In botany, a stoma called a stomate, is a pore, found in the epidermis of leaves and other organs, that facilitates gas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the stomatal opening; the term is used collectively to refer to the entire stomatal complex, consisting of the paired guard cells and the pore itself, referred to as the stomatal aperture. Air enters the plant through these openings by gaseous diffusion, contains carbon dioxide and oxygen, which are used in photosynthesis and respiration, respectively. Oxygen produced as a by-product of photosynthesis diffuses out to the atmosphere through these same openings. Water vapor diffuses through the stomata into the atmosphere in a process called transpiration. Stomata are present in the sporophyte generation of all land plant groups except liverworts. In vascular plants the number and distribution of stomata varies widely. Dicotyledons have more stomata on the lower surface of the leaves than the upper surface.
Monocotyledons such as onion and maize may have about the same number of stomata on both leaf surfaces. In plants with floating leaves, stomata may be found only on the upper epidermis and submerged leaves may lack stomata entirely. Most tree species have stomata only on the lower leaf surface. Leaves with stomata on both the upper and lower leaf are called. Size varies across species, with end-to-end lengths ranging from 10 to 80 µm and width ranging from a few to 50 µm. Carbon dioxide, a key reactant in photosynthesis, is present in the atmosphere at a concentration of about 400 ppm. Most plants require the stomata to be open during daytime; the air spaces in the leaf are saturated with water vapour, which exits the leaf through the stomata. Therefore, plants cannot gain carbon dioxide without losing water vapour. Ordinarily, carbon dioxide is fixed to ribulose-1,5-bisphosphate by the enzyme RuBisCO in mesophyll cells exposed directly to the air spaces inside the leaf; this exacerbates the transpiration problem for two reasons: first, RuBisCo has a low affinity for carbon dioxide, second, it fixes oxygen to RuBP, wasting energy and carbon in a process called photorespiration.
For both of these reasons, RuBisCo needs high carbon dioxide concentrations, which means wide stomatal apertures and, as a consequence, high water loss. Narrower stomatal apertures can be used in conjunction with an intermediary molecule with a high carbon dioxide affinity, PEPcase. Retrieving the products of carbon fixation from PEPCase is an energy-intensive process, however; as a result, the PEPCase alternative is preferable only where water is limiting but light is plentiful, or where high temperatures increase the solubility of oxygen relative to that of carbon dioxide, magnifying RuBisCo's oxygenation problem. A group of desert plants called "CAM" plants open their stomata at night, use PEPcarboxylase to fix carbon dioxide and store the products in large vacuoles; the following day, they close their stomata and release the carbon dioxide fixed the previous night into the presence of RuBisCO. This saturates RuBisCO with carbon dioxide; this approach, however, is limited by the capacity to store fixed carbon in the vacuoles, so it is preferable only when water is limited.
However, most plants do not have the aforementioned facility and must therefore open and close their stomata during the daytime, in response to changing conditions, such as light intensity and carbon dioxide concentration. It is not certain how these responses work. However, the basic mechanism involves regulation of osmotic pressure; when conditions are conducive to stomatal opening, a proton pump drives protons from the guard cells. This means that the cells' electrical potential becomes negative; the negative potential opens potassium voltage-gated channels and so an uptake of potassium ions occurs. To maintain this internal negative voltage so that entry of potassium ions does not stop, negative ions balance the influx of potassium. In some cases, chloride ions enter, while in other plants the organic ion malate is produced in guard cells; this increase in solute concentration lowers the water potential inside the cell, which results in the diffusion of water into the cell through osmosis.
