File Name: structure and function of xylem and phloem .zip
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 also transports nutrients. The most distinctive xylem cells are the long tracheary elements that transport water.
Tracheids and vessel elements are distinguished by their shape; vessel elements are shorter, and are connected together into long tubes that are called vessels. Xylem also contains two other cell types: parenchyma and fibers. In transitional stages of plants with secondary growth , the first two categories are not mutually exclusive, although usually a vascular bundle will contain primary xylem only.
The branching pattern exhibited by xylem follows Murray's law. Primary xylem is formed during primary growth from procambium.
It includes protoxylem and metaxylem. Metaxylem develops after the protoxylem but before secondary xylem. Metaxylem has wider vessels and tracheids than protoxylem.
Secondary xylem is formed during secondary growth from vascular cambium. Although secondary xylem is also found in members of the gymnosperm groups Gnetophyta and Ginkgophyta and to a lesser extent in members of the Cycadophyta , the two main groups in which secondary xylem can be found are:. The xylem, vessels and tracheids of the roots, stems and leaves are interconnected to form a continuous system of water-conducting channels reaching all parts of the plants.
The system transports water and soluble mineral nutrients from the roots throughout the plant. It is also used to replace water lost during transpiration and photosynthesis. Xylem sap consists mainly of water and inorganic ions, although it can also contain a number of organic chemicals as well. The transport is passive, not powered by energy spent by the tracheary elements themselves, which are dead by maturity and no longer have living contents.
Transporting sap upwards becomes more difficult as the height of a plant increases and upwards transport of water by xylem is considered to limit the maximum height of trees. 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.
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 very small in diameter; otherwise, cavitation would break the water column.
And as water evaporates from leaves, more is drawn up through the plant to replace it. Even after an embolism has occurred, plants are able to refill the xylem and restore the functionality.
The cohesion-tension theory is a theory of intermolecular attraction that explains the process of water flow upwards against the force of gravity through the xylem of plants. Water is a polar molecule.
When two water molecules approach one another, the slightly negatively charged oxygen atom of one forms a hydrogen bond with a slightly positively charged hydrogen atom in the other. This attractive force, along with other intermolecular forces , is one of the principal factors responsible for the occurrence of surface tension in liquid water. It also allows plants to draw water from the root through the xylem to the leaf.
Water is constantly lost through transpiration from the leaf. When one water molecule is lost another is pulled along by the processes of cohesion and tension. Transpiration pull, utilizing capillary action and the inherent surface tension of water, is the primary mechanism of water movement in plants. However, it is not the only mechanism involved. Any use of water in leaves forces water to move into them. Transpiration in leaves creates tension differential pressure in the cell walls of mesophyll cells.
Because of this tension, water is being pulled up from the roots into the leaves, helped by cohesion the pull between individual water molecules, due to hydrogen bonds and adhesion the stickiness between water molecules and the hydrophilic cell walls of plants.
This mechanism of water flow works because of water potential water flows from high to low potential , and the rules of simple diffusion. Over the past century, there has been a great deal of research regarding the mechanism of xylem sap transport; today, most plant scientists continue to agree that the cohesion-tension theory best explains this process, but multiforce theories that hypothesize several alternative mechanisms have been suggested, including longitudinal cellular and xylem osmotic pressure gradients , axial potential gradients in the vessels, and gel- and gas-bubble-supported interfacial gradients.
Until recently, the differential pressure suction of transpirational pull could only be measured indirectly, by applying external pressure with a pressure bomb to counteract it. More recent measurements do tend to validate the classic theory, for the most part. Xylem transport is driven by a combination  of transpirational pull from above and root pressure from below, which makes the interpretation of measurements more complicated.
Xylem appeared early in the history of terrestrial plant life. Fossil plants with anatomically preserved xylem are known from the Silurian more than million years ago , and trace fossils resembling individual xylem cells may be found in earlier Ordovician rocks.
This is the only type of xylem found in the earliest vascular plants, and this type of cell continues to be found in the protoxylem first-formed xylem of all living groups of vascular plants. Several groups of plants later developed pitted tracheid cells independently through convergent evolution. In living plants, pitted tracheids do not appear in development until the maturation of the metaxylem following the protoxylem.
In most plants, pitted tracheids function as the primary transport cells. The other type of vascular element, found in angiosperms, is the vessel element. Vessel elements are joined end to end to form vessels in which water flows unimpeded, as in a pipe. The presence of xylem vessels is considered to be one of the key innovations that led to the success of the angiosperms.
Cronquist considered the vessels of Gnetum to be convergent with those of angiosperms. To photosynthesize, plants must absorb CO 2 from the atmosphere. However, this comes at a price: while stomata are open to allow CO 2 to enter, water can evaporate. Specialized water transport tissues soon evolved in the form of hydroids, tracheids, then secondary xylem, followed by an endodermis and ultimately vessels.
The high CO 2 levels of Silurian-Devonian times, when plants were first colonizing land, meant that the need for water was relatively low.
As CO 2 was withdrawn from the atmosphere by plants, more water was lost in its capture, and more elegant transport mechanisms evolved. This transition from poikilohydry to homoiohydry opened up new potential for colonization.
