Lichens are symbiotic associations mushrooms (mycobiont) and microscopic green algae and/or cyanobacteria (photobiont, or phycobiont); the mycobiont forms a thallus (thallus), inside which the photobiont cells are located. The fungus in this case is either marsupial or basidial, and the algae is either green or blue-green. Lichens usually settle on bare rocks or tree trunks. Algae supplies the fungus with organic products of photosynthesis, and the fungus provides water and mineral salts.
Lichens grow very slowly and are sensitive to environmental pollution, so they are an ideal indicator of atmospheric pollution, especially sulfur dioxide. Lichen thallus has different shapes, sizes and colors.
The attachment organs of lichens are rhizoids and rizina (connected in strands of rhizoids).
Diversity of lichens
Lichens are white, grey, yellow, orange, green, black ; this is determined by the nature of the pigment in the hyphal sheath. Pigmentation helps to protect against excessive light or, conversely, helps to absorb more light (the black pigment of Antarctica lichens).
According to the form and nature of attachment to the substrate, three groups lichens:
- scale forms - have the appearance of a crust or plaque, tightly adhering to the substrate (edible lecanora, graphis, lecidea);
- leafy forms - have the form of plates with dissected, branching lobes; their resemblance to the leaves is very distant (xanthoria - wall goldfish, parmelia);
- bushy lichens - upright or hanging bushes. (cladonia, reindeer moss - deer moss, cetraria - Icelandic moss, bearded man).
According to the anatomical structure, lichens are divided into homeomeric (algae scattered throughout the body of the lichen) and heteromeric (algae form a separate layer in the thallus).
Lichens with heteromeric thallus are the majority. In a heteromeric thallus, the top layer is cortical composed of fungal hyphae. It protects the thallus from drying out and mechanical influences. The next layer from the surface - gonidial, or algal, it contains a photobiont. Located in the center core, consisting of randomly intertwined hyphae of the fungus. Moisture is mainly stored in the core, it also plays the role of a skeleton. At the lower surface of the thallus is often located lower cortex, with the help of the outgrowths of which ( rizin) the lichen is attached to the substrate. A complete set of layers is not found in all lichens.
Lichen reproduction
Reproduction of lichens occurs by spores or vegetatively: fragments of the thallus (isidia and soredia). Sexual reproduction is provided by special sections of the thallus that form spores. The spore germinates into a hyphae, and upon encountering a suitable algae, a new lichen is formed.
The role of lichens in nature and human life
The role of lichens in nature hard to overestimate. They are "pioneers" in the formation of plant communities. By releasing organic acids, lichens destroy the parent rock, and when their organics die, together with it, they form the primary soil on which plants can settle. Lichens serve as food for many animals (reindeer moss or reindeer moss), are a habitat for many invertebrates.
Role in human life. Lichens serve as indicators of air pollution. Some species are used by humans for food (lichen manna). Also, lichens are used in industry (making litmus), in perfumery (obtaining aromatic substances), in the pharmaceutical industry (obtaining drugs against tuberculosis, furunculosis, epilepsy, etc.). Lichen acids also have antibiotic properties.
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Walking through the forest, on stones, snags, trees, you can notice relief outgrowths or “bushes” of different colors and shapes. This is what a lichen looks like. For a long time he was a real mystery to doctors and researchers of nature. Since ancient times, people have used lichens in medicine, eaten, dyed fabrics with their help. The science that studies lichens is called lichenology. This article discusses the general characteristics of lichens as organisms.
Lichens belong to the kingdom of fungi, but scientists consider them as a separate unique group. There are many in nature, but at the moment approximately 25 thousand species have been discovered.
The body of the plant is called the thallus, thallus, slan. The variety of its colors, shapes and sizes is amazing. The thallus can grow with a crust and a leaf-like plate, as well as a bush, tube or ball. The plant can be as tall as a human being, or it can measure from 3 to 7 cm.
Lichens grow incredibly slowly, scientists have discovered a specimen that is over 4,000 years old.
All lichens are divided by lichenology into three groups depending on the shape of the thallus. The first group - scale (crustal), looks like a crust that fits snugly to the surface of the place where it grows. Representatives of this group are located on rocks and stones.
The second group, leafy, located on wood, soil and stones, are similar to plates and have wavy edges. They are firmly attached to the surface with a short, thick leg.
