Sargassum is a genus of brown macroalgae in the order Fucales. Numerous species are distributed throughout the temperate and tropical oceans of the world, where they inhabit shallow water and coral reefs, the genus is known for its planktonic species. Most species within the class Phaeophyceae are predominantly cold-water organisms that benefit from nutrients upwelling, but the genus Sargassum appears to be an exception. Any number of the benthic species may take on a planktonic pelagic existence after being removed from reefs during rough weather; the Atlantic Ocean's Sargasso Sea was named after the algae, as it hosts a large amount of Sargassum. Sargassum was named by the Portuguese sailors who found it in the Sargasso Sea after the wooly rock rose that grew in their water wells at home, and, called sargaço in Portuguese; the Florida Keys and its smaller islands are well known for their high levels of Sargassum covering their shores. Gulfweed was observed by Columbus. Although it was thought to cover the entirety of the Sargasso Sea, making navigation impossible, it has since been found to occur only in drifts.
Sargassum species ares cultivated and cleaned for use as an herbal remedy. Many Chinese herbalists prescribe powdered Sargassum—either the species S. pallidum, or more hijiki, S. fusiforme—in doses of 0.5 gram dissolved in warm water and drunk as a tea. It is called 海藻. Species of this genus of algae may grow to a length of several metres, they are brown or dark green in color and consist of a holdfast, a stipe, a frond. Oogonia and antheridia occur in conceptacles embedded in receptacles on special branches; some species have berrylike gas-filled bladders that help the fronds float to promote photosynthesis. Many have a rough, sticky texture that, along with a robust but flexible body, helps it withstand strong water currents. Large, pelagic mats of Sargassum in the Sargasso Sea act as one of the only habitats available for ecosystem development; the Sargassum patches act as a refuge for many species in different parts of their development, but as a permanent residence for endemic species that can only be found living on and within the Sargassum.
These endemic organisms have specialized patterns and colorations that mimic the Sargassum and allow them to be impressively camouflaged in their environment. In total, these Sargassum mats are home to over 100 different species. There is a total of 81 fish species that reside in the Sargassum or utilize it for parts of their life cycles. Other marine organisms, such as young sea turtles, will use the Sargassum as shelter and a resource for food until they reach a size at which they can survive elsewhere; this community is being affected by humans due to overfishing and other types of pollution, boat traffic, which could lead to the demise of this diverse and unique habitat. Below is a list of organisms. Organisms found in the pelagic Sargassum patches, Arthropods Amphipods Skeleton shrimp Crabs Copepods Shrimp Sea Spiders Worms Annelid worms Flatworms Mollusks Nudibranchs Snails Squid Fish Sargassum fish Porcupinefish Triplefin Plainhead filefish Others Sea turtlesSargassum is found in the beach drift near Sargassum beds, where they are known as gulfweed, a term that can mean all seaweed species washed up on shore.
Sargassum species are found throughout tropical areas of the world and are the most obvious macrophyte in near-shore areas where Sargassum beds occur near coral reefs. The plants grow subtidally and attach to coral, rocks, or shells in moderately exposed or sheltered rocky or pebble areas; these tropical populations undergo seasonal cycles of growth and decay in concert with seasonal changes in sea temperature. In tropical Sargassum species that are preferentially consumed by herbivorous fishes and echinoids, a low level of phenolics and tannins occurs. Sargassum muticum is a large brown seaweed of the class Phaeophyceae, it grows attached to rocks by a perennial holdfast up to 5 cm in diameter. From this holdfast the main axis grows to a maximum of 5 cm high; the leaf-like laminae and primary lateral branches grow from this stipe. In warm waters, it can grow to 12 m long, however in British waters it gives rise to a single main axis with secondary and tertiary branches that the plant sheds annually.
