The most important differences to the stem axis are:
- The root grows into a substrate (mostly the soil), while the shoot (shoot axis with leaves) grows out of it and towards the light.
- The root has root hairs and a root cap (kalyptra) at the tip.
- The branches are of endogenous origin.
- The primary vascular bundles are arranged radially.
- Roots do not have leaves.
The root serves primarily to absorb water and the minerals dissolved in it as well as to secure the plant in its location. In many cases it also takes on other functions, particularly often as a storage organ for reserve substances . The area of the soil influenced by the roots is the rhizosphere .
As a root system ( roots or root network ), the entirety of the roots of a is plant referred to. In principle one differentiates:
- "Heterogeneous root systems" ( allorhizia ) with a main root growing vertically downwards and lateral roots branching off from it. They occur with the dicots .
- "Homogeneous root systems" ( Homorhizia ), which consist of numerous equal, similarly shaped roots. These are characteristic of the ferns and the monocots .
The transition area to the trunk of a tree is the rootstock in the forest language ; In the case of herbaceous plants, however, the gardener means the root structure or the root ball (the entire root system of a potted plant). The rhizome (colloquially "root stock"), on the other hand, does not belong to the root system, but to the stem axis system .
- Taproot system : a vertically growing, thickened main root dominates ( oak , pine , fir , elm ).
- Heart root system : Several roots of different strengths are formed on the rhizome ( beech , birch , hornbeam , larch , linden ).
- Horizontal root system: The main root is only present in young trees. There are several horizontally ( plagiotropic ) growing side roots ( poplars ).
- A mixed form is the sinker root system , in which sinker roots grow vertically into the ground from strong horizontal roots ( ash , spruce in the aging phase).
Depending on whether the roots penetrate deeply into the ground or extend horizontally just below the surface of the earth, one differentiates:
- Deep-rooters who drive their taproots towards the groundwater (in addition to the trees mentioned above, for example, mullein or radish )
- Shallow-rooted species that are more adapted to absorbing surface water thatseeps into the ground
- Heart roots, whose roots penetrate in all directions, neither pronounced deep nor shallow. The cross-section of the plant roots has a heart shape.
The root system can be very different in the individual plants - depending on the site conditions.
Another distinction is that between - often lignified - coarse roots and fine roots :
- The coarse roots form the root structure , give the plant support and define the rooted soil area.
- The thin fine roots often only have a short lifespan and are responsible for absorbing water and nutrients. The upper limit for fine roots is, depending on the definition and the plant, between 0.8 millimeters in diameter for arable plants and two millimeters for trees. The fine roots are also the most physiologically active roots.
The growth of the root takes place by means of root tips meristem . In ferns , this is a tetragonal, four- edged apical cell that divides cells in all four directions . In the other higher plants, the meristem is formed by a group of initial cells.
The meristem is protected by the root cap (kalyptra). The root cap consists of parenchymal cells, the middle lamellae of which become slimy and thus facilitate the penetration of the roots into the soil. The cells are continuously shed and newly formed from the point of vegetation. The slime also changes the soil structure by sticking with clay particles. The negative charge of the slimes also play a role in the absorption of nutrients and potentially harmful ions (aluminum, cadmium). The mucus is only formed by the root cap, but as the roots continue to grow, it also sticks to the exodermis and the root hairs. As a result, grasses often form a root sheath of stuck together soil particles around the young root parts.
The formation and differentiation of the tissues proceeds from the meristem backwards. The cell elongation zone, which is only a few millimeters long, follows the meristematic zone. This is where the cells reach their final size. The internal differentiation of the tissues also takes place in the subsequent root hair zone.
The rhizodermis is the single-layer closing tissue. In contrast to the epidermis of the shoot, it has no cuticle and no stomata . In the root hair zone, the rhizodermis cells form papillae or tube-like protuberances, the root hairs . They serve to increase the surface area and thus the more effective absorption of water and minerals. The cell walls are thin and slimy (for easier penetration of the soil), the cells have large vacuoles , and the nucleus and plasma are often located at the tip of the hair. All cells of the rhizodermis can grow into root hairs, or only certain hair formers ( trichoblast ). Root hairs and the other rhizodermis cells only have a lifespan of a few days and then die. It serves as protection at the very tip. It also has a very large epidermis.
