This underground plant organ is widely used by humans in economic activities. But sometimes we don’t even realize what kind of metamorphoses we are dealing with.
Types of roots
Roots are the underground parts of a plant. Their structure most often has several parts. The main root develops from the radicle of the embryo. It is clearly expressed and is always the same in the plant. Lateral roots develop on the main root. They are numerous and make the process of water absorption from the soil more intense. Roots that grow directly from the stem are called adventitious. They can also develop lateral ones.
Types of root systems
Plants never form only one type of root. This would not be enough to provide mineral nutrition. The collection of roots of one plant is called the root system.
In dandelion, it is formed by the main root, which penetrates deep into the soil, and lateral ones. Such a root system is called a taproot. It is characteristic of all dicotyledonous plants.
Wheat has many roots. They are all almost the same length and grow in a bunch from the shoot. This root system is called fibrous. Its presence is a systematic feature of monocots.
Why does the root form modifications?
Remember modifications of roots in biology tables (6th grade)? These are carrots, beets, radishes... It turns out, yes. The root performs vital functions in the plant. It provides mineral nutrition, holds it firmly in the soil, and ensures vegetative propagation. But its usual structure is not enough to perform additional functions. Therefore, modifications of the root are formed.
Modifications of roots: table
The types of metamorphosis of the underground organ differ in annual and perennial plants and depend on the place of growth, climate characteristics, and location in relation to the support. The structure and functions of root modifications are presented in the table.
| Root modification | Structural features | Function | Example |
| Roots | Thickening of the main root and lower parts of the stem | Supply of water and organic nutrients | Carrots, beet, rutabaga |
| Root tubers | Thickening of lateral and adventitious roots | Storage, vegetative propagation | Dahlia, chistyak, yam (sweet potato) |
| Trailer roots | Modification of adventitious roots | Attaching to a support | Ivy |
Respiratory
Root vegetables and root tubers provide plants with the necessary nutrition during unfavorable periods. Such modifications of the roots (shown in the table) are formed exclusively in bi- and perennial species. In the first year of development, only vegetative organs are formed from their seeds. In the fall, the stem and leaves die, and the underground part overwinters due to the supply of water and substances. The next year, such plants bear fruit and form seeds.It is the modifications of the roots that help them survive the cold winter. The difference in root systems and growing conditions provides a variety of metamorphoses. Thus, respiratory roots are formed in plants that grow in waterlogged soils. Since their oxygen content is limited, this gas is absorbed directly from the air. This makes the breathing process possible. The plant can grow only under conditions of intense photosynthesis. Sometimes this requires placement even on completely vertical surfaces. For example, ivy can even grow on the walls of houses.
Significance in economy and natureMany of the plant names from the root modification table are familiar to everyone. First of all, these are root vegetables. Humans use them as staple foods and seasonings. These are radishes, parsnips, beets, carrots, parsley, horseradish. Turnips and turnips are used as pet food. And sugar beets are a sought-after raw material for the food industry. Root tubers or cones have accessory buds. Therefore, with their help, vegetative propagation is carried out. These two types of modifications are classified as storage roots. |
Plant organs: their functions, structure and metamorphoses.
Root and root systems. Root metamorphosis.
Stem and shoot. Metamorphoses of the shoot.
Leaf and its metamorphoses.
1.Root and root systems. Root metamorphosis.
Plant organs that serve to maintain the individual life of the plant (root, stem, leaf) are called vegetative. They are in their infancy in every seed.
Generative organs ensure the process of sexual reproduction. A flower is a modified unbranched shoot with limited growth, adapted for sexual reproduction with subsequent formation of seeds and fruit. Flower organs are modified leaves: integumentary leaves form sepals and petals, and spore-forming leaves give rise to stamens and pistils. The structural features of the flower are related to the methods of pollination.
Metamorphoses of vegetative organs.
The main vegetative organs of a plant are the root, stem and leaf. In addition to typical vegetative organs, there are often their modifications that arose in the long process of evolution. These phenomena are otherwise called metamorphosis, which means transformation. Modified organs are sometimes so unique that their origin cannot be immediately determined.
Sometimes the shape of a particular plant organ (for example, a beet root) changes as a result of human activity.
