Capillary action
Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, or in opposition to, external forces like gravity. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper and plaster, in some non-porous materials such as sand and liquefied carbon fiber, or in a biological cell, it occurs because of intermolecular forces between surrounding solid surfaces. If the diameter of the tube is sufficiently small the combination of surface tension and adhesive forces between the liquid and container wall act to propel the liquid; the first recorded observation of capillary action was by Leonardo da Vinci. A former student of Galileo, Niccolò Aggiunti, was said to have investigated capillary action. In 1660, capillary action was still a novelty to the Irish chemist Robert Boyle, when he reported that "some inquisitive French Men" had observed that when a capillary tube was dipped into water, the water would ascend to "some height in the Pipe".
Boyle reported an experiment in which he dipped a capillary tube into red wine and subjected the tube to a partial vacuum. He found that the vacuum had no observable influence on the height of the liquid in the capillary, so the behavior of liquids in capillary tubes was due to some phenomenon different from that which governed mercury barometers. Others soon followed Boyle's lead; some thought that liquids rose in capillaries because air could not enter capillaries as as liquids, so the air pressure was lower inside capillaries. Others thought that the particles of liquid were attracted to each other and to the walls of the capillary. Although experimental studies continued during the 18th century, a successful quantitative treatment of capillary action was not attained until 1805 by two investigators: Thomas Young of the United Kingdom and Pierre-Simon Laplace of France, they derived the Young–Laplace equation of capillary action. By 1830, the German mathematician Carl Friedrich Gauss had determined the boundary conditions governing capillary action.
In 1871, the British physicist William Thomson determined the effect of the meniscus on a liquid's vapor pressure—a relation known as the Kelvin equation. German physicist Franz Ernst Neumann subsequently determined the interaction between two immiscible liquids. Albert Einstein's first paper, submitted to Annalen der Physik in 1900, was on capillarity. Capillary penetration in porous media shares its dynamic mechanism with flow in hollow tubes, as both processes are resisted by viscous forces. A common apparatus used to demonstrate the phenomenon is the capillary tube; when the lower end of a glass tube is placed in a liquid, such as water, a concave meniscus forms. Adhesion occurs between the fluid and the solid inner wall pulling the liquid column along until there is a sufficient mass of liquid for gravitational forces to overcome these intermolecular forces; the contact length between the top of the liquid column and the tube is proportional to the radius of the tube, while the weight of the liquid column is proportional to the square of the tube's radius.
So, a narrow tube will draw a liquid column along further than a wider tube will, given that the inner water molecules cohere sufficiently to the outer ones. Capillary action is seen in many plants. Water is brought high up in trees by branching. Capillary action for uptake of water has been described in some small animals, such as Ligia exotica and Moloch horridus. In the built environment, evaporation limited capillary penetration is responsible for the phenomenon of rising damp in concrete and masonry, while in industry and diagnostic medicine this phenomenon is being harnessed in the field of paper-based microfluidics. In physiology, capillary action is essential for the drainage of continuously produced tear fluid from the eye. Two canaliculi of tiny diameter are present in the inner corner of the eyelid called the lacrimal ducts. Wicking is the absorption of a liquid by a material in the manner of a candle wick. Paper towels absorb liquid through capillary action, allowing a fluid to be transferred from a surface to the towel.
The small pores of a sponge act as small capillaries, causing it to absorb a large amount of fluid. Some textile fabrics are said to use capillary action to "wick" sweat away from the skin; these are referred to as wicking fabrics, after the capillary properties of candle and lamp wicks. Capillary action is observed in thin layer chromatography, in which a solvent moves vertically up a plate via capillary action. In this case the pores are gaps between small particles. Capillary action draws ink to the tips of fountain pen nibs from a reservoir or cartridge inside the pen. With some pairs of materials, such as mercury and glass, the intermolecular forces within the liquid exceed those between the solid and the liquid, so a convex meniscus forms and capillary action works in reverse. In hydrology, capillary action describes the attraction of water molecules to soil particles. Capillary action is responsible for moving groundwater from wet areas of the soil to dry a
Soil
Soil is a mixture of organic matter, gases and organisms that together support life. Earth's body of soil, called the pedosphere, has four important functions: as a medium for plant growth as a means of water storage and purification as a modifier of Earth's atmosphere as a habitat for organismsAll of these functions, in their turn, modify the soil; the pedosphere interfaces with the lithosphere, the hydrosphere, the atmosphere, the biosphere. The term pedolith, used to refer to the soil, translates to ground stone in the sense "fundamental stone". Soil consists of a solid phase of minerals and organic matter, as well as a porous phase that holds gases and water. Accordingly, soil scientists can envisage soils as a three-state system of solids and gases. Soil is a product of several factors: the influence of climate, relief and the soil's parent materials interacting over time, it continually undergoes development by way of numerous physical and biological processes, which include weathering with associated erosion.