Before the rhizodermis dies, a secondary tissue, the exodermis, forms from the outer cortical parenchyma (also called hypodermis) . This is a fabric from one or more cell layers, whose cells without intercellular spaces are interconnected and Akkrustierungen of suberin own. In addition to these corked cells, some roots also contain so-called passage cells without suberin accumulation (so-called short cell exodermis). The exodermis forms the outer edge of the primary root and prevents the loss of water and nutrients from the root.
The root cortex is a usually colorless parenchyma with large, schizogenic intercellular spaces. The bark is a storage tissue and also serves the exchange of substances between the rhizodermis and the xylem . The root bark is usually colonized by mycorrhizal fungi . In the case of roots without secondary growth in thickness, the bark also forms strengthening tissue ( sclerenchyma and collenchyma ).
The endodermis is the innermost layer of the cortex and completely encloses the central cylinder as a single layer of living cells. The cell walls are specially developed and prevent the apoplastic influx of water into the central cylinder. Thus, the endodermis controls the passage of water and nutrient salts. At the same time, the endodermis is a barrier for the mycorrhizal fungi. A distinction is made between three states of the endodermis:
The primary endodermis is characterized by the Casparian stripe , a stripe-like zone of the radial and horizontal walls in which a suberine-like polymer ("endodermine") and lignin are deposited, making the cell wall impermeable to water. However, depending on the nature of the Caspary strip, which can vary from species to species, it can also be permeable to water (example: Clivia miniala, no suberin storage in the primary state). Thus, in general, apoplastic water transport through the endodermis is not impossible. The Caspary strip, on the other hand, always rejects ions. In angiosperms with secondary growth in thickness, this is the final state of the endodermis, since it is rejected during growth in thickness.
In the secondary endodermis all walls are covered with suberine-like substance. Some uncorked cells, the “passage cells”, enable the exchange of substances between the cortex and the central cylinder. In the case of conifers , this is the final stage of development, otherwise the transition stage to the tertiary endodermis.
Thick cellulose walls are deposited on the suberine lamellae , whereby the protoplast remains alive. In the case of all-round thickening, one speaks of O-endoderms, if the tangential outer wall remains unthickened, of C- or U-endoderms. These cellulose layers can also lignify. There are also the passage cells that typically attach to the xylem. The tertiary endodermis is the typical end state in monocot plants.
The pericycle, also known as the pericambium, connects to the endodermis and forms the outermost part of the central cylinder. It is a mostly single-layer sheath made up of seamlessly adjoining cells that are stretched lengthways. The cells are residual meristematic or parenchymatic. In monocots and conifers, the pericyclic is often multilayered, in the former it is often sklerenchymatic. The peridermal and lateral root formation originate from the pericycle. In addition, the pericycle is involved in forming the cambium ring at the beginning of the secondary growth in thickness of the root.
The central cylinder is usually organized as an actinostele , the parts are arranged radially. In the middle there is usually xylem . This extends in two or many ridges to the pericyclic, with the phloem in between . Monocot usually have many xylem strands, one speaks of polyarch roots. Dicots and conifers usually have a few xylem strands (oligoarchic roots), a distinction is made again between two, three, four-pointed, etc. roots (di-, tri-, tetrarch). Phloem and xylem differentiate from the outside inwards (centripetal), in contrast to the scion. The different arrangement of xylem and phloem in shoot and root requires a rotation and reorientation of the vascular bundles in the transition area of the hypocotyl .
In the root center there are usually wide lumen vessels of the xylem. Storage parenchyma or strands of sclerenchyma can also be found here.
The side roots (also secondary roots) arise endogenously, i.e. inside the root, in contrast to the side shoots of the stem axis, which are formed exogenously. Usually they arise from the pericycle before the xylemprimans. Therefore the lateral roots are in rows (rhizo stitches), the number of which corresponds to the number of xylem rays in the central cylinder.
The cells of the pericycel can divide again and become the pericambium. They form a conical hump of tissue that grows through the bark and tears it open. Young side roots initially grow vertically away from the main root. They only react positively geotropically later . The endodermis of the main root initially grows with it and connects to the endodermis of the side root. The guide elements of the lateral root are also connected to those of the main root.
Secondary growth in thickness
In conifers and dicots, the roots as well as the stem axes grow secondarily in the thickness. The growth in thickness begins simultaneously with that of the stem axis.