The third group, bushy, as you might guess from the name, look like a standing and hanging bush, branched or not. Such bushes grow on the soil, to which they are attached with the help of filamentous rhizoids. They also grow on the branches of trees, to which they attach themselves with the help of several sections of the thallus.
According to the place of growth, lichens can be divided into epigeic (on the soil), epiphytic (on tree trunks and branches) and epilithic (on stones and rocks). These plants come in a wide variety of colors.
The bark, which is a dense formation of mycelium hyphae, is covered with pores. With the help of which the plant breathes. With the help of the bark, lichens also absorb moisture from the air and protect themselves from hypothermia and overheating.
Internal structure
A lichen is an organism composed of mycelium and algae (sometimes cyanobacteria). What are the structural features of lichens, you can find out by examining the plant under a microscope. Already at a magnification of 15 × 8, one can see in it how the threads of the mycelium braid the algae cells.
Nutrition system and reproduction
The nutrition of lichens occurs due to the vital activity of both symbionts. The mycelium absorbs water and absorbs the nutrients contained in it, and the algae (cyanobacteria) feeds on chlorophylls and photosynthesis. As mentioned above, algae are classified as autotrophic organisms, that is, capable of synthesizing organic substances from inorganic ones, and fungi are heterotrophic, which do not have the ability to photosynthesis or chemosynthesis. The fact that these two organisms exist side by side is a distinctive feature of lichens as a species.
Lichen is a plant that reproduces vegetatively and sexually. During sexual reproduction, the plant forms spores, which, during germination, expect to meet with a suitable algae in order to form a new thallus with it.
For vegetative propagation in some lichens, special isidia can be found that look like small processes or twigs. They break off easily, a new thallus is formed from them. Some plants of this species form soredia that are easily dispersed by the wind. Soredia is an algae cell that is densely entwined with hyphae.
Spreading
Lichen is a bioindicator organism. This definition is very suitable for this species. After all, they grow only in places with good environmental conditions. Therefore, in a city polluted by the waste of cars and enterprises, you will never meet this plant. As soon as harmful impurities appear in the air, it dies.
The lichen can settle in conditions where any other plant would not survive.. Thanks to their bark, they absorb every molecule of water from any available source: fog, dew, air. Their habitat can be the tundra, the tropics, the swamp and even the desert. They are one of the few plants in Antarctica.
Role in nature and human life
Lichens are pioneers in colonizing the surfaces of bare rocks and stony soils. They contribute to the process of destruction of rocks with the help of acids that they produce. After death, they take part in the process of soil formation, serve as food for various organisms. Lichens, located on the branches and trunks of trees, are an excellent defense against fungal pests that penetrate into the bark of a tree and destroy it from the inside.
Reindeer moss and reindeer moss are of great importance. During the winter months, these plants are the only food for the reindeer. The rest of the ungulates also do not deprive the attention of multi-colored thalli. Yet, half of this plant is a mushroom, which, as you know, are a source of proteins and vitamins.
Certain types serve as the basis of some dishes. For example, in Iceland, when bread is baked, lichen powder is added to the flour. In Japan, some lichens are considered a real delicacy.
In ancient Egypt, lichens were used to treat diseases, and in the 18th century, they are mentioned in many official reference books of medicines. All this is due to the ability to kill pathogens.
These unusual plants have found their way into the perfume industry to create unique fragrances. In the textile industry, they are used as natural dyes, while the chemical and food industries use them as sources of alcohol and sugars.
Lichens are a very interesting and peculiar group of lower plants. Lichens (lat. Lichenes) - symbiotic associations of fungi (mycobiont) and microscopic green algae and / or cyanobacteria (photobiont, or phycobiont); the mycobiont forms a thallus (thallus), inside which the photobiont cells are located. The group includes from 17,000 to 26,000 species in about 400 genera. And every year, scientists discover and describe dozens and hundreds of new unknown species.
Fig.1. Lichen Cladonia stellate Cladonia stellaris
The lichen combines two organisms with opposite properties: an algae (usually green), which creates organic matter in the process of photosynthesis, and a fungus that consumes this substance.