This increases the cell's turgor pressure. Because of rings of cellulose microfibrils that prevent the width of the guard cells from swelling, thus only allow the extra turgor pressure to elongate the guard cells, whose ends are held in place by surrounding epidermal cells, the two guard cells lengthen by bowing apart from one another, creating an open pore through which gas can move; when the roots begin to sense a water shortage in the soil, abscisic acid is released. ABA binds to receptor proteins in the guard cells' plasma membrane and cytosol, which first raises the pH of the cytosol of the cells and cause the concentration of free Ca2+ to increase in the cytosol due to influx from outside the cell and release of Ca2+ from internal stores such as the endoplasmic reticulum and vacuoles; this caus
Natural Resources Conservation Service
Natural Resources Conservation Service known as the Soil Conservation Service, is an agency of the United States Department of Agriculture that provides technical assistance to farmers and other private landowners and managers. Its name was changed in 1994 during the presidency of Bill Clinton to reflect its broader mission, it is a small agency comprising about 12,000 employees. Its mission is to improve and conserve natural resources on private lands through a cooperative partnership with state and local agencies. While its primary focus has been agricultural lands, it has made many technical contributions to soil surveying and water quality improvement. One example is the Conservation Effects Assessment Project, set up to quantify the benefits of agricultural conservation efforts promoted and supported by programs in the Farm Security and Rural Investment Act of 2002. NRCS is the leading agency in this project; the agency was founded through the efforts of Hugh Hammond Bennett, a soil conservation pioneer who worked for the Department of Agriculture from 1903 to 1952.
Bennett's motivation was based on his knowledge of the detrimental effects of soil erosion and the impacts on U. S lands. On September 13, 1933, the Soil Erosion Service was formed in the Department of the Interior, with Bennett as chief; the service was transferred to the Department of Agriculture on March 23, 1935, was shortly thereafter combined with other USDA units to form the Soil Conservation Service by the Soil Conservation and Domestic Allotment Act of 1935. The Soil Conservation Service was in charge of 500 Civilian Conservation Corps camps between 1933 and 1942; the primary purpose of these camps was erosion control. Hugh Bennett continued as chief, a position he held until his retirement in 1952. On October 20, 1994, the agency was renamed to the Natural Resources Conservation Service as part of the Federal Crop Insurance Reform and Department of Agriculture Reorganization Act of 1994. NRCS offers financial assistance to farmers and ranchers; the financial assistance is authorized by the Farm Bill, a law, renewed every five years.
The 2014 Farm Bill consolidated 23 programs into 15. NRCS offers these services to private land owners, conservation districts and other types of organizations. NRCS collects and shares information on the nation's soil, water and plants; the Conservation Title of the Farm Bill provides the funding to agricultural producers, a conservation plan must be included. All of these programs are voluntary; the main programs include: The purpose of EQIP is to provide assistance to landowners to help them improve their soil and related natural resources, including grazing lands and wildlife habitat. Conservation Stewardship Program CSP is targeted to a producers who maintain a higher level of environmental stewardship. Regional Conservation Partnership Program RCPP consolidated four programs from the prior 2008 Farm Bill, it aims at more watershed scale projects, rather than individual farms and ranches. Agricultural Conservation Easement Program ACEP was another consolidation effort of the 2014 Farm Bill, which includes the former Grasslands Reserve Program and Ranch Lands Protection Program, Wetlands Reserve Program.
ACEP includes technical and financial help to maintain or improve land for agriculture or environmental benefits. Landowners volunteer to protect forests in 30 or 10 year contracts; this program hands assisting funds to participants. The objectives of HFRP are to: Promote the recovery of endangered and threatened species under the Endangered Species Act Improve plant and animal biodiversity Enhance carbon sequestration Serves 10 states in the Midwest United States in helping to reduce Nitrate levels in soil due to runoff from fertilized farmland; the project began in 2010 and focused on the Mississippi Basin area. The main goal of the project is to implement better methods of managing water drainage from agricultural uses, in place of letting the water drain as it had done in the past. In October 2011, The National "Managing Water, Harvesting Results" Summit was held to promote the drainage techniques used in hopes of people adopting them nationwide. Includes water supply forecasts and the Surface Water Supply Index for Alaska and other Western states.