Numerous small 2–6 mm stalked air vesicles provide buoyancy. The reproductive receptacles are stalked, develop in the axils of leafy laminae, it is self-fertile. The Gulf has the second largest concentration of sargassum of any body of water in the world. A fair amount of it washes out through the Straits of Florida in the Gulf Stream and ends up in the Sargasso Sea in the Atlantic Ocean off the East Coast of the United States. We rounded Hatteras in fair weather, saw the line between the brilliant blue Gulf Stream full of gulf weed and the muddy grayish shore water as defined as that between the sidewalk and the roadway in a street. In summer 2015, large quantities of different species of Sargassum accumulated along the shores of many of the countries on the Caribbean Sea; some of the affected islands and regions include the Caribbean coast of Mexico, the Dominican Republic and Tobago. Another large outbreak
Cellular differentiation is the process where a cell changes from one cell type to another. The cell changes to a more specialized type. Differentiation occurs numerous times during the development of a multicellular organism as it changes from a simple zygote to a complex system of tissues and cell types. Differentiation continues in adulthood as adult stem cells divide and create differentiated daughter cells during tissue repair and during normal cell turnover; some differentiation occurs in response to antigen exposure. Differentiation changes a cell's size, membrane potential, metabolic activity, responsiveness to signals; these changes are due to controlled modifications in gene expression and are the study of epigenetics. With a few exceptions, cellular differentiation never involves a change in the DNA sequence itself. Thus, different cells can have different physical characteristics despite having the same genome. A specialized type of differentiation, known as'terminal differentiation', is of importance in some tissues, for example vertebrate nervous system, striated muscle and gut.
During terminal differentiation, a precursor cell capable of cell division, permanently leaves the cell cycle, dismantles the cell cycle machinery and expresses a range of genes characteristic of the cell's final function. Differentiation may continue to occur after terminal differentiation if the capacity and functions of the cell undergo further changes. Among dividing cells, there are multiple levels of cell potency, the cell's ability to differentiate into other cell types. A greater potency indicates a larger number of cell types. A cell that can differentiate into all cell types, including the placental tissue, is known as totipotent. In mammals, only the zygote and subsequent blastomeres are totipotent, while in plants, many differentiated cells can become totipotent with simple laboratory techniques. A cell that can differentiate into all cell types of the adult organism is known as pluripotent; such cells are called meristematic cells in higher plants and embryonic stem cells in animals, though some groups report the presence of adult pluripotent cells.
Virally induced expression of four transcription factors Oct4, Sox2, c-Myc, KIF4 is sufficient to create pluripotent cells from adult fibroblasts. A multipotent cell is one that can differentiate into multiple different, but related cell types. Oligopotent cells are more restricted than multipotent, but can still differentiate into a few related cell types. Unipotent cells can differentiate into only one cell type, but are capable of self-renewal. In cytopathology, the level of cellular differentiation is used as a measure of cancer progression. "Grade" is a marker of. Three basic categories of cells make up the mammalian body: germ cells, somatic cells, stem cells; each of the 37.2 trillion cells in an adult human has its own copy or copies of the genome except certain cell types, such as red blood cells, that lack nuclei in their differentiated state. Most cells are diploid; such cells, called somatic cells, make up most such as skin and muscle cells. Cells differentiate to specialize for different functions.
Germ line cells are any line of cells that give rise to gametes—eggs and sperm—and thus are continuous through the generations. Stem cells, on the other hand, have the ability to divide for indefinite periods and to give rise to specialized cells, they are best described in the context of normal human development. Development begins when a sperm fertilizes an egg and creates a single cell that has the potential to form an entire organism. In the first hours after fertilization, this cell divides into identical cells. In humans four days after fertilization and after several cycles of cell division, these cells begin to specialize, forming a hollow sphere of cells, called a blastocyst; the blastocyst has an outer layer of cells, inside this hollow sphere, there is a cluster of cells called the inner cell mass. The cells of the inner cell mass go on to form all of the tissues of the human body. Although the cells of the inner cell mass can form every type of cell found in the human body, they cannot form an organism.
These cells are referred to as pluripotent. Pluripotent stem cells undergo further specialization into multipotent progenitor cells that give rise to functional cells. Examples of stem and progenitor cells include: Radial glial cells that give rise to excitatory neurons in the fetal brain through the process of neurogenesis. Hematopoietic stem cells from the bone marrow that give rise to red blood cells, white blood cells, platelets Mesenchymal stem cells from the bone marrow that give rise to stromal cells, fat cells, types of bone cells Epithelial stem cells that give rise to the various types of skin cells Muscle satellite cells that contribute to differentiated muscle tissue. A pathway, guided by the cell adhesion molecules consisting of four amino acids, glycine and serine, is created as the cellular blastomere differentiates from the single-layered blastula to the three primary layers of germ cells in mammals, namely the ectoderm and endoderm; the ectoderm ends up forming the skin and the nervous system, the mesoderm forms the bones and muscular tissue, the endoderm forms the internal organ tissues.