Given its complexity and strong internal connectedness, soil ecologists regard soil as an ecosystem. Most soils have a dry bulk density between 1.1 and 1.6 g/cm3, while the soil particle density is much higher, in the range of 2.6 to 2.7 g/cm3. Little of the soil of planet Earth is older than the Pleistocene and none is older than the Cenozoic, although fossilized soils are preserved from as far back as the Archean. Soil science has two basic branches of study: pedology. Edaphology studies the influence of soils on living things. Pedology focuses on the formation and classification of soils in their natural environment. In engineering terms, soil is included in the broader concept of regolith, which includes other loose material that lies above the bedrock, as can be found on the Moon and on other celestial objects as well. Soil is commonly referred to as earth or dirt. Soil is a major component of the Earth's ecosystem; the world's ecosystems are impacted in far-reaching ways by the processes carried out in the soil, from ozone depletion and global warming to rainforest destruction and water pollution.
With respect to Earth's carbon cycle, soil is an important carbon reservoir, it is one of the most reactive to human disturbance and climate change. As the planet warms, it has been predicted that soils will add carbon dioxide to the atmosphere due to increased biological activity at higher temperatures, a positive feedback; this prediction has, been questioned on consideration of more recent knowledge on soil carbon turnover. Soil acts as an engineering medium, a habitat for soil organisms, a recycling system for nutrients and organic wastes, a regulator of water quality, a modifier of atmospheric composition, a medium for plant growth, making it a critically important provider of ecosystem services. Since soil has a tremendous range of available niches and habitats, it contains most of the Earth's genetic diversity. A gram of soil can contain billions of organisms, belonging to thousands of species microbial and in the main still unexplored. Soil has a mean prokaryotic density of 108 organisms per gram, whereas the ocean has no more than 107 procaryotic organisms per milliliter of seawater.
Organic carbon held in soil is returned to the atmosphere through the process of respiration carried out by heterotrophic organisms, but a substantial part is retained in the soil in the form of soil organic matter. Since plant roots need oxygen, ventilation is an important characteristic of soil; this ventilation can be accomplished via networks of interconnected soil pores, which absorb and hold rainwater making it available for uptake by plants. Since plants require a nearly continuous supply of water, but most regions receive sporadic rainfall, the water-holding capacity of soils is vital for plant survival. Soils can remove impurities, kill disease agents, degrade contaminants, this latter property being called natural attenuation. Soils maintain a net absorption of oxygen and methane and undergo a net release of carbon dioxide and nitrous oxide. Soils offer plants physical support, water, temperature moderation and protection from toxins. Soils provide available nutrients to plants and animals by converting dead organic matter into various nutrient forms.
A typical soil is about 50% solids, 50% voids of which half is occupied by water and half by gas. The percent soil mineral and organic content can be treated as a constant, while the percent soil water and gas content is considered variable whereby a rise in one is balanced by a reduction in the other; the pore space allows for the infiltration and movement of air and water, both of which are critical for life existing in soil. Compaction, a common problem with soils, reduces this space, preventing air and water from reaching plant roots and soil organisms. Given sufficient time, an undifferentiated soil will evolve a soil profile which consists of two or more layers, referred to as soil horizons, that differ in one or more properties such as in their texture, density, consistency, temperature and reactivity; the horizons differ in thickness and gene
Osmosis
Osmosis is the spontaneous net movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides. It may be used to describe a physical process in which any solvent moves across a selectively permeable membrane separating two solutions of different concentrations. Osmosis can be made to do work. Osmotic pressure is defined as the external pressure required to be applied so that there is no net movement of solvent across the membrane. Osmotic pressure is a colligative property, meaning that the osmotic pressure depends on the molar concentration of the solute but not on its identity. Osmosis is a vital process in biological systems. In general, these membranes are impermeable to large and polar molecules, such as ions and polysaccharides, while being permeable to non-polar or hydrophobic molecules like lipids as well as to small molecules like oxygen, carbon dioxide and nitric oxide.
