walls of mesophyll cells. Because of this tension, water is being pulled up from the roots into the leaves, helped by cohesion (the pull between individual water molecules, due to hydrogen bonds) and adhesion (the stickiness between water molecules and

2604-577: Is unusual in that it branches dichotomously, lives underground and possesses vascular tissue. The nutrition of the gametophyte appears to be myco-heterotrophic , assisted by endophytic fungi . The genus Psilotum was first formally described in 1801 by Olof Swartz and the description was published in Journal für die Botanik (Schrader) . The name of the genus is from the Ancient Greek word psilos meaning "bare", "smooth" or "bald" referring to

Psilotum - Misplaced Pages Continue

2688-428: Is well worth plants' while to avoid cavitation occurring. For this reason, pits in tracheid walls have very small diameters, to prevent air entering and allowing bubbles to nucleate. Freeze-thaw cycles are a major cause of cavitation. Damage to a tracheid's wall almost inevitably leads to air leaking in and cavitation, hence the importance of many tracheids working in parallel. Once cavitation has occurred, plants have

2772-646: The Cretaceous , contemporaneous with the rise of flowering plants that came to dominate the world's flora. 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 ( Pteridium aquilinum ) and water fern ( Azolla filiculoides ), are significant weeds worldwide. Some fern genera, such as Azolla , can fix nitrogen and make

2856-710: The Silurian (more than 400 million years ago), and trace fossils resembling individual xylem cells may be found in earlier Ordovician rocks. The earliest true and recognizable xylem consists of tracheids with a helical-annular reinforcing layer added to the cell wall . This is the only type of xylem found in the earliest vascular plants, and this type of cell continues to be found in the protoxylem (first-formed xylem) of all living groups of vascular plants. Several groups of plants later developed pitted tracheid cells independently through convergent evolution . In living plants, pitted tracheids do not appear in development until

2940-479: The class Filices, and later in a Division of the Plant Kingdom named Pteridophyta or Filicophyta. Pteridophyta is no longer recognised as a valid taxon because it is paraphyletic . The ferns are also referred to as Polypodiophyta or, when treated as a subdivision of Tracheophyta (vascular plants), Polypodiopsida, although this name sometimes only refers to leptosporangiate ferns. Traditionally, all of

3024-603: The hydrophilic cell walls of plants). This mechanism of water flow works because of water potential (water flows from high to low potential), and the rules of simple diffusion . Over the past century, there has been a great deal of research regarding the mechanism of xylem sap transport; today, most plant scientists continue to agree that the cohesion-tension theory best explains this process, but multiforce theories that hypothesize several alternative mechanisms have been suggested, including longitudinal cellular and xylem osmotic pressure gradients , axial potential gradients in

3108-402: The "next generation" of transport cell design, have a more rigid structure than hydroids, allowing them to cope with higher levels of water pressure. Tracheids may have a single evolutionary origin, possibly within the hornworts, uniting all tracheophytes (but they may have evolved more than once). Water transport requires regulation, and dynamic control is provided by stomata . By adjusting

3192-461: The U.S., P. nudum is found from Florida to Texas, and P. complanatum in Hawaii. Psilotum superficially resembles certain extinct early vascular plants, such as the rhyniophytes and the trimerophyte genus Psilophyton . The unusual features of Psilotum that suggest an affinity with early vascular plants include dichotomously branching sporophytes, aerial stems arising from horizontal rhizomes,

3276-473: The amount of gas exchange, they can restrict the amount of water lost through transpiration. This is an important role where water supply is not constant, and indeed stomata appear to have evolved before tracheids, being present in the non-vascular hornworts. An endodermis probably evolved during the Silu-Devonian, but the first fossil evidence for such a structure is Carboniferous. This structure in

3360-502: The arrangement of protoxylem and metaxylem in stems and roots. The other three terms are used where there is more than one strand of primary xylem. In his book De plantis libri XVI (On Plants, in 16 books) (1583), the Italian physician and botanist Andrea Cesalpino proposed that plants draw water from soil not by magnetism ( ut magnes ferrum trahit , as magnetic iron attracts) nor by suction ( vacuum ), but by absorption, as occurs in

