A variety of modified plant organs. Modifications of plant organs. Types of root systems

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 has special meaning, 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 not available 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. A number of perennial plants Aboveground 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.
Many climbing plants, such as ivy, have special adventitious roots-trailers that penetrate cracks and unevenness in wood, rocks, or walls and firmly hold 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 tap root. Is this a kind of perennial organ of shoot origin? usually lignified lower portions of shoots that develop into a woody tap root.

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: mantle, 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 on the onion. 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

Special interesting case metamorphosis of a bud into a succulent organ—a head of cabbage—is observed in ordinary cultivated cabbage. As you know, cabbage is 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 often get divorced and room conditions. 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 special type shoots used for vegetative propagation, such as lashes and stolons. They are classified as above-ground creeping plants.

How helpful was this material?

Common modifications of angiosperm shoots are rhizomes, bulbs and tubers. They are usually formed in perennial herbaceous plants as organs in which reserve nutrients are deposited. In such plants, the green above-ground parts die off during the winter, but modified shoots remain in the soil. In the spring, due to the nutrients they contain, the plants again develop ordinary above-ground shoots.

In addition to storing nutrients, modified shoots also perform another function. With their help, plants can reproduce vegetatively.

Rhizome

Modified escape rhizome found in many perennial plants (nettle, lily of the valley, wheatgrass, etc.). The rhizome is located in the upper layers of the soil, resembles a root, but spreads horizontally.

The rhizome is a shoot, since it has apical and axillary buds, as well as leaves that are modified into scales. The resemblance to a root is given to the rhizome by adventitious roots that grow from it along its entire length.

During the growing season, the plant deposits reserve nutrients in the rhizome. Due to them, new young shoots grow from the buds of the rhizome next year.

With the help of parts of the rhizome containing buds and roots, vegetative propagation of plants is possible.

Bulb

Modified escape bulb characteristic of onions, tulips, lilies and other plants. At the bottom of the bulb there is a flattened stem called bottom. Two types of leaves, modified into scales, grow from the bottom. The outer leaves are changed into dry scales, which perform a protective function. The inner thick and juicy scales contain reserve nutrients (the bulbs contain many different sugars, among other substances) and water. The bulbs also grow buds from the bottom.

In favorable conditions, adventitious roots grow from below the bottom of the bulb, resulting in the formation of a fibrous root system. Shoots can grow from the buds, but they can also develop into so-called baby bulbs. Each such bulb can give rise to a separate new plant. Thus, vegetative propagation is carried out using bulbs.

Tuber

Modified escape tuber can be observed in plants such as potatoes and Jerusalem artichokes, as well as some others.

The tuber is formed at the top of another modified shoot - stolon. Stolons grow from the lower parts of above-ground shoots and go into the soil. Organic substances synthesized by the green parts of the plant during photosynthesis move along the stolons to their tops, and thus tubers are formed here. A lot of starch accumulates in the tubers.

A tuber is, although modified, a shoot. It has short, but thick internodes and many buds, which are called eyes. The leaves of the tubers are reduced. The eyes are located in the recesses of the tuber, and in each such recess there may be several eye-buds.

The part of the tuber that is connected to the stolon is called the base of the tuber. On the opposite side of the base is the top of the tuber. There are more eyes closer to the top. Most often, the apical eye-bud develops into a young green shoot.

Slide 3

Root zones