During the Silurian, CO 2 was readily available, so little water needed expending to acquire it. By the end of the Carboniferous, when CO 2 levels had lowered to something approaching today's, around 17 times more water was lost per unit of CO 2 uptake. This early water transport took advantage of the cohesion-tension mechanism inherent in water. Water has a tendency to diffuse to areas that are drier, and this process is accelerated when water can be wicked along a fabric with small spaces.
Therefore, transpiration alone provided the driving force for water transport in early plants. Early cuticle may not have had pores but did not cover the entire plant surface, so that gas exchange could continue. To be free from the constraints of small size and constant moisture that the parenchymatic transport system inflicted, plants needed a more efficient water transport system. During the early Silurian , they developed specialized cells, which were lignified or bore similar chemical compounds  to avoid implosion; this process coincided with cell death, allowing their innards to be emptied and water to be passed through them.
The earliest macrofossils to bear water-transport tubes are Silurian plants placed in the genus Cooksonia. Plants continued to innovate new ways of reducing the resistance to flow within their cells, thereby increasing the efficiency of their water transport. Bands on the walls of tubes, in fact apparent from the early Silurian onwards,  are an early improvisation to aid the easy flow of water. These, the "next generation" of transport cell design, have a more rigid structure than hydroids, allowing them to cope with higher levels of water pressure.
Water transport requires regulation, and dynamic control is provided by stomata. This is an important role where water supply is not constant, and indeed stomata appear to have evolved before tracheids, being present in the non-vascular hornworts.
An endodermis probably evolved during the Silu-Devonian, but the first fossil evidence for such a structure is Carboniferous. The endodermis can also provide an upwards pressure, forcing water out of the roots when transpiration is not enough of a driver.
Once plants had evolved this level of controlled water transport, they were truly homoiohydric, able to extract water from their environment through root-like organs rather than relying on a film of surface moisture, enabling them to grow to much greater size.
During the Devonian, maximum xylem diameter increased with time, with the minimum diameter remaining pretty constant. While wider tracheids with robust walls make it possible to achieve higher water transport pressures, this increases the problem of cavitation.
A tracheid, once cavitated, cannot have its embolism removed and return to service except in a few advanced angiosperms   which have developed a mechanism of doing so. Therefore, it is well worth plants' while to avoid cavitation occurring. For this reason, pits in tracheid walls have very small diameters, to prevent air entering and allowing bubbles to nucleate. Freeze-thaw cycles are a major cause of cavitation. Damage to a tracheid's wall almost inevitably leads to air leaking in and cavitation, hence the importance of many tracheids working in parallel.
Cavitation is hard to avoid, but once it has occurred plants have a range of mechanisms to contain the damage. These torus-margo structures have a blob floating in the middle of a donut; when one side depressurizes the blob is sucked into the torus and blocks further flow. Maples use root pressure each spring to force sap upwards from the roots, squeezing out any air bubbles.
Defunct tracheids were retained to form a strong, woody stem, produced in most instances by a secondary xylem. However, in early plants, tracheids were too mechanically vulnerable, and retained a central position, with a layer of tough sclerenchyma on the outer rim of the stems. Tracheids end with walls, which impose a great deal of resistance on flow;  vessel members have perforated end walls, and are arranged in series to operate as if they were one continuous vessel.
An embolism is where an air bubble is created in a tracheid. This may happen as a result of freezing, or by gases dissolving out of solution. Once an embolism is formed, it usually cannot be removed but see later ; the affected cell cannot pull water up, and is rendered useless. End walls excluded, the tracheids of prevascular plants were able to operate under the same hydraulic conductivity as those of the first vascular plant, Cooksonia. Vessels first evolved during the dry, low CO 2 periods of the late Permian, in the horsetails, ferns and Selaginellales independently, and later appeared in the mid Cretaceous in angiosperms and gnetophytes.
Xylem development can be described by four terms: centrarch, exarch, endarch and mesarch. As it develops in young plants, its nature changes from protoxylem to metaxylem i. The patterns in which protoxylem and metaxylem are arranged is important in the study of plant morphology.
Vascular tissue is a complex conducting tissue , formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue: the vascular cambium and the cork cambium. All the vascular tissues within a particular plant together constitute the vascular tissue system of that plant. The cells in vascular tissue are typically long and slender. Since the xylem and phloem function in the conduction of water, minerals, and nutrients throughout the plant, it is not surprising that their form should be similar to pipes.
These two tissues extend from the leaves to the roots, and are vital conduits for water and nutrient transport. In a sense, they are to plants what veins and arteries are to animals. The structure of xylem and phloem tissue depends on whether the plant is a flowering plant including dicots and monocots or a gymnosperm polycots.
Xylem and phloem form the vascular system of plants to transport water and other substances throughout the plant. The first fossils that show the presence of vascular tissue date to the Silurian period, about million years ago. The simplest arrangement of conductive cells shows a pattern of xylem at the center surrounded by phloem. Together, xylem and phloem tissues form the vascular system of plants.
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 also transports nutrients.
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