As organisms, lichens were known to scientists and the people long before the discovery of their essence. Even the great Theophrastus (371 - 286 BC), "the father of botany", gave a description of two lichens - usnea (Usnea) and rocella (Rocce11a). The latter was already used to obtain dyes. The beginning of lichenology (the science of lichens) is considered to be 1803, when the student of Carl Linnaeus, Eric Acharius, published his work “Methodus, qua omnes detectos lichenes ad genera redigere tentavit” (“Methods by which everyone can identify lichens”). He identified them as an independent group and created a system based on the structure of the fruiting bodies, which included 906 species described at that time. The first to point out the symbiotic nature in 1866, using the example of one of the species, was the physician and mycologist Anton de Bari. In 1869, the botanist Simon Schwendener extended these ideas to all species. In the same year, Russian botanists Andrei Sergeevich Famintsyn and Osip Vasilievich Baranetsky discovered that the green cells in lichen are unicellular algae. These discoveries were perceived by contemporaries as "amazing".
Lichens are divided into three unequal groups:
1. It includes a greater number of lichens, a class of marsupial lichens, since they are formed by marsupial fungi
2. A small group, a class of basidial lichens, since they are formed by basidial fungi (less resistant fungi)
3. “Imperfect lichens” got their name due to the fact that fruiting bodies with spores were not found in them.
External and internal structure of lichens
The vegetative body of the lichen - thallus, or thallus, is very diverse in shape and color. Lichens are painted in a variety of colors: white, pink, bright yellow, orange, orange-red, gray, bluish-gray, grayish-green, yellowish-green, olive brown, brown, black and some others. The color of the lichen thallus depends on the presence of pigments that are deposited in the hyphae membranes, less often in the protoplasm. The hyphae of the crustal layer of lichens and various parts of their fruiting bodies are the richest in pigments. Lichens have five groups of pigments: green, blue, purple, red, brown. The mechanism of their formation has not yet been elucidated, but it is quite obvious that the most important factor influencing this process is light.
Sometimes the color of the thallus depends on the color of lichen acids, which are deposited in the form of crystals or grains on the surface of the hyphae. Most lichen acids are colorless, but some of them are colored, and sometimes very brightly - in yellow, orange, red and other colors. The color of the crystals of these substances determines the color of the entire thallus. And here the most important factor contributing to the formation of lichen substances is light. The brighter the lighting in the place where the lichen grows, the brighter it is colored. As a rule, lichens of the highlands and polar regions of the Arctic and Antarctic are very brightly colored. This is also related to lighting conditions. The high-mountain and polar regions of the globe are characterized by a high transparency of the atmosphere and a high intensity of direct solar radiation, which provide significant illumination here. Under such conditions, a large amount of pigments and lichen acids are concentrated in the outer layers of thalli, causing the bright color of lichens. It is believed that the colored outer layers protect the underlying algae cells from excessive light intensity.
Due to the low temperature, precipitation in Antarctica falls only in the form of snow. In this form, they cannot be used by plants. This is where the dark color of lichens comes to their aid.
Dark-colored thalli of Antarctic lichens, due to high solar radiation, quickly heat up to a positive temperature even at negative air temperatures. The snow falling on these heated thalli melts, turning into water, which the lichen immediately absorbs. Thus, it provides itself with the water necessary for the implementation of the processes of respiration and photosynthesis.
How diverse are the thalli of lichens in color, they are just as diverse in shape. The thallus may take the form of a crust, a leaf-shaped plate or a bush. Depending on the appearance, there are three main morphological types:
Scale. The thallus of scale lichens is a crust (“scale”), the lower surface is tightly fused with the substrate and does not separate without significant damage. This allows them to live on steep mountain slopes, trees, and even on concrete walls. Sometimes scale lichen develops inside the substrate and is completely invisible from the outside. As a rule, scale thalli are small in size, their diameter is only a few millimeters or centimeters, but sometimes it can reach 20–30 cm. rocks or tree trunks large spots, reaching a diameter of several tens of centimeters.