A lichen is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship. The combined lichen has properties different from those of its component organisms. Lichens come in many colors and forms; the properties are sometimes plant-like, but lichens are not plants. Lichens may have tiny, leafless branches, flat leaf-like structures, flakes that lie on the surface like peeling paint, a powder-like appearance, or other growth forms. A macrolichen is a lichen, either bush-like or leafy. Here, "macro" and "micro" do not refer to size, but to the growth form. Common names for lichens may contain the word moss, lichens may superficially look like and grow with mosses, but lichens are not related to mosses or any plant. Lichens do not have roots that absorb water and nutrients as plants do, but like plants, they produce their own nutrition by photosynthesis; when they grow on plants, they do not live as parasites, but instead use the plants as a substrate.
Lichens occur from sea level to high alpine elevations, in many environmental conditions, can grow on any surface. Lichens are abundant growing on bark, mosses, on other lichens, hanging from branches "living on thin air" in rain forests and in temperate woodland, they grow on rock, gravestones, exposed soil surfaces, in the soil as part of a biological soil crust. Different kinds of lichens have adapted to survive in some of the most extreme environments on Earth: arctic tundra, hot dry deserts, rocky coasts, toxic slag heaps, they can live inside solid rock, growing between the grains. It is estimated that 6% of Earth's land surface is covered by lichens. There are about 20,000 known species of lichens; some lichens have lost the ability to reproduce sexually, yet continue to speciate. Lichens can be seen as being self-contained miniature ecosystems, where the fungi, algae, or cyanobacteria have the potential to engage with other microorganisms in a functioning system that may evolve as an more complex composite organism.
Lichens may be long-lived, with some considered to be among the oldest living things. They are among the first living things to grow on fresh rock exposed after an event such as a landslide; the long life-span and slow and regular growth rate of some lichens can be used to date events. In American English, "lichen" is pronounced the same as the verb "liken". In British English, both this pronunciation and one rhyming with "kitchen" ) are used. English lichen derives from Greek λειχήν leichēn via Latin lichen; the Greek noun, which means "licker", derives from the verb λείχειν leichein, "to lick". Lichens grow in a wide range of shapes and forms; the shape of a lichen is determined by the organization of the fungal filaments. The nonreproductive tissues, or vegetative body parts, are called the thallus. Lichens are grouped by thallus type, since the thallus is the most visually prominent part of the lichen. Thallus growth forms correspond to a few basic internal structure types. Common names for lichens come from a growth form or color, typical of a lichen genus.
Common groupings of lichen thallus growth forms are: fruticose – growing like a tuft or multiple-branched leafless mini-shrub, upright or hanging down, 3-dimensional branches with nearly round cross section or flattened foliose – growing in 2-dimensional, leaf-like lobes crustose – crust-like, adhering to a surface like a thick coat of paint squamulose – formed of small leaf-like scales crustose below but free at the tips leprose – powdery gelatinous – jelly like filamentous – stringy or like matted hair byssoid – wispy, like teased wool structurelessThere are variations in growth types in a single lichen species, grey areas between the growth type descriptions, overlapping between growth types, so some authors might describe lichens using different growth type descriptions. When a crustose lichen gets old, the center may start to crack up like old-dried paint, old-broken asphalt paving, or like the polygonal "islands" of cracked-up mud in a dried lakebed; this is called being rimose or areolate, the "island" pieces separated by the cracks are called areolas.
The areolas appear separated, but are connected by an underlying "prothallus" or "hypothallus". When a crustose lichen grows from a center and appears to radiate out, it is called crustose placodioid; when the edges of the areolas lift up from the substrate, it is called squamulose. These growth form groups are not defined. Foliose lichens may sometimes branch and appear to be fruticose. Fruticose lichens may have flattened branching parts and appear leafy. Squamulose lichens may appear where the edges lift up. Gelatinous lichens may appear leafy when dry. Means of telling them apart in these cases are in the sections below. Structures involved in reproduction appear as discs, bumps, or squiggly lines on the surface of the thallus; the thallus is not always the part of the lichen, most visually noticeable. Some lichens can grow inside solid rock between the grains, with only the sexual fruiting part visible growing outside the rock; these may be dramatic in color or appearance. Forms of these sexual parts are not in the above growth form categories.