Root morphology and root systems.
The root is a specialized organ of soil nutrition. It performs the following functions:
absorbs water and mineral elements
serves for fixation in the soil;
has motor activity (stretch zone);
may also have reserve functions, taking the form of root tubers (dahlia);
the fulfillment of new functions leads to the emergence of: a) respiratory roots in marsh plants; b) roots - trailers (ivy); c) aerial roots of orchids and other modifications.
But the main function of the root is soil nutrition. This function determines the peculiarity of the structure. Firstly, the root must have as large a surface of contact with soil particles as possible and grow tightly together with them. Secondly, the suction working areas of the root cannot remain in place - they must move, exploring new spaces and overcoming the resistance of dense soil.
Movement in dense soil is made possible by apical root growth and protective devices that allow the delicate apical meristem to push through between soil particles.
The suction tissue performs the most important function of the root - soil nutrition. It consists of a single layer of cells located on the surface of the young root. The entire outer layer of cells covering the young root is called rhizoderm.
The cells of the suction layer have thin membranes and fit tightly to the soil particles. They actively influence the soil and absorb the necessary substances. This activity requires significant energy expenditure, which is ensured, firstly, by a constant influx of organic substances and, secondly, by the intense oxidation of these substances, i.e. breathing with oxygen consumption. Therefore, the system of intercellular spaces filled with gases and facilitating gas exchange is fundamentally well developed.
The cells of the suction layer form long outgrowths - root hairs, which increase the surface of the root several times.
Root hairs appear only at some distance from the root. This is explained by the fact that the area of the root between the hairs and the sheath experiences strong stretching and slides between the soil particles. Any irregularities and protrusions in this area of the root would make it difficult to penetrate the soil.
The first root to appear when a seed germinates is the main root, which develops from the embryonic root. The main root is a first order axis. Lateral roots extend from it, these are second-order axes, third-order roots come from them, etc. As a result, a root system is formed.
On plants, adventitious roots often form from stems or leaves. Their structure and functions are the same as those of the main and lateral roots.
Since the stem is thicker than the root, the boundary between them is usually noticeable. The junction of the stem and the root is called the root collar, and the section of the stem located between the root collar and the cotyledons is called the hypocotyl, or subcotyledon. Adventitious roots often arise from it. Their formation is facilitated by hilling of plants. Due to adventitious roots, the root system increases, which improves the nutrition of the plant and makes it more stable.
The root system can be taproot if the main root stands out among other roots due to its size, and fibrous if the main root is poorly developed and does not differ from the other roots.
The shape of tap roots is: cone-shaped (parsley); turnips (turnips, beets); filamentous (flax sprouts); fusiform (some varieties of carrots).
The length of the roots varies widely. In cultivated cereals, the main mass of them develops in the arable horizon, but individual roots will fall to a depth of 1.5 - 2 m.
The total length of the roots of one rye or wheat plant (without root hairs), grown in the field, is 600 m - 70 km.
There are growing and sucking roots. The first one grows quickly, soon becomes covered with a plug and does not absorb water. Sucking plants grow slowly, remain tender for a long time and absorb soil solutions well. They are the endings of roots of higher orders.
Metamorphoses of roots.
The root crop is formed from the main root due to the deposition of a large amount of nutrients in it. Root crops are formed mainly under conditions of cultural cultivation of plants. They are found in beets, carrots, radishes, etc. In a root crop, they are distinguished: a) a head bearing a rosette of leaves; b) neck – middle part; c) the root itself, from which the lateral roots extend.
Root tubers, or root cones, are fleshy compactions of lateral as well as adventitious roots. Sometimes they reach a very large size and are a receptacle for reserve substances, mainly carbohydrates. In the root tubers of chistyacha, orchids, starch serves as a reserve substance. Inulin accumulates in the adventitious roots of dahlias, which have turned into root tubers.
Among the cultivated plants, the sweet potato belongs to the bindweed family. Its root tubers usually reach 2 - 3 kg, but can be larger. Cultivated in subtropical and tropical areas to produce starch and sugar.
Aerial roots are formed in some tropical plants. They develop as adventitious stems, are brown in color and hang freely in the air. Characterized by the ability to absorb atmospheric moisture. They can be seen in orchids.