Leafy. Leafy lichens have the form of plates of various shapes and sizes. They are more or less tightly attached to the substrate with the help of outgrowths of the lower cortical layer. The simplest thallus of leafy lichens has the appearance of one large rounded leaf-shaped plate, reaching a diameter of 10–20 cm. Such a plate is often dense, leathery, painted in dark gray, dark brown or black.
bushy. According to the organizational level, fruticose lichens represent the highest stage in the development of the thallus. In fruticose lichens, the thallus forms many rounded or flat branches. Grow on the ground or hang from trees, wood debris, rocks. The thallus of fruticose lichens has the appearance of an upright or hanging bush, less often unbranched upright outgrowths. This allows fruticose lichens to take the best position by bending branches in different directions, in which algae can maximize the use of light for photosynthesis. Thallus of fruticose lichens can be of different sizes. The height of the smallest is only a few millimeters, and the largest is 30-50 cm. Hanging thalli of fruticose lichens can sometimes reach colossal sizes.
The internal structure of the lichen: crustal layer, gonidial layer, core, lower cortex, rhizoids. The body of lichens (thallus) is an interweaving of fungal hyphae, between which there is a population of photobiont.
Rice. 2. Anatomical structure of the lichen thallus
1 - heteromeric thallus (a - upper crustal layer, b - algae layer, c - core, d - lower crustal layer); 2 - homeomeric thallus of the slimy collema lichen (Collema flaccidum); 3 - homeomeric thallus of the slimy lichen leptogium (Leptogium saturninum) (a - crustal layer from the upper and lower sides of the thallus, b - rhizoids)
Each of the listed anatomical layers of the thallus performs a specific function in the life of a lichen and, depending on this, has a completely specific structure.
The crustal layer plays a very important role in the life of the lichen. It performs two functions at once: protective and strengthening. It protects the inner layers of the thallus from the effects of the external environment, especially algae from excessive lighting. Therefore, the crustal layer of lichens is usually dense in structure and is colored grayish, brown, olive, yellow, orange or reddish. The crust layer also serves to strengthen the thallus. The higher the thallus rises above the substrate, the more it needs to be strengthened. Strengthening mechanical functions in such cases are often performed by a thick crustal layer. Attachment organs usually form on the lower crustal layer of lichens. Sometimes they look like very thin threads, consisting of one row of cells. These threads are called rhizoids. Each such thread originates from one cell of the lower crustal layer. Often, several rhizoids are combined into thick rhizoidal strands.
In the zone of algae, the processes of assimilation of carbon dioxide and the accumulation of organic substances are carried out. As you know, for the implementation of photosynthesis processes, algae do not bypass sunlight. Therefore, the layer of algae is usually located near the upper surface of the thallus, directly under the upper crustal layer, and in vertically standing fruticose lichens, also above the lower crustal layer. The layer of algae is most often thin, and the algae are placed in it so that they are in almost the same lighting conditions. Algae in the lichen thallus can form a continuous layer, but sometimes mycobiont hyphae divide it into separate sections. To carry out the processes of carbon dioxide assimilation and respiration, algae also need normal gas exchange. Therefore, fungal hyphae in the zone of algae do not form dense plexuses, but are located loosely at some distance from each other.
Under the layer of algae is the core layer. Usually the core is much thicker than the crustal layer and the algae zone. The thickness of the thallus itself depends on the degree of development of the core. The main function of the core layer is to conduct air to algae cells containing chlorophyll. Therefore, most lichens are characterized by a loose arrangement of hyphae in the core. The air entering the thallus easily penetrates to the algae through the gaps between the hyphae. The core hyphae are weakly branched, with sparse transverse septa, with smooth, weakly gelatinous thick walls and a rather narrow lumen filled with protoplasm. In most lichens, the core is white, since the hyphae of the core layer are colorless.
According to the internal structure, lichens are divided into:
Homeomeric (Collema), photobiont cells are randomly distributed among fungal hyphae throughout the entire thickness of the thallus;
Heteromeric (Peltigera canina), thallus in cross section can be clearly divided into layers.
Lichens with heteromeric thallus are the majority. In the heteromeric thallus, the upper layer is cortical, composed of fungal hyphae. It protects the thallus from drying out and mechanical influences. The next layer from the surface is gonidial, or algal, in which the photobiont is located. In the center is the core, consisting of randomly intertwined hyphae of the fungus. Moisture is mainly stored in the core, it also plays the role of a skeleton. At the lower surface of the thallus there is often a lower bark, with the help of outgrowths of which (rhizine) the lichen is attached to the substrate. A complete set of layers is not found in all lichens.
As in the case of two-component lichens, the algal component - phycobiont - of three-component lichens is evenly distributed over the thallus, or forms a layer under the upper bark. Some three-component cyanolichens form specialized superficial or internal compact structures (cephalodia) in which the cyanobacterial component is concentrated.