3444-485: The branched sporophyte is the dominant phase. Ferns have complex leaves called megaphylls that are more complex than the microphylls of clubmosses . Most ferns are leptosporangiate ferns . They produce coiled fiddleheads that uncoil and 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

Psilotum - Misplaced Pages Continue

3528-463: The case of linen, sponges, or powders. The Italian biologist Marcello Malpighi was the first person to describe and illustrate xylem vessels, which he did in his book Anatome plantarum ... (1675). Although Malpighi believed that xylem contained only air, the British physician and botanist Nehemiah Grew , who was Malpighi's contemporary, believed that sap ascended both through the bark and through

3612-405: The disc is sucked into the pore on that side, and blocks further flow. Other plants simply tolerate cavitation. For instance, oaks grow a ring of wide vessels at the start of each spring, none of which survive the winter frosts. Maples use root pressure each spring to force sap upwards from the roots, squeezing out any air bubbles. Growing to height also employed another trait of tracheids –

3696-435: The fourth power of diameter, so increased diameter has huge rewards; vessel elements , consisting of a number of cells, joined at their ends, overcame this limit and allowed larger tubes to form, reaching diameters of up to 500 μm, and lengths of up to 10 m. Vessels first evolved during the dry, low CO 2 periods of the late Permian, in the horsetails, ferns and Selaginellales independently, and later appeared in

3780-431: The fronds are branched more than once, it can also be a combination of the pinnatifid are pinnate shapes. If the leaf blades are divided twice, the plant has bipinnate fronds, and tripinnate fronds if they branch three times, and all the way to tetra- and pentapinnate fronds. In tree ferns, the main stalk that connects the leaf to the stem (known as the stipe), often has multiple leaflets. The leafy structures that grow from

3864-514: The fusion of three sporangia and which produce the spores . When mature, the synangia release yellow to whitish spores which develop into a gametophyte less than 2 mm (0.08 in) long. The gametophyte lives underground as a mycoheterotroph, tapping into mycorrhizal networks to access carbon and other nutrients. When the gametophyte is mature, it is monoicous , producing both egg and sperm cells. The sperm cells swim using several flagella and when they reach an egg cell, unite with it to form

3948-470: The height of a plant increases and upwards transport of water by xylem is considered to limit the maximum height of trees. Three phenomena cause xylem sap to flow: The primary force that creates the capillary action movement of water upwards in plants is the adhesion between the water and the surface of the xylem conduits. Capillary action provides the force that establishes an equilibrium configuration, balancing gravity. When transpiration removes water at

4032-494: The inclusion of Equisetaceae in the ferns, notably relating to the construction of their sperm and peculiarities of their roots. The leptosporangiate ferns are sometimes called "true ferns". This group includes most plants familiarly known as ferns. Modern research supports older ideas based on morphology that the Osmundaceae diverged early in the evolutionary history of the leptosporangiate ferns; in certain ways this family

4116-413: The key innovations that led to the success of the angiosperms . However, the occurrence of vessel elements is not restricted to angiosperms, and they are absent in some archaic or "basal" lineages of the angiosperms: (e.g., Amborellaceae , Tetracentraceae , Trochodendraceae , and Winteraceae ), and their secondary xylem is described by Arthur Cronquist as "primitively vesselless". Cronquist considered

4200-728: The lack of the usual plant organs, and the seeming lack of leaves. There are two species, Psilotum nudum and Psilotum complanatum , with a hybrid between them known, Psilotum × intermedium W. H. Wagner . The distribution of Psilotum is tropical and subtropical, in the New World , Asia , and the Pacific , with a few isolated populations in south-west Europe. The highest latitudes known are in South Carolina , Cádiz province in Spain , and southern Japan for P. nudum . In

4284-446: The leptosporangiate ( Polypodiidae ) and eusporangiate ferns , the latter group including horsetails , whisk ferns , marattioid ferns , and ophioglossoid ferns . The fern crown group , consisting of the leptosporangiates and eusporangiates, is estimated to have originated in the late Silurian period 423.2 million years ago, but Polypodiales , the group that makes up 80% of living fern diversity, did not appear and diversify until