Permeability depends on charge, or chemistry, as well as solute size. Water molecules travel through the plasma membrane, tonoplast membrane or protoplast by diffusing across the phospholipid bilayer via aquaporins. Osmosis provides the primary means by which water is transported out of cells; the turgor pressure of a cell is maintained by osmosis across the cell membrane between the cell interior and its hypotonic environment. Some kinds of osmotic flow have been observed since ancient times, e.g. on the construction of Egyptian pyramids. Jean-Antoine Nollet first documented observation of osmosis in 1748; the word "osmosis" descends from the words "endosmose" and "exosmose", which were coined by French physician René Joachim Henri Dutrochet from the Greek words ἔνδον, ἔξω, ὠσμός. In 1867, Moritz Traube invented selective precipitation membranes, advancing the art and technique of measurement of osmotic flow. Osmosis is the movement of a solvent across a semipermeable membrane toward a higher concentration of solute.
In biological systems, the solvent is water, but osmosis can occur in other liquids, supercritical liquids, gases. When a cell is submerged in water, the water molecules pass through the cell membrane from an area of low solute concentration to high solute concentration. For example, if the cell is submerged in saltwater, water molecules move out of the cell. If a cell is submerged in freshwater, water molecules move into the cell; when the membrane has a volume of pure water on both sides, water molecules pass in and out in each direction at the same rate. There is no net flow of water through the membrane; the mechanism responsible for driving osmosis has been represented in biology and chemistry texts as either the dilution of water by solute or by a solute's attraction to water. Both of these notions have been conclusively refuted; the diffusion model of osmosis is rendered untenable by the fact that osmosis can drive water across a membrane toward a higher concentration of water. The "bound water" model is refuted by the fact that osmosis is independent of the size of the solute molecules—a colligative property—or how hydrophilic they are.
It is hard to describe osmosis without a mechanical or thermodynamic explanation, but there is an interaction between the solute and water that counteracts the pressure that otherwise free solute molecules would exert. One fact to take note of is that heat from the surroundings is able to be converted into mechanical energy. Many thermodynamic explanations go into the concept of chemical potential and how the function of the water on the solution side differs from that of pure water due to the higher pressure and the presence of the solute counteracting such that the chemical potential remains unchanged; the virial theorem demonstrates that attraction between the molecules reduces the pressure, thus the pressure exerted by water molecules on each other in solution is less than in pure water, allowing pure water to "force" the solution until the pressure reaches equilibrium. Osmotic pressure is the main cause of support in many plants; the osmotic entry of water raises the turgor pressure exerted against the cell wall, until it equals the osmotic pressure, creating a steady state.
When a plant cell is placed in a solution, hypertonic relative to the cytoplasm, water moves out of the cell and the cell shrinks. In doing so, the cell becomes flaccid. In extreme cases, the cell becomes plasmolyzed – the cell membrane disengages with the cell wall due to lack of water pressure on it; when a plant cell is placed in a solution, hypotonic relative to the cytoplasm, water moves into the cell and the cell swells to become turgid. Osmosis is responsible for the ability of plant roots to draw water from the soil. Plants concentrate solutes in their root cells by active transport, water enters the roots by osmosis. Osmosis is responsible for controlling the movement of guard cells. Osmosis can be demonstrated; the water from inside the potato moves out to the solution, causing the potat
Xylem
Xylem is one of the two types of transport tissue in vascular plants, phloem being the other. The basic function of xylem is to transport water from roots to stems and leaves, but it transports nutrients; the word "xylem" is derived from the Greek word ξύλον, meaning "wood". The term was introduced by Carl Nägeli in 1858; the most distinctive xylem cells are the long tracheary elements. Tracheids and vessel elements are distinguished by their shape. Xylem contains two other cell types: parenchyma and fibers. Xylem can be found: in vascular bundles, present in non-woody plants and non-woody parts of woody plants in secondary xylem, laid down by a meristem called the vascular cambium in woody plants as part of a stelar arrangement not divided into bundles, as in many ferns. In transitional stages of plants with secondary growth, the first two categories are not mutually exclusive, although 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 metaxylem. Metaxylem develops before secondary xylem. Metaxylem has wider tracheids than protoxylem. Secondary xylem is formed during secondary growth from vascular cambium. Although secondary xylem is 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: conifers: there are some six hundred species of conifers. All species have secondary xylem, uniform in structure throughout this group. Many conifers become tall trees: the secondary xylem of such trees is used and marketed as softwood. Angiosperms: there are some quarter of a million to four hundred thousand species of angiosperms. Within this group secondary xylem is rare in the monocots. Many non-monocot angiosperms become trees, the secondary xylem of these is used and marketed as hardwood; the xylem and tracheids of the roots 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 the roots throughout the plant. It is used to replace water lost during transpiration and photosynthesis. Xylem sap consists of water and inorganic ions, although it can 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 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: Pressure flow hypothesis: Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a solute pressure differential versus the xylem system carrying a far lower load of solutes- water and minerals; the phloem pressure can rise to several MPa, far higher than atmospheric pressure. Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure.