NRCS agents collect data from snowpack and mountain sites to predict spring runoff and summer streamflow amounts. These predictions are used in decision making for agriculture, wildlife management and development, several other areas; these predictions are available within the first 5 days of each month from January to June. Is a blanket program which involves conservation efforts on soil and water conservation, as well as management of agricultural wastes and general longterm sustainability. NRCS and related agencies work with landowners, communities, or developers to protect the environment. Serve to guide people to comply with acts such as the Highly Erodible Land and Conservation Compliance Provisions acts; the CTA can cover projects by state and federal governments. Is a program to assist gulf bordering states improve water quality and use sustainable methods of farming and other industry; the program will deliver up to 50 million dollars over 2011-2013 to apply these sustainable methods, as well as wildlife habitat management systems that do not hinder agricultural productivity, prevent future over use of water resources to protect native endangered spe
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
A sporophyte is the diploid multicellular stage in the life cycle of a plant or alga. It develops from the zygote produced when a haploid egg cell is fertilized by a haploid sperm and each sporophyte cell therefore has a double set of chromosomes, one set from each parent. All land plants, most multicellular algae, have life cycles in which a multicellular diploid sporophyte phase alternates with a multicellular haploid gametophyte phase. In the seed plants, flowering plants, the sporophyte phase is more prominent than the gametophyte, is the familiar green plant with its roots, stem and cones or flowers. In flowering plants the gametophytes are reduced in size, are represented by the germinated pollen and the embryo sac; the sporophyte produces spores by meiosis, a process known as "reduction division" that reduces the number of chromosomes in each spore mother cell by half. The resulting meiospores develop into a gametophyte. Both the spores and the resulting gametophyte are haploid, meaning they only have one set of chromosomes.
The mature gametophyte produces female gametes by mitosis. The fusion of male and female gametes produces a diploid zygote which develops into a new sporophyte; this cycle is known as alternation of alternation of phases. Bryophytes have a dominant gametophyte phase on which the adult sporophyte is dependent for nutrition; the embryo sporophyte develops by cell division of the zygote within the female sex organ or archegonium, in its early development is therefore nurtured by the gametophyte. Because this embryo-nurturing feature of the life cycle is common to all land plants they are known collectively as the embryophytes. Most algae have dominant gametophyte generations, but in some species the gametophytes and sporophytes are morphologically similar. An independent sporophyte is the dominant form in all clubmosses, ferns and angiosperms that have survived to the present day. Early land plants had sporophytes that produced identical spores but the ancestors of the gymnosperms evolved complex heterosporous life cycles in which the spores producing male and female gametophytes were of different sizes, the female megaspores tending to be larger, fewer in number, than the male microspores.
During the Devonian period several plant groups independently evolved heterospory and subsequently the habit of endospory, in which the gametophytes develop in miniaturized form inside the spore wall. By contrast in exosporous plants, including modern ferns, the gametophytes break the spore wall open on germination and develop outside it; the megagametophytes of endosporic plants such as the seed ferns developed within the sporangia of the parent sporophyte, producing a miniature multicellular female gametophyte complete with female sex organs, or archegonia. The oocytes were fertilized in the archegonia by free-swimming flagellate sperm produced by windborne miniaturized male gametophytes in the form of pre-pollen; the resulting zygote developed into the next sporophyte generation while still retained within the pre-ovule, the single large female meiospore or megaspore contained in the modified sporangium or nucellus of the parent sporophyte. The evolution of heterospory and endospory were among the earliest steps in the evolution of seeds of the kind produced by gymnosperms and angiosperms today.
P. Kenrick & P. R. Crane The origin and early evolution of plants on land. Nature 389, 33-39. T. N. Taylor, H. Kerp and H. Hass Life history biology of early land plants: Deciphering the gametophyte phase. Proceedings of the National Academy of Sciences 102, 5892-5897. P. R. Bell & A. R. Helmsley Green plants, their Origin and Diversity. Cambridge University Press ISBN 0-521-64673-1
Thallus, from Latinized Greek θαλλός, meaning "a green shoot" or "twig", is the undifferentiated vegetative tissue of some organisms in diverse groups such as algae, some liverworts and the Myxogastria. Many of these organisms were known as the thallophytes, a polyphyletic group of distantly related organisms. An organism or structure resembling a thallus is called thalloid, thalliform, thalline, or thallose. A thallus names the entire body of a multicellular non-moving organism in which there is no organization of the tissues into organs. Though thalli do not have organized and distinct parts as do the vascular plants, they may have analogous structures that resemble their vascular "equivalents"; the analogous structures have similar function or macroscopic structure, but different microscopic structure. In exceptional cases such as the Lemnoideae, where the structure of a vascular plant is in fact thallus-like, it is referred to as having a thalloid structure, or sometimes as a thalloid. Although a thallus is undifferentiated in terms of its anatomy, there can be visible differences and functional differences.