The parenchyma between the xylem and phloem strands becomes meristematic and forms the newly emerging cambium. The parts of the pericycle overlying the xylem also become meristematic. This creates a closed cambium jacket with a star-shaped cross section.
This cambium now subdivides xylem elements inwards and phloem elements outwards. The initially stronger xylem growth leads to a rounding of the cambium, which finally becomes cylindrical. During growth, primary medullary rays and secondary rays are formed , similar to the axis of the shoot . The first rays emerge over the primary xylempole.
The exodermis and bark do not follow the secondary growth in thickness. The tissue tears open, the cells die. At the beginning, the endodermis continues to grow in thickness through dilation, but later also tears. A periderm is created as a tertiary closing tissue , which is formed by the pericycle, which has now completely become the pericambium. In woody plants, a bark similar to that of the stem axis forms.
Roots that are several years old hardly differ from shoot axes. The only differences are found in the radial arrangement of the primary guide elements in the center of the root. The wood of the root is usually more spacious and therefore resembles the early wood of the trunk. For this reason, the annual rings are also less pronounced.
The main functions of the root are to absorb water and minerals from the soil and to secure the plant in the soil. Other functions usually go in parallel with significant modifications and are discussed further below .
The water is absorbed through the root hairs and the fine lateral roots. Older roots are corked and only serve as a water pipe.
Roots usually develop a negative water potential of only a few tenths of a mega pascal (MPa). Hygrophytes can usually reach a maximum of −1 MPa, mesophytes −4 MPa and xerophytes −6 MPa. Forest trees do not reach more than −2 to −4 MPa.
The root system can only absorb water from the soil as long as the water potential of the fine roots is lower than that of the surrounding soil. The water uptake during a unit of time is proportional to the exchange area (active root area) and the potential difference between root and soil. It is inversely proportional to the transfer resistances for water in the soil (leakage resistance) and at the transition from the soil to the plant (permeation resistance):
The water reaches the endodermis from the ground via the cortical parenchyma. The transport can take place in this area both within the cells (symplastic) or in the intercellular space (in the apoplast). The apoplastic transport in the endodermis is blocked by the Casparian strip. The water only reaches the central body via the symplasts and here into the xylem vessels, from where it reaches the leaves through long-distance transport.
With a few exceptions, plants take up the minerals they need for growth through their roots. The exceptions include the aquatic plants, which absorb water and nutrients over the entire plant surface. In some areas, the uptake of nutrients through the leaves also plays a certain role.
The extraction from the ground takes place through three processes:
- By absorbing nutrient ions from the soil solution : these ions are already freely in solution and are immediately available to the plant. However, the concentrations are usually very low: nitrate often 5 to 10 mmol / l, phosphate mostly below 4 µmol / l.
- By exchange absorption of sorbed nutrient ions: These ions are relatively loosely bound to clay and humic particles . By releasing hydrogen ions and hydrogen carbonate, the dissociation products of respiratory carbon dioxide in water, the plant promotes the ion exchange on these particles. As a result, the nutrient ions go into solution and can be absorbed.
- By mobilizing chemically bound nutrient reserves : This is done by excreting organic acids and chelating agents. The hydrogen ions of the dissociated acids dissolve nutrients from minerals. The chelating agents are organic acids (e.g. malic acid , citric acid ) and phenols (e.g. caffeic acid ), which form metal chelates especially with the important micronutrients such as iron and thus protect them from being redefined.
The nutrient ions first enter the apoplasmic space of the root or root hairs with the water . The ion uptake into the cytoplasm takes place largely in the cortical parenchyma , since the apoplastic transport ends at the Casparian strip of the endodermis . Since the concentration of nutrients in the plant cell is usually higher than in the soil solution, active transport processes are necessary for the ions to be absorbed by the plant cell. These are made possible by the development of a chemiosmotic potential through membrane ATPases . The ions are absorbed by specific ionophores and tunnel proteins. It is through these mechanisms that the plant acquires soil solution enrichment and choice, i.e. that is, it may prefer certain ions present in low concentration over other, more common ions.
The ions are transported symplastically (from cell to cell). The ions are passively released into the main vessels of the xylem , the trachea and tracheids , due to the concentration gradient. In addition, they are actively secreted into the vessels by the adjacent parenchymal cells. From here on the long-distance transport takes place.