Transpirational pull: Similarly, the evaporation of water from the surfaces of mesophyll cells to the atmosphere creates a negative pressure at the top of a plant. This causes millions of minute menisci to form in the mesophyll cell wall; the resulting surface tension causes a negative pressure or tension in the xylem that pulls the water from the roots and soil. Root pressure: If the water potential of the root cells is more negative than that of the soil due to high concentrations of solute, water can move by osmosis into the root from the soil; this causes a positive pressure. In some circumstances, the sap will be forced from the leaf through a hydathode in a phenomenon known as guttation. Root pressure is highest in the morning before the stomata allow transpiration to begin. Different plant species can have different root pressures in a similar environment; 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 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 a hundred meters from ground level to a tree's highest branches. Transpirational pull requires that the vessels transporting the water be small in diameter, 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 the roots the gases come out of solution and form a bubble – an embolism forms, which will spread to other adjacent cells, unless bordered pits are present (these have
Stoma
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
Diffusion
Diffusion is the net movement of molecules or atoms from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in chemical potential of the diffusing species. A gradient is the change in the value of a quantity e.g. concentration, pressure, or temperature with the change in another variable distance. A change in concentration over a distance is called a concentration gradient, a change in pressure over a distance is called a pressure gradient, a change in temperature over a distance is called a temperature gradient; the word diffusion derives from the Latin word, which means "to spread way out.” A distinguishing feature of diffusion is that it depends on particle random walk, results in mixing or mass transport without requiring directed bulk motion. Bulk motion, or bulk flow, is the characteristic of advection; the term convection is used to describe the combination of both transport phenomena. An example of a situation in which bulk motion and diffusion can be differentiated is the mechanism by which oxygen enters the body during external respiration known as breathing.
The lungs are located in the thoracic cavity, which expands as the first step in external respiration. This expansion leads to an increase in volume of the alveoli in the lungs, which causes a decrease in pressure in the alveoli; this creates a pressure gradient between the air outside the body at high pressure and the alveoli at low pressure. The air moves down the pressure gradient through the airways of the lungs and into the alveoli until the pressure of the air and that in the alveoli are equal i.e. the movement of air by bulk flow stops once there is no longer a pressure gradient. The air arriving in the alveoli has a higher concentration of oxygen than the “stale” air in the alveoli; the increase in oxygen concentration creates a concentration gradient for oxygen between the air in the alveoli and the blood in the capillaries that surround the alveoli. Oxygen moves by diffusion, down the concentration gradient, into the blood; the other consequence of the air arriving in alveoli is that the concentration of carbon dioxide in the alveoli decreases.
This creates a concentration gradient for carbon dioxide to diffuse from the blood into the alveoli, as fresh air has a low concentration of carbon dioxide compared to the blood in the body. The pumping action of the heart transports the blood around the body; as the left ventricle of the heart contracts, the volume decreases, which increases the pressure in the ventricle. This creates a pressure gradient between the heart and the capillaries, blood moves through blood vessels by bulk flow down the pressure gradient; as the thoracic cavity contracts during expiration, the volume of the alveoli decreases and creates a pressure gradient between the alveoli and the air outside the body, air moves by bulk flow down the pressure gradient. The concept of diffusion is used in: physics, biology, sociology and finance. However, in each case, the object, undergoing diffusion is “spreading out” from a point or location at which there is a higher concentration of that object. There are two ways to introduce the notion of diffusion: either a phenomenological approach starting with Fick's laws of diffusion and their mathematical consequences, or a physical and atomistic one, by considering the random walk of the diffusing particles.
In the phenomenological approach, diffusion is the movement of a substance from a region of high concentration to a region of low concentration without bulk motion. According to Fick's laws, the diffusion flux is proportional to the negative gradient of concentrations, it goes from regions of higher concentration to regions of lower concentration. Sometime various generalizations of Fick's laws were developed in the frame of thermodynamics and non-equilibrium thermodynamics. From the atomistic point of view, diffusion is considered as a result of the random walk of the diffusing particles. In molecular diffusion, the moving molecules are self-propelled by thermal energy. Random walk of small particles in suspension in a fluid was discovered in 1827 by Robert Brown; the theory of the Brownian motion and the atomistic backgrounds of diffusion were developed by Albert Einstein. The concept of diffusion is applied to any subject matter involving random walks in ensembles of individuals. Biologists use the terms "net movement" or "net diffusion" to describe the movement of ions or molecules by diffusion.