Lichen feeding methods
Lichens are a complex object for physiological studies, since they consist of two physiologically opposite components - a heterotrophic fungus and an autotrophic algae. Therefore, it is first necessary to separately study the vital activity of the myco- and phycobiont, which is done with the help of cultures, and then the life of the lichen as an integral organism. It is clear that such a “triple physiology” is a difficult path of research, and it is not surprising that there is still a lot of mystery in the life of lichens. However, the general patterns of their metabolism are still elucidated.
Quite a lot of research is devoted to the process of photosynthesis in lichens. Since only a small part of their thallus (5 - 10% of the volume) is formed by algae, which, nevertheless, is the only source of supply of organic substances, a significant question arises about the intensity of photosynthesis in lichens.
Measurements have shown that the intensity of photosynthesis in lichens is much lower than in higher autotrophic plants.
For normal photosynthetic activity, the thallus must contain a certain amount of water, depending on the anatomical and morphological type of lichen. In general, in thick thalli, the optimal water content for active photosynthesis is lower than in thin and loose thalli. At the same time, it is very significant that many species of lichens, especially in dry habitats, are generally rarely or at least very irregularly supplied with an optimal amount of intrathallus water. After all, the regulation of the water regime in lichens occurs in a completely different way than in higher plants that have a special apparatus that can control the receipt and consumption of water. Lichens assimilate water (in the form of rain, snow, fog, dew, etc.) very quickly, but passively with the entire surface of their body and partly with the rhizoids of the underside. This absorption of water by the thallus is a simple physical process, such as the absorption of water by filter paper. Lichens are able to absorb water in very large quantities, usually up to 100 - 300% of the dry mass of the thallus, and some slimy lichens (kollems, leptogiums, etc.) even up to 800 - 3900%.
The minimum water content in lichens under natural conditions is approximately 2 - 15% of the dry mass of the thallus.
The release of water by the thallus also occurs quite quickly. Lichens saturated with water in the sun after 30-60 minutes lose all their water and become brittle, that is, the water content in the thallus becomes lower than the minimum required for active photosynthesis. From this follows a kind of “arrhythmia” of lichen photosynthesis - its productivity changes during the day, season, a number of years, depending on general environmental conditions, especially hydrological and temperature ones.
There are observations that many lichens photosynthesize more actively in the morning and evening hours and that photosynthesis continues in them in winter, and in ground forms even under a thin snow cover.
An important component in the nutrition of lichens is nitrogen. Those lichens that have green algae as a phycobiont (and most of them) perceive nitrogen compounds from aqueous solutions when their thalli are saturated with water. It is possible that lichens also take part of the nitrogenous compounds directly from the substrate - soil, tree bark, etc. An ecologically interesting group is the so-called nitrophilic lichens growing in habitats rich in nitrogenous compounds - on "bird stones", where there is a lot of bird excrement , on tree trunks, etc. (types of xanthoria, fiscia, caloplaki, etc.). Lichens that have blue-green algae (especially nostocs) as a phycobiont are able to fix atmospheric nitrogen, since the algae contained in them have this ability. In experiments with such species (from the genera collema, leptogium, peltiger, lobaria, stikta, etc.), it was found that their thalli quickly and actively absorb atmospheric nitrogen. These lichens often settle on substrates that are very poor in nitrogenous compounds. Most of the nitrogen fixed by the algae goes to the mycobiont, and only a small part is used by the phycobiont itself. There is evidence that the mycobiont in the lichen thallus actively controls the assimilation and distribution of nitrogenous compounds fixed from the atmosphere by the phycobiont.
The rhythm of life described above is one of the reasons for the very slow growth of most lichens. Sometimes lichens grow only a few tenths of a millimeter per year, mostly less than one centimeter. Another reason for slow growth is that the photobiont, often making up less than 10% of the volume of the lichen, takes over the supply of nutrients to the mycobiont. In good conditions, with optimal humidity and temperature, such as in cloudy or rainy tropical forests, lichens grow several centimeters per year.
The growth zone of lichens in scale forms is located along the edge of the lichen, in foliose and fruticose forms at each top.
Lichens are among the longest living organisms and can be several hundred years old, and in some cases over 4500 years old, such as Rhizocapron geographicum living in Greenland.