The most visually noticeable reproductive parts are circular, plate-like or disc-like outgrowths, with crinkly edges, are described in sections below. Lichens come in many colors. Coloratio
In biology, tissue is a cellular organizational level between cells and a complete organ. A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. Organs are formed by the functional grouping together of multiple tissues; the English word "tissue" is derived from the French "tissu", meaning something, "woven", from the verb tisser, "to weave". The study of human and animal tissues is known as histology or, in connection with disease, histopathology. For plants, the discipline is called plant anatomy; the classical tools for studying tissues are the paraffin block in which tissue is embedded and sectioned, the histological stain, the optical microscope. In the last couple of decades, developments in electron microscopy, immunofluorescence, the use of frozen tissue sections have enhanced the detail that can be observed in tissues. With these tools, the classical appearances of tissues can be examined in health and disease, enabling considerable refinement of medical diagnosis and prognosis.
Animal tissues are grouped into four basic types: connective, muscle and epithelial. Collections of tissues joined in structural units to serve a common function compose organs. While all eumetazoan animals can be considered to contain the four tissue types, the manifestation of these tissues can differ depending on the type of organism. For example, the origin of the cells comprising a particular tissue type may differ developmentally for different classifications of animals; the epithelium in all birds and animals is derived from the ectoderm and endoderm, with a small contribution from the mesoderm, forming the endothelium, a specialized type of epithelium that composes the vasculature. By contrast, a true epithelial tissue is present only in a single layer of cells held together via occluding junctions called tight junctions, to create a selectively permeable barrier; this tissue covers all organismal surfaces that come in contact with the external environment such as the skin, the airways, the digestive tract.
It serves functions of protection and absorption, is separated from other tissues below by a basal lamina. Connective tissues are fibrous tissues, they are made up of cells separated by non-living material, called an extracellular matrix. This matrix can be rigid. For example, blood contains plasma as its matrix and bone's matrix is rigid. Connective tissue holds them in place. Blood, tendon, ligament and areolar tissues are examples of connective tissues. One method of classifying connective tissues is to divide them into three types: fibrous connective tissue, skeletal connective tissue, fluid connective tissue. Muscle cells form the active contractile tissue of the body known as muscle tissue or muscular tissue. Muscle tissue functions to produce force and cause motion, either locomotion or movement within internal organs. Muscle tissue is separated into three distinct categories: visceral or smooth muscle, found in the inner linings of organs. Cells comprising the central nervous system and peripheral nervous system are classified as nervous tissue.
In the central nervous system, neural tissues form spinal cord. In the peripheral nervous system, neural tissues form the cranial nerves and spinal nerves, inclusive of the motor neurons; the epithelial tissues are formed by cells that cover the organ surfaces, such as the surface of skin, the airways, the reproductive tract, the inner lining of the digestive tract. The cells comprising an epithelial layer are linked via tight junctions. In addition to this protective function, epithelial tissue may be specialized to function in secretion and absorption. Epithelial tissue helps to protect organs from microorganisms and fluid loss. Functions of epithelial tissue: The cells of the body's surface form the outer layer of skin. Inside the body, epithelial cells form the lining of the mouth and alimentary canal and protect these organs. Epithelial tissues help in absorption of water and nutrients. Epithelial tissues help in the elimination of waste. Epithelial tissues hormones in the form of glands; some epithelial tissue perform secretory functions.
They secrete a variety of substances such as sweat, enzymes, etc. There are many kinds of epithelium, nomenclature is somewhat variable. Most classification schemes combine a description of the cell-shape in the upper layer of the epithelium with a word denoting the number of layers: either simple or stratified. However, other cellular features, such as cilia may be described in the classification system; some common kinds of epithelium are listed below: Simple squamous epithelium Stratified squamous epithelium Simple cuboidal epithelium Transitional epithelium Pseudostratified columnar epithelium Columnar epithelium Glandular epithelium Ciliated columnar epithelium In plant anatomy, tissues are categorized broadly into three tissue systems: the epidermis, the ground tissue, the vascular tissue. Epidermis - Cells forming the outer surface of the leaves and of the young plant body. Vascular tissue - The primary components of vascular tissue are the xylem and phloem; these transport nutrients internally.