Another basic function of the roots is to anchor the plant in the ground. Corresponding to the tensile stress on the root, the fixed elements (xylem) are concentrated in the middle, resulting in a particularly tensile , anatomical structure that corresponds to the cable construction.
Metamorphoses of the Roots
Plants encounter the various ecological conditions of their environment with corresponding transformations and modifications of the basic structure of their organs ( metamorphoses ). The metamorphoses of the root are determined by the special tasks that the root has to perform.
Many plants store reserve substances in their roots, especially plants with pronounced tap roots.
Fast-growing, often poorly anchored trees form buttress roots through excessive secondary growth in the thickness of the tops of roots growing horizontally just below the surface of the earth. This often occurs with trees in the tropical rainforest.
Maize plants form support roots at the lowest nodes of the shoot, as they have no secondary growth in thickness. The support roots prevent it from falling over, as the lower part of the rung alone would be too weak.
Plants growing in swamp or silt form respiratory roots with negative gravitropic growth, pneumatophores , which supply the root system with oxygen via the intercellular system of the cortical tissue. Examples are again the mangrove trees . But breathing knees are also formed by cypress trees.
Some plants, especially epiphytic plants, have the ability to take up water via the aerial roots as they do not reach the water reservoirs of the soil. The aerial roots have a special tissue for this, the velamen radicum . This velamen lies outside the exodermis and contains many cells that died prematurely and therefore many air spaces. These cells soak up rainwater by capillary action and pass it through passage cells in the exodermis into the root body.
Some aerial roots have photosynthetically active chloroplasts in the cortex cells . In some epiphytic orchids the leaves have been reduced and the band-shaped, broadened aerial roots have taken over the task of photosynthesis.
Plant parasites tap into their host plants via root haustoria . The white berry mistletoe forms bark roots in the bast of the host tree, from which it drives sinkers into the sapwood of the host, where it establishes a direct connection to the host's xylem via short trachea. The shedroot ( Lathraea ) taps the xylem of tree roots, while the summerroot species ( Orobanche ) tap the phloem of the host roots and can even cause the root parts above the tapping point to die off. Other parasites with root house houses are eyebright , rattlespot , quail wheat and lice herb .
In certain palm trees , the ends of some aerial roots are transformed into root spines to protect the base of the trunk.
Rhizomes are not part of the subterranean root system, but rather thickened shoot axes , i.e. part of the shoot axis system, which may be kept at a constant depth by contraction of pulling roots. Although their growth is unlimited, their length remains the same, as they die off as new limbs appear at the point of vegetation. The sprouts appear in a different place on the ground every year. Root stocks serve both as a nutrient store and for vegetative reproduction, as they form two plants instead of one after each branch.
Draft roots or contractile roots pull earth sprouts - i.e. bulbs, bulbs or rhizomes - deeper into the soil through root contractions. The cortex cells of these roots are probably stretched lengthways ( axially ), the fibers of the cell walls also run lengthways. Therefore, as the turgor increases, the cells shorten and thicken. The roots are fixed by the thickening, so that the shortening pulls the stems down. This function occurs in many geophytes . However, the exact mechanism has not been conclusively clarified.
So-called "water roots"
Between water roots of plants in hydroponic be drawn and ground roots there is no difference. Both form fine white root hairs that absorb water, the nutrient ions dissolved in it and oxygen for root breathing .
Heterorhizia or different roots means that two different types of roots are formed. There are z. B. next to the nutrient roots separate and morphologically distinguishable traction roots are formed, as in crocuses , or long, non- mycorrhizal and short, mycorrhizal roots as in pines .
In the symbiotic relationship with nitrogen-fixing bacteria (eg. B. Actinomycetales ) leads to the formation of root nodules , enlarged exist in these growths local growths of the cortical tissue, polyploid parenchymal cells in which the symbionts as nodule bacteria or Bacteroides , in particular vacuoles live . This symbiosis enables alders , for example , to live in places with few nutrients.
The symbiosis with the hyphae of soil fungi (mycorrhiza) is even more widespread. Since the hyphae of the fungus have an enormous absorption capacity, roots with hyphae contact do not form root hairs. The mycorrhiza enables some plants, such as the spruce asparagus, to forego photosynthesis of their own, which turns these plants into parasites.
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