Lichen reproduction
Lichens reproduce either by spores, which are formed by the mycobiont sexually or asexually, or vegetatively - by fragments of the thallus, soredia and isidia.
During sexual reproduction on lichen thalli, as a result of the sexual process, sexual sporulation is formed in the form of fruiting bodies. Among the fruit bodies in lichens, apothecia, perithecia and gasterothecia are distinguished. Most lichens form open fruiting bodies in the form of apothecia - disc-shaped formations. Some have fruiting bodies in the form of perithecium - a closed fruiting body that looks like a small jug with a hole at the top. A small number of lichens form narrow elongated fruiting bodies, which are called gasterothecia.
In apothecia, perithecia and gasterothecia, spores develop inside bags - special sac-like formations. Lichens that form spores in bags are combined into a large group of marsupial lichens. They originated from fungi of the class Ascomycetes and represent the main evolutionary line of development of lichens.
In a small group of lichens, spores are formed not inside the bags, but exogenously, on top of elongated club-shaped hyphae - basidia, at the ends of which four spores develop. Lichens with such spore formation are combined into a group of basidial lichens.
The female genital organ of lichens - the archicarp - consists of two parts. The lower part is called askogon and is a spirally twisted hyphae, thicker than other hyphae and consisting of 10–12 one- or many nuclear cells. Trichogyne extends upward from the askogon - a thin, elongated hypha that passes through the algae zone and the crustal layer and emerges on the surface of the thallus, towering above it with its sticky top.
The development and maturation of the fruit body in lichens is a very slow process that lasts 4 to 10 years. The formed fruiting body is also perennial, capable of producing spores for a number of years. How many spores are capable of producing lichen fruiting bodies? It has been calculated, for example, that in the lichen Solorin, 31 thousand bags are formed in an apothecia with a diameter of 5 mm, and 4 spores usually develop in each bag. Therefore, the total number of spores produced by one apothecia is 124,000. Within one day, from 1200 to 1700 spores are ejected from such an apothecia. Of course, not all spores thrown out of the fruiting body germinate. Many of them, once in adverse conditions, die. For spore germination, first of all, sufficient humidity and a certain temperature are necessary.
In lichens, asexual sporulation is also known - conidia, pycnoconidia and stylospores that occur exogenously on the surface of conidiophores. At the same time, conidia are formed on conidiophores that develop directly on the surface of the thallus, and pycnoconidia and stylospores in special receptacles - pycnidia.
From asexual sporulation, lichens most often form pycnidia with pycnoconidia. Pycnidia are often found on the thalli of many fruticose and leafy lichens, less often they can be observed in scale forms.
In each of the pycnidia, small unicellular spores, pycnoconidia, are formed in large numbers. The role of these widespread sporulation in the life of the lichen has not yet been elucidated. Some scientists, calling these spores spermatozoa, and pycnidia - spermagonia, consider them male germ cells, although there is still no experimental or cytological data proving that pycnoconidia are really involved in the sexual process of lichens.
vegetative reproduction. If scale lichens, as a rule, form fruiting bodies, then among the more highly organized leafy and bushy lichens there are many representatives that reproduce exclusively by vegetative means. In this case, such formations are more important for the reproduction of lichens, in which fungal hyphae and algae cells are simultaneously present. These are soredia and isidia. They serve to reproduce the lichen as a whole organism. Once in favorable conditions, they give rise directly to a new thallus. Soredia and isidia are more common in foliose and fruticose lichens.
Soredia are tiny formations in the form of dust particles, consisting of one or more algae cells surrounded by fungal hyphae. Their formation usually begins in the gonidial layer. Due to the mass formation of soredia, their number increases, they put pressure on the upper bark, tear it and end up on the surface of the thallus, from where they are easily blown away with any movement of air or washed off with water. Clusters of soredia are called sorals. The presence and absence of soredia and sorals, their location, shape and color are constant for certain lichens and serve as a defining feature.
Sometimes, when lichens die, their thallus turns into a powdery mass consisting of soredia. These are the so-called leprosy forms of lichens (from the Greek word "lepros" - "rough", "uneven"). In this case, it is almost impossible to determine the lichen.