SECTION 50

#1732801990532

4368-474: The leptosporangiate ferns. The Marattiaceae are a primitive group of tropical ferns with large, fleshy rhizomes and are now thought to be a sibling taxon to the leptosporangiate ferns. Several other groups of species were considered fern allies: the clubmosses , spikemosses , and quillworts in Lycopodiophyta ; the whisk ferns of Psilotaceae ; and the horsetails of Equisetaceae . Since this grouping

4452-405: The likelihood of cavitation. Cavitation occurs when a bubble of air forms within a vessel, breaking the bonds between chains of water molecules and preventing them from pulling more water up with their cohesive tension. A tracheid, once cavitated, cannot have its embolism removed and return to service (except in a few advanced angiosperms which have developed a mechanism of doing so). Therefore, it

4536-444: The long tracheary elements that transport water. Tracheids and vessel elements are distinguished by their shape; vessel elements are shorter, and are connected together into long tubes that are called vessels . Xylem also contains two other type of cells: parenchyma and fibers . Xylem can be found: In transitional stages of plants with secondary growth , the first two categories are not mutually exclusive, although usually

4620-555: The maternal gametophyte . The green , photosynthetic part of the plant is technically a megaphyll and in ferns, it is often called a frond . New leaves typically expand by the unrolling of a tight spiral called a crozier or fiddlehead into fronds . This uncurling of the leaf is termed circinate vernation . Leaves are divided into two types: sporophylls and tropophylls. Sporophylls produce spores; tropophylls do not. Fern spores are borne in sporangia which are usually clustered to form sori . The sporangia may be covered with

4704-408: The maturation of the metaxylem (following the protoxylem ). In most plants, pitted tracheids function as the primary transport cells. The other type of vascular element, found in angiosperms, is the vessel element . Vessel elements are joined end to end to form vessels in which water flows unimpeded, as in a pipe. The presence of xylem vessels (also called trachea ) is considered to be one of

4788-403: The mid Cretaceous in angiosperms and gnetophytes. Vessels allow the same cross-sectional area of wood to transport around a hundred times more water than tracheids! This allowed plants to fill more of their stems with structural fibers, and also opened a new niche to vines , which could transport water without being as thick as the tree they grew on. Despite these advantages, tracheid-based wood

4872-433: The need for water was relatively low. As CO 2 was withdrawn from the atmosphere by plants, more water was lost in its capture, and more elegant transport mechanisms evolved. As water transport mechanisms, and waterproof cuticles, evolved, plants could survive without being continually covered by a film of water. This transition from poikilohydry to homoiohydry opened up new potential for colonization. Plants then needed

4956-480: The overall transport rate depends also on the overall cross-sectional area of the xylem bundle itself. The increase in vascular bundle thickness further seems to correlate with the width of plant axes, and plant height; it is also closely related to the appearance of leaves and increased stomatal density, both of which would increase the demand for water. While wider tracheids with robust walls make it possible to achieve higher water transport tensions, this increases

5040-430: The rank of a fifth class, separating the whisk ferns and ophioglossoid ferns. The ferns are related to other groups as shown in the following cladogram: Lycophytes [REDACTED] Ferns [REDACTED] Gymnosperms [REDACTED] Angiosperms [REDACTED] The classification of Smith et al. in 2006 treated ferns as four classes: In addition they defined 11 orders and 37 families. That system

5124-415: The resistance to flow within their cells, thereby increasing the efficiency of their water transport. Bands on the walls of tubes, in fact apparent from the early Silurian onwards, are an early improvisation to aid the easy flow of water. Banded tubes, as well as tubes with pitted ornamentation on their walls, were lignified and, when they form single celled conduits, are considered to be tracheids . These,

SECTION 60

#1732801990532

5208-465: The rhyniophytes and trimerophytes are that the development of its vascular strand is exarch , while it is centrarch in rhyniophytes and trimerophytes. The sporangia of Psilotum are trilocular synangia resulting from the fusion of three adjacent sporangia, and these are borne laterally on the axes. In the rhyniophytes and trimerophytes the sporangia were single and in a terminal position on branches. Molecular evidence strongly confirms that Psilotum