Ground tissue - Ground tissue is less differentiated than other tissues. Ground tis
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
Vascular plants known as tracheophytes, form a large group of plants that are defined as those land plants that have lignified tissues for conducting water and minerals throughout the plant. They have a specialized non-lignified tissue to conduct products of photosynthesis. Vascular plants include the clubmosses, ferns and angiosperms. Scientific names for the group include Tracheophyta and Equisetopsida sensu lato; the term higher plants should be avoided as a synonym for vascular plants as it is a remnant of the abandoned concept of the great chain of being. Vascular plants are defined by three primary characteristics: Vascular plants have vascular tissues which distribute resources through the plant; this feature allows vascular plants to evolve to a larger size than non-vascular plants, which lack these specialized conducting tissues and are thereby restricted to small sizes. In vascular plants, the principal generation phase is the sporophyte, which produce spores and is diploid. By contrast, the principal generation phase in non-vascular plants is the gametophyte, which produces gametes and is haploid.
They have true roots and stems if one or more of these traits are secondarily lost in some groups. The formal definition of the division Tracheophyta encompasses both these characteristics in the Latin phrase "facies diploida xylem et phloem instructa". One possible mechanism for the presumed switch from emphasis on the haploid generation to emphasis on the diploid generation is the greater efficiency in spore dispersal with more complex diploid structures. In other words, elaboration of the spore stalk enabled the production of more spores, enabled the development of the ability to release them higher and to broadcast them farther; such developments may include more photosynthetic area for the spore-bearing structure, the ability to grow independent roots, woody structure for support, more branching. A proposed phylogeny of the vascular plants after Kenrick and Crane is as follows, with modification to the gymnosperms from Christenhusz et al. Pteridophyta from Smith et al. and lycophytes and ferns by Christenhusz et al.
This phylogeny is supported by several molecular studies. Other researchers state that taking fossils into account leads to different conclusions, for example that the ferns are not monophyletic. Water and nutrients in the form of inorganic solutes are drawn up from the soil by the roots and transported throughout the plant by the xylem. Organic compounds such as sucrose produced by photosynthesis in leaves are distributed by the phloem sieve tube elements; the xylem consists of vessels in flowering plants and tracheids in other vascular plants, which are dead hard-walled hollow cells arranged to form files of tubes that function in water transport. A tracheid cell wall contains the polymer lignin; the phloem however consists of living cells called sieve-tube members. Between the sieve-tube members are sieve plates, which have pores to allow molecules to pass through. Sieve-tube members lack such organs as nuclei or ribosomes, but cells next to them, the companion cells, function to keep the sieve-tube members alive.
The most abundant compound in all plants, as in all cellular organisms, is water which serves an important structural role and a vital role in plant metabolism. Transpiration is the main process of water movement within plant tissues. Water is transpired from the plant through its stomata to the atmosphere and replaced by soil water taken up by the roots; the movement of water out of the leaf stomata creates a transpiration pull or tension in the water column in the xylem vessels or tracheids. The pull is the result of water surface tension within the cell walls of the mesophyll cells, from the surfaces of which evaporation takes place when the stomata are open. Hydrogen bonds exist between water molecules; the draw of water upwards may be passive and can be assisted by the movement of water into the roots via osmosis. Transpiration requires little energy to be used by the plant. Transpiration assists the plant in absorbing nutrients from the soil as soluble salts. Living root cells passively absorb water in the absence of transpiration pull via osmosis creating root pressure.
It is possible for there to be no evapotranspiration and therefore no pull of water towards the shoots and leaves. This is due to high temperatures, high humidity, darkness or drought. Xylem and phloem tissues are involved in the conduction processes within plants. Sugars are conducted throughout the plant in the phloem and other nutrients through the xylem. Conduction occurs from a source to a sink for each separate nutrient. Sugars are produced in the leaves by photosynthesis and transported to the growing shoots and roots for use in growth, cellular respiration or storage. Minerals are transported to the shoots to allow cell division and growth. Fern allies Non-vascular plant “Higher plants” or “vascular plants”
A gametophyte is one of the two alternating phases in the life cycle of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes; the gametophyte is the sexual phase in the life cycle of plants and algae. It develops sex organs that produce gametes, haploid sex cells that participate in fertilization to form a diploid zygote which has a double set of chromosomes. Cell division of the zygote results in a new diploid multicellular organism, the second stage in the life cycle known as the sporophyte; the sporophyte can produce haploid spores by meiosis. In some multicellular green algae, red algae and brown algae and gametophytes may be externally indistinguishable. In Ulva the gametes are isogamous, all of one size and general morphology. In land plants, anisogamy is universal; as in animals and male gametes are called eggs and sperm. In extant land plants, either the sporophyte or the gametophyte may be reduced. In bryophytes, the gametophyte is the most visible stage of the life cycle.