Clinging roots, with the help of which weak stems of vines rise up tree trunks, along walls, slopes. Such adventitious roots, growing into the cracks, secure the plant well and give it the opportunity to rise to great heights. This group of vines includes ivy, which is widespread in the Crimea and the Caucasus.
Breathing roots. Swamp plants, to which air access to ordinary roots is very difficult, grow special roots directed upward from the ground. They are located above water and receive air from the atmosphere. Swamp cypress has breathing roots. (Caucasus, Florida).
Organs are similar and homologous.
Charles Darwin introduced the concept of analogous and homologous organs.
Similar organs perform the same functions, but have different origins (hawthorn spines and cactus spines).
Homologous organs - have the same origin, but perform different functions. (Pear thorn, kupena rhizome).
Metamorphoses are hereditary modifications of organs associated with a change in their basic functions. Metamorphoses of vegetative organs of plants are extremely diverse.
Root metamorphosis
One of the most interesting phenomena in the biology of the root is its mutually beneficial cohabitation with fungi. Does this phenomenon have a special name? mycorrhiza (which literally means fungal root) and there is an extensive literature devoted to it. Mycorrhizae are characteristic of the vast majority of flowering plants (probably at least 90%). Due to such a wide distribution, mycorrhiza is the rule rather than the exception, but, following the established tradition, we still consider it as a modification of the root.
On the other hand, many plants, such as orchids, especially those leading a saprophytic lifestyle, are so closely related to fungi that they cannot even develop without being “infected” by a specific mycorrhizal fungus.
Mycorrhizae come in two different types.
The first type includes ectomycorrhiza(external mycorrhiza). In this case, the hyphae of the fungus entwine the roots of the plant with a thick cover (hyphal mantle) and, in addition, penetrate into the intercellular spaces (but not the cells of the cortex). Under the influence of hormones secreted by the fungus, young roots branch profusely and their ends thicken. Ectomycorrhiza is characteristic of many temperate zone trees, including species of oak, birch, willow, maple, conifer, poplar and others. Ectomycorrhiza is formed almost exclusively by basidiomycetes, less often by ascomycetes.
It has a wider, almost universal distribution. endomycorrhiza(internal mycorrhiza). It can be observed, for example, in apple trees, pears, strawberries, tomatoes, cereals, orchids and many other species. It is characteristic of most flowering plants. With endomycorrhiza, a fungal cover does not form around the root, root hairs do not die, but the hyphae penetrate much deeper into the root tissue and penetrate into the cells of the parenchyma of the cortex. There are several different types of endomycorrhizae, some of which are quite different from each other.
The most specialized type of endomycorrhiza can be observed in orchids. In orchids there is no hyphal mantle around the roots and the mycelium is almost entirely located inside the root. The hyphae of the fungus form peculiar balls in the cortex cells. These tangles are subsequently digested by the host plant. Fungi that form orchid mycorrhizae are capable of decomposing complex organic substances and supplying the roots with products of their decomposition. And this is of particular importance with a saprophytic method of nutrition. The fungal component of this type of endomycorrhiza is almost always represented by oomycetes.
Mycorrhiza is of great importance in tropical rain forests. Due to the strong leaching regime (daily precipitation), these forests are practically devoid of soil (all nutrients are washed out of the soil). Plants are faced with an acute problem of nutrition. At the same time, there is a lot of fresh organic matter: fallen branches, leaves, fruits, seeds. But this organic matter is inaccessible to higher plants, and they come into close contact with saprotrophic fungi. Thus, the main source of minerals under these conditions is not the soil, but soil fungi. Minerals enter the root directly from the hyphae of mycorrhizal fungi, which is why rain forest plants have a shallow root system. How effectively mycorrhiza works can be judged by the fact that tropical rain forests are the most productive communities on Earth; the maximum possible biomass develops here.
Cohabitation of plant roots with nitrogen-fixing bacteria is much less common. True, the biological significance of this phenomenon is extremely high. In addition to the well-known legumes, root nodules have also been observed in representatives of other families, for example, in some conifers, in alder, in a number of casuarinaceae, buckthornaceae, and suckers. We will not dwell in detail on this well-known phenomenon.