5292-416: The roots (if, for example, the soil is dry), then the gases come out of solution and form a bubble – an embolism forms, which will spread quickly to other adjacent cells, unless bordered pits are present (these have a plug-like structure called a torus, that seals off the opening between adjacent cells and stops the embolism from spreading). Even after an embolism has occurred, plants are able to refill

5376-472: The roots covers the water transport tissue and regulates ion exchange (and prevents unwanted pathogens etc. from entering the water transport system). The endodermis can also provide an upwards pressure, forcing water out of the roots when transpiration is not enough of a driver. Once plants had evolved this level of controlled water transport, they were truly homoiohydric, able to extract water from their environment through root-like organs rather than relying on

5460-445: The roots throughout the plant. It is also used to replace water lost during transpiration and photosynthesis. Xylem sap consists mainly of water and inorganic ions, although it can also contain a number of organic chemicals as well. The transport is passive, not powered by energy spent by the tracheary elements themselves, which are dead by maturity and no longer have living contents. Transporting sap upwards becomes more difficult as

5544-470: The size of the earliest plants. This process demands a steady supply of water from one end, to maintain the chains; to avoid exhausting it, plants developed a waterproof cuticle . Early cuticle may not have had pores but did not cover the entire plant surface, so that gas exchange could continue. However, dehydration at times was inevitable; early plants cope with this by having a lot of water stored between their cell walls, and when it comes to it sticking out

5628-476: 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 also denominated pteridophytes ( sensu stricto ). Traditionally, three discrete groups have been denominated ferns: two groups of eusporangiate ferns, the families Ophioglossaceae ( adder's tongues , moonworts , and grape ferns) and Marattiaceae ; and

5712-417: The spore wall and are dependent on the parent sporophyte for their nutrition. A fern gametophyte typically consists of: The lifecycle of a fern involves two stages, as in club mosses and horsetails . In stage one, the spores are produced by sporophytes in sporangia , which are clustered together in sori ( s.g. sorus ), developing on the underside of fertile fronds. In stage two, the spores germinate into

5796-536: The spread of embolism likely facilitated increases in plant size and the colonization of drier habitats during the Devonian radiation . Conifers, by the Jurassic, developed bordered pits had valve-like structures to isolate cavitated elements. These torus-margo structures have an impermeable disc (torus) suspended by a permeable membrane (margo) between two adjacent pores. When a tracheid on one side depressurizes,

5880-645: The stems being the organs containing photosynthetic and conducting tissue. There are only two species in Psilotum and a hybrid between the two. They differ from those in Tmesipteris in having stems with many branches and a synangium with three lobes rather than two. Whisk ferns in the genus Psilotum lack true roots but are anchored by creeping rhizomes . The stems have many branches with paired enations , which look like small leaves but have no vascular tissue . Above these enations there are synangia formed by

5964-496: The sterile leaves, and may have no green tissue at all, as in the Blechnaceae and Lomariopsidaceae . The anatomy of fern leaves can be anywhere from simple to highly divided, or even indeterminate (e.g. Gleicheniaceae , Lygodiaceae ). The divided forms are pinnate , where the leaf segments are completely separated from one other, or pinnatifid (partially pinnate), where the leaf segments are still partially connected. When

6048-463: The stipe are known as pinnae and are often again divided into smaller pinnules. Fern stems are often loosely called rhizomes , even though they grow underground only in some of the species. Epiphytic species and many of the terrestrial ones have above-ground creeping stolons (e.g., Polypodiaceae ), and many groups have above-ground erect semi-woody trunks (e.g., Cyatheaceae , the scaly tree ferns). These can reach up to 20 meters (66 ft) tall in

6132-462: The support offered by their lignified walls. Defunct tracheids were retained to form a strong, woody stem, produced in most instances by a secondary xylem. However, in early plants, tracheids were too mechanically vulnerable, and retained a central position, with a layer of tough sclerenchyma on the outer rim of the stems. Even when tracheids do take a structural role, they are supported by sclerenchymatic tissue. Tracheids end with walls, which impose

6216-450: The top, the flow is needed to return to the equilibrium. Transpirational pull results from the evaporation of water from the surfaces of cells in the leaves . This evaporation causes the surface of the water to recess into the pores of the cell wall . By capillary action , the water forms concave menisci inside the pores. The high surface tension of water pulls the concavity outwards, generating enough force to lift water as high as