The bryophyte gametophyte is longer lived, nutritionally independent, the sporophytes are attached to the gametophytes and dependent on them. When a moss spore germinates it grows to produce a filament of cells; the mature gametophyte of mosses develops into leafy shoots that produce sex organs that produce gametes. Eggs develop in sperm in antheridia. In some bryophyte groups such as many liverworts of the order Marchantiales, the gametes are produced on specialized structures called gametophores. All vascular plants are sporophyte dominant, a trend toward smaller and more sporophyte-dependent female gametophytes is evident as land plants evolved towards reproduction by seeds. Vascular plants such as ferns that produce only one type of spore are said to be homosporous, they have exosporic gametophytes—that is, the gametophyte is free-living and develops outside of the spore wall. Exosporic gametophytes can either be bisexual, capable of producing both sperm and eggs in the same thallus, or specialized into separate male and female organisms.
In heterosporous vascular plants, the gametophyte develops endosporically, within the spore wall. These gametophytes are dioicous, producing eggs but not both. In most ferns, for example, in the leptosporangiate fern Dryopteris, the gametophyte is a photosynthetic free living autotrophic organism called a prothallus that produces gametes and maintains the sporophyte during its early multicellular development. However, in some groups, notably the clade that includes Ophioglossaceae and Psilotaceae, the gametophytes are subterranean and subsist by forming mycotrophic relationships with fungi. Extant lycophytes produce two different types of gametophytes. In the families Lycopodiaceae and Huperziaceae, spores germinate into free-living and mycotrophic gametophytes that derive nutrients from symbiosis with fungi. In Isoetes and Selaginella, which are heterosporous, the megaspore remains attached to the parent sporophyte and a reduced megagametophyte develops inside. At maturity, the megaspore cracks open at the trilete suture to allow the male gametes to access the egg cells in the archegonia inside.
The gametophytes of Isoetes appear to be similar in this respect to those of the extinct Carboniferous giant arborescent clubmosses and Lepidostrobus. The seed plants are heterosporous; the gametophytes develop into multicellular organisms while still enclosed within the spore wall, the megaspores are retained within the sporangium. In plants with heteromorphic gametophytes, there are two distinct kinds of gametophytes; because the two gametophytes differ in form and function, they are termed heteromorphic, from hetero- "different" and morph "form". The egg producing gametophyte is known as a megagametophyte, because it is larger, the sperm producing gametophyte is known as a microgametophyte. Gametophytes which produce egg and sperm on separate plants are termed dioicous. In heterosporous plants, there are two distinct sporangia, each of which produces a single kind of spore and single kind of gametophyte. However, not all heteromorphic gametophytes come from heterosporous plants; that is, some plants have distinct egg-producing and sperm-producing gametophytes, but these gametophytes develop from the same kind of spore inside the same sporangium.
In the seed plants, the microgametophyte is called pollen. Seed plant microgametophytes consists of two or three cells when the pollen grains exit the sporangium; the megagametophyte develops within the megaspore of extant seedless vascular plants and within the megasporangium in a cone or flower in seed plants. In seed plants, the microgametophyte travels to the vicinity of the egg cell, produces two sperm by mitosis. In gymnosperms the megagametophyte consists of several thousand cells and produces one to several archegonia, each with a single egg cell; the gametophyte becomes a food storage tissue in the seed. In angiosperms, the megagametophyte is reduced to only a few nuclei and cells, is sometimes called the embryo sac. A typical embryo sac contains seven cells and eight nuclei, one of, the egg cell. Two nuclei fuse with a sperm nucleus to form the endosperm, which becomes the food storage tissue in the seed. Sporophyte Alternation of generations Arc