Typically, a certain amount of reserve nutrients, mainly carbohydrates, is deposited in the roots of any plant: especially starch and sugar; in other species? inulin. But in some cases, the storage function is hypertrophied and comes to the fore. At the same time, the roots thicken and become fleshy. The most common structure is called the root. It is especially characteristic of biennial plants: beets, radishes, rutabaga, turnips, carrots, celery and many others. Exotics include ginseng and chicory. Root crops have a complex morphological nature, since both the root and the stem (more precisely, the hypocotyl) take part in the formation of the root crop. But in order to establish: which part of the root crop is represented by the root, and which by the stem? special anatomical studies are required.
Thickenings in the form of root cones are also found in plants with a fibrous root system, for example, dahlia.
In many cases, the roots are adapted for vegetative propagation. In a number of perennial plants, above-ground shoots develop from root adventitious buds. These shoots are called root shoots. Root shoot plants include: aspen, plum, cherry, lilac, loach, bergenia, sow thistle. The latter are malicious and difficult to eradicate weeds. Small pieces of horses cut during soil cultivation easily take root and give rise to new plants.
In many climbing plants, for example, ivy, special adventitious roots-trailers are formed on the stem, penetrating into cracks and unevenness of wood, rocks, or walls and firmly holding the plants.
There are even hook roots, like the strangler ficus.
The so-called contractile (contracting) or retracting roots have a completely different function. They are characteristic of many rhizomatous, bulbous and corm plants. A classic example of a plant with contractile roots is the crocus. In addition to the usual roots, the crocus develops longer contractile roots, which, when contracted, pull the corm into the ground. In this case, the contractile roots seem to fall off, become transversely wrinkled and, by this feature, are easily distinguished from ordinary roots.
In special cases, roots can be reservoirs for storing water. In many tropical epiphytic orchids (and epiphytes are plants that other plants use as a substrate for growth), the outer part of the bark, called velamen, consists of empty large cells that can absorb water like a sponge. During rainfalls, these cells are filled with water, which is stored in them and used by the plant as needed. By origin, velamen is a multilayered rhizoderm.
Many tropical woody plants, mainly mangroves (for example, Avicenia), growing in freshwater tropical swamps, as well as in shallow waters of ocean coasts, develop special ventilation or respiratory roots? pneumatophores. They appear on underground lateral roots and grow vertically upward, rising above water or soil. It is noteworthy that such roots are characterized by negative geotropism.
The same mangrove plants form another type of roots, called stilts. These are adventitious roots - supports. They appear on trunks and branches and grow downwards, embedded in the substrate and firmly holding the plant, for example, in soft mud. A widespread mangrove plant, rhizophora, has such roots. But the most impressive are the stilted roots of the ficus banyan tree ( Ficus benghalensis). Numerous adventitious roots of the banyan tree grow downwards, take root and develop their own root system. Thanks to this, one banyan tree grows into a whole grove. Such groves can occupy impressive areas.
No less interesting are the supporting plank-shaped roots, characteristic of large trees of the tropical rain forest. For known reasons, rain forest trees have a shallow root system, while at the same time, the trunks of the trees of the first tier reach enormous sizes. The roots of a normal structure cannot anchor the plant in the soil (which, moreover, does not exist), holding these giants during frequent storms and downpours. Therefore, the roots of such trees spreading along the soil surface develop special vertical outgrowths, like boards adjacent to the tree trunk.
At first, the plank-shaped roots are round in cross-section, but then strong one-sided secondary growth occurs. In tropical rain forests, the height of the plank-shaped roots often exceeds human height.
No less peculiar are the adventitious roots of our ordinary marsh plants - they have a “storey” structure. True, these roots do not have a special name.
Metamorphoses of the shoot
The shoot is the most variable organ of the plant. It is characterized by such properties as:
- multifunctionality;
- lability of behavior;
- plastic.
Already to a first approximation, shoots are divided into two types: 1) vegetative and 2) generative.
There is a clear change in the forms of growth and functions of the shoot in the process of its biological development. For example:
- capture of a new area (lash or rhizome);
- enhanced nutrition (rosette stage);
- formation of flowers and fruits (generative stage).
Let us consider the main types of specialized and metamorphosed organs of shoot origin.