A kelp, for example, may have its thallus divided into three regions. The parts of a kelp thallus include the holdfast and the blades; the thallus of a fungus is called a mycelium. The term thallus is commonly used to refer to the vegetative body of a lichen. In seaweed, thallus is sometimes called'frond'; the gametophyte of some non-thallophyte plants – clubmosses and ferns is termed "prothallus". Homothallism Heterothallism
Plants are multicellular, predominantly photosynthetic eukaryotes of the kingdom Plantae. Plants were treated as one of two kingdoms including all living things that were not animals, all algae and fungi were treated as plants. However, all current definitions of Plantae exclude the fungi and some algae, as well as the prokaryotes. By one definition, plants form the clade Viridiplantae, a group that includes the flowering plants and other gymnosperms and their allies, liverworts and the green algae, but excludes the red and brown algae. Green plants obtain most of their energy from sunlight via photosynthesis by primary chloroplasts that are derived from endosymbiosis with cyanobacteria, their chloroplasts contain b, which gives them their green color. Some plants are parasitic or mycotrophic and have lost the ability to produce normal amounts of chlorophyll or to photosynthesize. Plants are characterized by sexual reproduction and alternation of generations, although asexual reproduction is common.
There are about 320 thousand species of plants, of which the great majority, some 260–290 thousand, are seed plants. Green plants provide a substantial proportion of the world's molecular oxygen and are the basis of most of Earth's ecosystems on land. Plants that produce grain and vegetables form humankind's basic foods, have been domesticated for millennia. Plants have many cultural and other uses, as ornaments, building materials, writing material and, in great variety, they have been the source of medicines and psychoactive drugs; the scientific study of plants is known as a branch of biology. All living things were traditionally placed into one of two groups and animals; this classification may date from Aristotle, who made the distincton between plants, which do not move, animals, which are mobile to catch their food. Much when Linnaeus created the basis of the modern system of scientific classification, these two groups became the kingdoms Vegetabilia and Animalia. Since it has become clear that the plant kingdom as defined included several unrelated groups, the fungi and several groups of algae were removed to new kingdoms.
However, these organisms are still considered plants in popular contexts. The term "plant" implies the possession of the following traits multicellularity, possession of cell walls containing cellulose and the ability to carry out photosynthesis with primary chloroplasts; when the name Plantae or plant is applied to a specific group of organisms or taxon, it refers to one of four concepts. From least to most inclusive, these four groupings are: Another way of looking at the relationships between the different groups that have been called "plants" is through a cladogram, which shows their evolutionary relationships; these are not yet settled, but one accepted relationship between the three groups described above is shown below. Those which have been called "plants" are in bold; the way in which the groups of green algae are combined and named varies between authors. Algae comprise several different groups of organisms which produce food by photosynthesis and thus have traditionally been included in the plant kingdom.
The seaweeds range from large multicellular algae to single-celled organisms and are classified into three groups, the green algae, red algae and brown algae. There is good evidence that the brown algae evolved independently from the others, from non-photosynthetic ancestors that formed endosymbiotic relationships with red algae rather than from cyanobacteria, they are no longer classified as plants as defined here; the Viridiplantae, the green plants – green algae and land plants – form a clade, a group consisting of all the descendants of a common ancestor. With a few exceptions, the green plants have the following features in common, they undergo closed mitosis without centrioles, have mitochondria with flat cristae. The chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic cyanobacteria. Two additional groups, the Rhodophyta and Glaucophyta have primary chloroplasts that appear to be derived directly from endosymbiotic cyanobacteria, although they differ from Viridiplantae in the pigments which are used in photosynthesis and so are different in colour.
These groups differ from green plants in that the storage polysaccharide is floridean starch and is stored in the cytoplasm rather than in the plastids. They appear to have had a common origin with Viridiplantae and the three groups form the clade Archaeplastida, whose name implies that their chloroplasts were derived from a single ancient endosymbiotic event; this is the broadest modern definition of the term'plant'. In contrast, most other algae not only have different pigments but have chloroplasts with three or four surrounding membranes, they are not close relatives of the Archaeplastida having acquired chloroplasts separately from ingested or symbiotic green and red algae. They are thus not included in the broadest modern definition of the plant kingdom, although they were in the past; the green plants or Viridiplantae were traditionally divided into the green algae (including