Soredia, carried by wind and rainwater, once in favorable conditions, gradually form a new thallus. The renewal of a new thallus from the soredia is very slow. Thus, in species of the genus Cladonia, normal scales of the primary thallus develop from the soredia only after a period of 9 to 24 months. And for the development of a secondary thallus with apothecia, it takes from one to eight years, depending on the type of lichen and external conditions.
Isidia are found in a smaller number of lichen species than soredia and sorals. They are simple or coral-branched outgrowths, usually densely covering the upper side of the thallus (see figure). Unlike sorals, isidia are covered with bark on the outside, often darker than thallus. Inside, under the bark, they contain algae and fungal hyphae. Isidia easily break off from the surface of the thallus. Breaking off and spreading with the help of rain and wind, they, like soredia, can, under favorable conditions, form new lichen thalli.
Many lichens do not form apothecia, soredia and isidia and multiply in thallus areas that are easily broken off from lichens that are fragile in dry weather by wind or animals and are also carried by them. Particularly widespread is the reproduction of lichens in areas of the thallus in the Arctic regions, representatives of the genera Cetraria and Cladonia, many of which almost never form fruiting bodies.
Definition
Lichens- symbiotic associations of fungus and algae
Lichen structure
The vegetative body of a lichen is called thallus.
The hyphae of the fungus form the basis of the thallus, forming the lower crust attached to the substrate, and the surface crustal layer, causing the shape and color of the lichen.
Seaweed occupying cavities between hyphae,form a gonidial (algal) layer.
Under a layer of algae, fungal hyphae are located loosely, large gaps between them are filled with air - this is the core. The core is followed by the lower crust, which is similar in structure to the upper. Bundles of hyphae (rhizoids) pass through the lower cortex from the core, which attach the lichen to the substrate.
Crustose lichens do not have a lower bark, and the fungal hyphae of the core fuse directly with the substrate.
Rice. Lichen structure
Bark functions:
protective;
support;
attaching (nand rhizoids form in the lower crustal layer);
gas exchange (through perforations (dead areas of the crustal layer), cracks and breaks in the crustal layer).
Algae zone function:
photosynthesis;
accumulation of organic matter.
Core function:
conducting air to algae cells;
support function (atsome bushy lichens).
Lichens form mainly:
fungi - ascomycetes and basidiomycetes;
algae - most often green (cyanobacteria are less common).
The essence of symbiosis:
Algae gives the fungus organic substances obtained during photosynthesis.
The fungus, having an extensive mycelium, provides the algae with water and minerals.
Such symbioses of certain types of fungus and algae are so stable that they are perceived as a certain type of organism.
Lichen classification
According to the shape of the thallus, lichens are divided into:
scale: attached to the substrate with its entire surface (rhizocarpon);
leafy: attached to the substrate at separate points (parmelia, xanthoria).
In some leaf lichens, the thallus is attached with a short stalk. (gomfa), located in the central part of the thallus.
bushy:
attached at one point and branching (cladonia, reindeer moss, sleeping).
Bushy radially built lichens have a bark on the periphery of the transverse section, a gonidial layer under it, and a core inside.
Crustaceous lichens grow with their periphery, and fruticose ones grow with the ends of "twigs".
Rice. Scale lichen Fig. leafy lichen
Rice. fruticose lichen
Lichen reproduction
Lichens have vegetative, sexual and asexual reproduction.
Asexual reproduction:
fragmentation;
soredia- microscopic glomeruli, consisting of one or more algae cells surrounded by fungal hyphae; are formed inside the thallus, and after maturation they come to the surface and burst, scattering diaspores;
isidia- small, variously shaped outgrowths of the upper surface of the thallus, breaking off when ripe.
In both cases, the detachable structure includes both fungal and algal components.
Sexual reproduction:
the formation of fruiting bodies of various shapes, where spores of sexual reproduction ripen. The development and maturation of the fruiting body can last up to 10 years, and then for a number of years the fruiting body is able to produce spores. A lot of spores form, but not all of them germinate. For germination, conditions are needed, primarily certain temperature and humidity.
Features of the ecology of lichens
Lichens are characterized by very slow growth:from fractions of a millimeter to several centimeters per year.Most likely, this is due to the small relative volume of autotrophic algae synthesizing organic substances.
The lichens of tropical forests have the highest growth rate, the inhabitants of rocks and tundra have the lowest.