Caudex
Develops in perennial herbs and shrubs with a well-developed taproot. Is this a kind of perennial organ of shoot origin? usually lignified lower portions of shoots that develop into a woody taproot.
The caudex bears numerous renewal buds. In addition, the caudex usually serves as a site for the deposition of reserve nutrients.
As a rule, the caudex is underground and quite rare? above ground.
The shoot origin of the caudex can be determined by leaf scars and the regular arrangement of the buds. Caudex differs from rhizomes in the way it dies. Gradual death occurs from the center to the periphery, while the organ divides (cracks) longitudinally into separate sections - particulates. Accordingly, the process of division is called particulation. As a result, a structure is formed, which is often called: multi-headed rhizome, knobby rhizome, multi-headed stem rod, stem root. These names quite accurately reflect the appearance of the caudex and create its image.
It should be noted that particulation is characteristic of old (cyanyl) plants.
The caudex is especially pronounced in semi-desert, desert and alpine plants. In some species, caudexes reach enormous size and weight, for example, in representatives of the genus Pangos up to 15 kg.
Systematically, there are many caudex plants among legumes (alfalfa), umbelliferae (female), and Asteraceae (dandelion, wormwood).
Rhizome
Rhizome or rhizome (root-like)? This is a durable underground shoot that performs the functions of vegetative renewal, reproduction and often deposition of reserves. The rhizome usually does not bear green leaves, but has a clear metameric structure and this differs significantly from the root. The nodes are distinguished by leaf scales, scars or axillary buds.
As a rule, abundant adventitious roots are formed on the rhizome, located in nodes in lobes or singly.
Growing with the apex - the distal part, the rhizome gradually dies off in the old one? proximal part.
Depending on the degree of development of the internodes of the rhizomes, plants are divided into:
- long-rhizome;
- short-rhizome;
- dense turf.
When the rhizome branches, a clump of above-ground shoots is formed, which actually belong to one individual. Such clumps are called a clone.
So many plants are characterized by a clonal growth form. There are two ways to form rhizomes.
In some plants, initially the entire shoot is aboveground. It bears both scale-like and green rosette leaves. Subsequently, the leaves die off, and the stem part is drawn into the soil, where it thickens due to the deposition of reserve substances and turns into a rhizome.
Thus, two phases can be distinguished in the structure of the shoot: aboveground and underground. During ontogenesis, the shoot undergoes a real transformation, metamorphosis in the literal sense. Such rhizomes are called submersible or epigeogenic - aboveground. This picture is observed during the formation of rhizomes: cuff, gravilata, strawberry, lungwort and others.
In other plants, the rhizome begins the growth phase from a bud located underground. Such rhizomes of initially underground origin are called hypogeogenic. They are observed in many perennial herbs and shrubs: wheatgrass, crow's eye, kupena, long-leaved speedwell and others.
In this case, the rhizomes are thin and serve more for vegetative propagation.
Underground stolons and tubers
Tubers are thickened underground shoots like potatoes and Jerusalem artichoke. Do tuberous growths begin to develop at the ends of underground stems? stolons. Stolons are short-lived and are usually destroyed during the growing season, which is why they differ from rhizomes.
In tubers, predominantly parenchyma cells of the core grow. Conductive tissues are very poorly developed and are noticeable at the border of the pith and cortex. The outside of the tuber is covered with a periderm with a thick layer of cork, which helps withstand long winter dormancy.
The leaves on the tuber fall off very early, but leave scars in the form of so-called tuber eyes. Each eye contains 2-3 axillary buds, of which only one germinates. Under favorable conditions, the buds easily germinate, feeding on the reserve substances of the tuber and growing into an independent plant.
Thus, the third leading function of underground shoots? vegetative regeneration and reproduction.
Some plant species produce very distinctive leaf tubers (e.g., thin-leaved core). These are modified leaf blades sitting on the petioles of rhizomes. These leafy tubers have lobes, pinnate veining, and even mesophyll tissue, but are achlorophyll-free and adapted for storing storage starch.
Corms and bulbs
The gladiolus corm is similar in appearance to a bulb. However, a longitudinal section shows that its stem part is highly developed and turned into a tuber containing reserve substances. Numerous adventitious roots appear from the bottom of the corm, forming a fibrous system. Among them there are also contractile (retractable) roots.