For example, oxygen can diffuse through cell membranes so long as there is a higher concentration of oxygen outside the cell. However, because the movement of molecules is random oxygen molecules move out of the cell; because there are more oxygen molecules outside the cell, the probability that oxygen molecules will enter the cell is higher than the probability that oxygen molecules will leave the cell. Therefore, the "net" movement of oxygen molecules is into the cell. In other words, there is a net movement of oxygen molecules down the concentration gradient. In the scope of time, diffusion in solids was used. For example, Pliny the Elder had described the cementation process, which produces steel from the element iron through carbon diffusion. Another example is well known for many centuries, the diffusion of colors of stained glass or earthenware and Chinese ceramics. In modern science, the first systematic experimental study of di
Root
In vascular plants, the root is the organ of a plant that lies below the surface of the soil. Roots can be aerial or aerating, that is, growing up above the ground or above water. Furthermore, a stem occurring below ground is not exceptional either. Therefore, the root is best defined as the non-nodes bearing parts of the plant's body. However, important internal structural differences between stems and roots exist; the fossil record of roots—or rather, infilled voids where roots rotted after death—spans back to the late Silurian, about 430 million years ago. Their identification is difficult, because casts and molds of roots are so similar in appearance to animal burrows, they can be discriminated using a range of features. The first root that comes from a plant is called the radicle. A root's four major functions are: absorption of inorganic nutrients. In response to the concentration of nutrients, roots synthesise cytokinin, which acts as a signal as to how fast the shoots can grow. Roots function in storage of food and nutrients.
The roots of most vascular plant species enter into symbiosis with certain fungi to form mycorrhizae, a large range of other organisms including bacteria closely associate with roots. When dissected, the arrangement of the cells in a root is root hair, epiblem, endodermis, pericycle and, the vascular tissue in the centre of a root to transport the water absorbed by the root to other places of the plant; the most striking characteristic of roots is that, roots have an endogenous origin, i.e. it originates and develops from an inner layer of the mother axis. Whereas Stem-branching and leaves are exogenous, i.e. start to develop from the cortex, an outer layer. In its simplest form, the term root architecture refers to the spatial configuration of a plant’s root system; this system can be complex and is dependent upon multiple factors such as the species of the plant itself, the composition of the soil and the availability of nutrients. The configuration of root systems serves to structurally support the plant, compete with other plants and for uptake of nutrients from the soil.
Roots grow to specific conditions. For example, a root system that has developed in dry soil may not be as efficient in flooded soil, yet plants are able to adapt to other changes in the environment, such as seasonal changes. Root architecture plays the important role of providing a secure supply of nutrients and water as well as anchorage and support; the main terms used to classify the architecture of a root system are: Branch magnitude: the number of links. Topology: the pattern of branching, including:Herringbone: alternate lateral branching off a parent root Dichotomous: opposite, forked branches Radial: whorl of branches around a rootLink length: the distance between branches. Root angle: the radial angle of a lateral root’s base around the parent root’s circumference, the angle of a lateral root from its parent root, the angle an entire system spreads. Link radius: the diameter of a root. All components of the root architecture are regulated through a complex interaction between genetic responses and responses due to environmental stimuli.
These developmental stimuli are categorised as intrinsic, the genetic and nutritional influences, or extrinsic, the environmental influences and are interpreted by signal transduction pathways. The extrinsic factors that affect root architecture include gravity, light exposure and oxygen, as well as the availability or lack of nitrogen, sulphur and sodium chloride; the main hormones and respective pathways responsible for root architecture development include: Auxin – Auxin promotes root initiation, root emergence and primary root elongation. Cytokinins – Cytokinins regulate root apical meristem size and promote lateral root elongation. Gibberellins -- Together with ethylene they promote elongation. Together with auxin they promote root elongation. Gibberellins inhibit lateral root primordia initiation. Ethylene – Ethylene promotes crown root formation. Early root growth is one of the functions of the apical meristem located near the tip of the root; the meristem cells more or less continuously divide, producing more meristem, root cap cells, undifferentiated root cells.
The latter become the primary tissues of the root, first undergoing elongation, a process that pushes the root tip forward in the growing medium. These cells differentiate and mature into specialized cells of the root tissues. Growth from apical meristems is known as primary growth. Secondary growth encompasses all growth in diameter, a major component of woody plant tissues and many nonwoody plants. For example, storage roots of sweet potato are not woody. Secondary growth occurs at the lateral meristems, namely the vascular cork cambium; the former forms secondary phloem, while the latter forms the periderm. In plants with secondary growth, the vascular cambium, originating between the xylem and the phloem, forms a cylinder of tissue along the stem and root; the vascular cambium forms new cells on both the inside and outside of the cambium cylinder, with those on the inside forming secondary xylem cells, those on the outside forming secondary phloem cells. As