The low growth rate leads to the fact that lichens mainly grow in places where they do not meet competition from plants.First of all, these are mountainous areas, where they are pioneers on stones and rocks, creating primary soils.Lichens do not meet competitors in the tundra, where due to frozen soils plant roots cannot develop.Often lichens grow as epiphytes in tree crowns.
The ability of the fungus to absorb and retain water allows lichens to exist in extremely dry conditions. They can absorb water not only during rains, but also from fog and air saturated with water vapor.
Interestingly, the age of the thallus is often several hundred and thousands of years.
Many lichens are very demanding on the purity of the air, so the species diversity of lichens in cities is significantly lower than in the wild.
The meaning of lichens
formation of primary soils in primary biogeocenoses;
main producersin tundra communities.
Human use of lichens:
lichens are food for tundra reindeer herding;
some types of lichens are eaten;
are raw materials for the production of dyes (for example, litmus);
used in folk medicine (for example, sleeping);
are used in environmental monitoring due to their high sensitivity to chemical pollutants.
Lichens are a group of living organisms.
Their body is built using a combination of two microorganisms that are in a symbiotic relationship: a fungus (mycobiont) and an algae (phycobiont or cyanobacteria).
general characteristics
The science of lichenology, which is a department of botany, is engaged in the study of this species.
For a long time, lichens were a mystery to scientists, although their use was widespread throughout various sectors of human life. And only in 1867 the structure of this species was scientifically proven. Scientists-lichenologists were engaged in this.
At the moment, scientists have discovered more than 25 thousand species, but they all have a similar external and internal structure. The features by which each species should be distinguished are based on structural features.
What does a lichen look like
As already mentioned, the main part of the species is the body, which is distinguished by a variety of shapes and colors. In this case, growth can be a plate, a crust that looks like a leaf, in the form of a bush, tube or ball.
The height of the plant also varies within fairly large limits: from 3 centimeters to the height of a person.
Types and names of lichens
Lichenology has divided lichens into several groups in connection with the shape of the thallus:
In addition, based on the place where they grow, there are:
- epigeic (mainly on a land basis);
- epiphytic (on a tree base);
- epilithic (on stone).
Features of the internal structure
It seems possible to see the structure of the lichen under a magnifying device. A lichen is an organism consisting of a part of a fungus - a mycelium and algae, intertwined with each other.
Depending on how the cells of algae and fungi are distributed among themselves, another classification is distinguished:
- homeomeric, in which the phycobiont is located randomly among the cells of the mycobiont;
- heteromeric, in which there is a clear separation into layers.
Lichens with a layered structure are found everywhere and have the following layer structure:
- The cortical layer is composed of mycobiont cells and protects from external influences, especially from drying out.
- Superficial or gonidial: contains only phycobiont cells.
- The core consists of a fungus, performs the function of a skeleton, and also contributes to the retention of water.
- The lower cortex performs the function of attachment to the base.
It is worth noting: in some species, some types of layers may be absent or have a modified structure.
Where do they live
Lichens are distinguished by their ability to adapt to absolutely any conditions of existence. For example, they grow on bare stones, rocks, walls and roofs of buildings, tree bark, etc.
This is due to the mutually beneficial cooperation of the myco- and phycobionts included in the composition. The vital activity of one complements the existence of the other, and vice versa.
How do lichens eat
Nutrition is provided by symbiotes. Since fungi do not have the function of autotrophic nutrition, in which the process of converting organic components from inorganic ones takes place, algae supply the body with the necessary elements.
This happens through photosynthesis. And the fungus supplies the lichen with mineral salts, which it absorbs from the incoming fluid. In this way, the process of symbiosis occurs.
How they breed
They reproduce in two ways:
- Sexual - is carried out due to spore formation.
- Vegetative - for this, there are soredia (an algae cell braided with a mycelium thread that spreads through the wind) and isidia (outgrowths that form the surface layer of the thallus).
The value of lichens in nature and human life
They have the following positive effects:
Lichens are famous for their longevity, because only the growth period can reach 4 thousand years.
As a result, they can be used to approximately determine the age of the rock.
It is popular to use them as a fertilizer in the agricultural industry. In addition, their use began in ancient times. Lichens were used as natural dyes.
Lichens are a unique species that carry a lot of useful properties and qualities that are applicable in virtually any area of human life.
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