The bulb represents another type of greatly shortened underground shoot. In contrast to the tuber, does it have a relatively small stem part? bottom. Numerous succulent leaves are attached to the bottom, overlapping each other and called bulbous scales.
In garden onions, for example, the fleshy scales on the outside are covered with protective membranous dry scales, so the entire bulb of this type is called membranous tunicate or concentric. In lilies, fleshy scales overlap each other imbricately, and the bulb is called imbricate.
The succulent scales of the bulb are only the lower feeding leaves of the shoot. The upper green leaves are located in the apical bud of the bottom.
All bulbs are grouped into two categories: with rhizomes and without rhizomes. Bulbs with rhizomes have the ability to reproduce by shoots: rhizomes grow from the bottom of the bulb, which extend horizontally in the soil and, at some distance from the mother bulb, form a new bulb - shoots. The bulb takes root and may bloom in a few years. Some tulips and wild onions fall into this category.
Bulbs without rhizomes are familiar to everyone because they serve as common planting material in vegetable and floriculture.
In the axils of the succulent leaves, daughter bulbs (babies or cloves) are laid, which develop before flowering. Many bulbs develop, for example, in garlic.
The storage of water in the parenchyma cells of succulent scales is ensured due to the production by these cells of special mucous substances that swell strongly in water and retain water.
In most cases, bulbous plants behave like ephemeroids. Their aboveground shoots exist for a very short time: they appear in early spring and die off in early summer. Plants seem to be “running away from drought.”
Succulents
Succulents are plants that have succulent, fleshy leaves or stems that serve as unique reservoirs for storing moisture. Succulents use this moisture very carefully and economically during the dry period.
Succulents are divided into two large groups:
Stem succulents
Stem succulents have fleshy stems, but the leaves, as a rule, have turned into spines (to reduce transpiration). Examples of stem succulents include the well-known American cacti and African spurges, which are very similar to them.
Leaf succulents
Leaf succulents – have thick, fleshy leaves. These include Crassulaceae: sedum, golden root; liliaceae, amaryllis, agaves, aloe, gasteria, haworthia.
Other shoot metamorphoses
A particularly interesting case of metamorphosis of a bud into a succulent organ—a head of cabbage—is observed in ordinary cultivated cabbage. As you know, cabbage is a biennial plant. In the first year, rosette, slightly succulent leaves appear, then the bud sharply increases in size and turns into a head of cabbage. In the second year after overwintering, cabbage, like a typical biennial, produces an elongated flowering shoot.
Plants have a wide variety of thorns and prickles, which, moreover, have different origins. For example, in cacti and barberries spines are modified leaves. Typically, such spines are intended primarily to reduce transpiration, while the protective function in most cases is secondary.
Other plants (hawthorn, wild apple) have spines of shoot origin- These are modified shortened shoots. Often they begin to develop as normal leafy shoots, and then become woody and lose their leaves.
A further step in the underdevelopment of leaves and the transfer of their functions to green stems leads to the formation of such metamorphosed organs as phylloclades and cladodes.
Phyllocladia
Phyllocladia (Greek phyllon - leaf, klados - branch) are flat leaf-like stems and even entire shoots. The most well-known example of plants that have metamorphoses of this kind are butcher's broom (Ruscus). These plants grow in the Crimea and the Caucasus; They are often bred indoors. It is very interesting that scale-like leaves and inflorescences develop on leaf-like shoots of butcher's broom, which never happens on normal leaves. In addition, phyllocladies, like leaves, have limited growth.
Cladodes
Cladodes are flattened stems that, unlike phyllocladians, have retained the ability for long-term growth. These are quite rare modifications and are found, for example, in Australian muhlenbeckias.
For many climbing plants(peas, chins, pumpkins, etc.) there is a modification of the leaves into tendrils, which have the ability to twist around a support. The stem of such plants is usually thin and weak, unable to maintain an upright position.
creeping plants(strawberries, stone fruits, etc.) form a special type of shoots that serve for vegetative propagation, such as lashes and stolons. They are classified as above-ground creeping plants.
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