6300-543: The tough times by putting life "on hold" until more water is supplied. To be free from the constraints of small size and constant moisture that the parenchymatic transport system inflicted, plants needed a more efficient water transport system. During the early Silurian , they developed specialized cells, which were lignified (or bore similar chemical compounds) to avoid implosion; this process coincided with cell death, allowing their innards to be emptied and water to be passed through them. These wider, dead, empty cells were

6384-471: The vessels of Gnetum to be convergent with those of angiosperms. Whether the absence of vessels in basal angiosperms is a primitive condition is contested, the alternative hypothesis states that vessel elements originated in a precursor to the angiosperms and were subsequently lost. To photosynthesize, plants must absorb CO 2 from the atmosphere. However, this comes at a price: while stomata are open to allow CO 2 to enter, water can evaporate. Water

6468-408: The vessels, and gel- and gas-bubble-supported interfacial gradients. Until recently, the differential pressure (suction) of transpirational pull could only be measured indirectly, by applying external pressure with a pressure bomb to counteract it. When the technology to perform direct measurements with a pressure probe was developed, there was initially some doubt about whether the classic theory

6552-477: The whisk ferns and ophioglossoid ferns are demonstrably a clade , and the horsetails and Marattiaceae are arguably another clade. Smith et al. (2006) carried out the first higher-level pteridophyte classification published in the molecular phylogenetic era, and considered the ferns as monilophytes, as follows: Molecular data, which remain poorly constrained for many parts of the plants' phylogeny, have been supplemented by morphological observations supporting

6636-400: The xylem and restore the functionality. The cohesion-tension theory is a theory of intermolecular attraction that explains the process of water flow upwards (against the force of gravity ) through the xylem of plants. It was proposed in 1894 by John Joly and Henry Horatio Dixon . Despite numerous objections, this is the most widely accepted theory for the transport of water through

6720-406: The xylem. However, according to Grew, capillary action in the xylem would raise the sap by only a few inches; to raise the sap to the top of a tree, Grew proposed that the parenchymal cells become turgid and thereby not only squeeze the sap in the tracheids but force some sap from the parenchyma into the tracheids. In 1727, English clergyman and botanist Stephen Hales showed that transpiration by

6804-399: The young sporophyte . A mature sporophyte may grow to a height of 30 cm (10 in) or more but has no apparent leaves. The stem has a core of thick-walled protostele in its centre surrounded by an endodermis which regulates the flow of water and nutrients. The surface of the stem is covered with stomata which allow gas exchange with the surroundings. The gametophyte of Psilotum

6888-844: Was a consensus of a number of studies, and was further refined. The phylogenetic relationships are shown in the following cladogram (to the level of orders). This division into four major clades was then confirmed using morphology alone. Lycopodiophytes (club mosses, spike mosses, quillworts) Spermatophytes (seed plants) Equisetales (horsetails) [REDACTED] Ophioglossales (grapeferns etc.) Psilotales (whisk ferns) [REDACTED] Marattiales [REDACTED] Osmundales [REDACTED] Hymenophyllales (filmy ferns) [REDACTED] Gleicheniales [REDACTED] Schizaeales Salviniales (heterosporous) Cyatheales (tree ferns) [REDACTED] Polypodiales [REDACTED] Subsequently, Chase and Reveal considered both lycopods and ferns as subclasses of

6972-468: Was correct, because some workers were unable to demonstrate negative pressures. More recent measurements do tend to validate the classic theory, for the most part. Xylem transport is driven by a combination of transpirational pull from above and root pressure from below, which makes the interpretation of measurements more complicated. Xylem appeared early in the history of terrestrial plant life. Fossil plants with anatomically preserved xylem are known from

7056-412: Was lost per unit of CO 2 uptake. However, even in these "easy" early days, water was at a premium, and had to be transported to parts of the plant from the wet soil to avoid desiccation . This early water transport took advantage of the cohesion-tension mechanism inherent in water. Water has a tendency to diffuse to areas that are drier, and this process is accelerated when water can be wicked along

#531468