Green algae departmentcurrently belongs to the protists and includes unicellular colonial and multicellular plants - about 13 thousand species in total.
Unicellular algae include Chlamydomonas and Chlorella. Volvox and Pandorina form colonies. Multicellular green algae include Ulva, Ulotrix, and Spirogyra. Common to all green algae is the presence of a chromatophore containing chlorophyll. Chromatophores vary in shape. They can be closed, open (ulotrix), spiral (spirogyra), etc. Ulothrix also includes pleurococcus, a microscopic algae that usually lives on trees and fences.
Green algae reproduce asexually and sexually. Asexual reproduction is carried out using flagellated zoospores formed inside the mother cell or parts of the thallus. The sexual process is associated with the formation of gametes and their subsequent fusion to form a zygote.
However, not all algae have gametes that are divided into male and female; in some algae, two identical gametes merge. The zygote produces either a new individual or zoospores. In the life cycle of algae, the haploid phase predominates over the diploid phase.
Brown algae departmentincludes about 1,500 species of seaweed. The most common of them is kelp sugar (seaweed), containing a reserve polysaccharide - laminarin, the body of which consists of thallus and rhizoids, and the color is explained by the presence of carotenoids in the chromatophores along with chlorophyll.
Laminaria reproduces vegetatively - by parts of the thallus, spores and sexually.
An adult plant has presented It is a diploid sporophyte on which sporangia mature. In sporangia, as a result of meiosis, spores mature and grow into shoots - gametophytes. Gametes are formed in the antheridia and archegonia of the growths. After fertilization, a zygote is formed and develops into a new plant.
Purple algae (red algae) - mostly multicellular, filamentous, bush-like, lamellar plants attached to the substrate by rhizoids. They contain chlorophyll, carotenoids, brown, blue and red pigments, the ratio of which varies depending on the depth of the algae, have disc-shaped chromatophores and do not have light-sensitive eyes. Purples serve as food for marine animals. They reproduce asexually and sexually.
Lichens- lower plants, the organism of which was formed as a result of the symbiosis of a fungus and an algae: a fungus is a heterotrophic component of a lichen, a green or blue-green algae is its autotrophic component. The fungus provides the algae with water and mineral salts and protects it from drying out. The algae supplies the fungus with organic substances. Lichens reproduce both asexually and sexually (vegetative reproduction, carried out by sections of the thallus), and are found in all geographical zones, especially in areas with temperate and cold climates.
The department Green algae currently belongs to the protists and includes unicellular colonial and multicellular plants. There are about 13 thousand species in total. Single-celled organisms include Chlamydomonas and Chlorella.
Colonies are formed by Volvox and Pandorina cells. Multicellular green algae include Ulva, Ulothrix, and Spirogyra. Common to all green algae is the presence of a chromatophore containing chlorophyll. Chromatophores vary in shape. They can be closed, open (ulotrix), spiral (spirogyra), etc. Ulothrix also includes pleurococcus, a microscopic algae that often settles on trees and fences.
Green algae reproduce asexually and sexually. Asexual reproduction is carried out by flagellated zoospores formed inside the mother cell, or by parts of the thallus. The sexual process is associated with the formation of gametes and their subsequent fusion to form a zygote. However, not all algae have gametes that are divided into male and female: in some algae, two identical gametes merge. The zygote either produces a new individual or zoospores. In the life cycle, the haploid phase predominates over the diploid phase
The Brown Algae department includes about 1,500 species of seaweed, of which the most common is kelp, or seaweed. Its body consists of thallus and rhizoids. The color is explained by the presence of carotenoids in the chromatophores along with chlorophyll. Contains a reserve polysaccharide - kelp.
Laminaria reproduces vegetatively - by parts of the thallus, spores and sexually. An adult plant is a diploid sporophyte on which sporangia mature. In sporangia, as a result of meiosis, spores mature and grow into shoots - gametophytes. Gametes are formed in the antheridia and archegonia of the growths. After fertilization, a zygote is formed and develops into a new plant.
Purple algae, or red algae, are mainly multicellular, filamentous, bushy, plate-shaped plants. Attached to the substrate by rhizoids. Contains chlorophyll, carotenoids, brown, blue and red pigments.
Their ratio varies depending on the depth of algae habitat. Chromatophores are disc-shaped. There are no light-sensitive eyes. Purples serve as food for marine animals. They reproduce asexually and sexually.
Lichens. Lower plants, the organism of which was formed as a result of the symbiosis of a fungus and algae. A fungus is a heterotrophic component of a lichen, a green or blue-green algae is an autotrophic component. The fungus provides the algae with water and mineral salts and protects it from drying out. The algae supplies the fungus with organic substances. Lichens reproduce both asexually and sexually. Vegetative propagation is carried out by sections of the thallus. They are found in all geographical zones, especially in temperate and cold areas.
There are about 200 species. The most famous are Cladonia, or deer moss, Xanthoria wallaria, or wall goldenrod, Parmelia and Cetraria.
EXAMPLES OF TASKS No. 22
1. Choose the correct statements:
a) algae are higher plants; b) kelp lives in the northern seas; c) kelp is attached to the bottom by rhizoids; d) red algae are capable of photosynthesis; e) iodine accumulates in seaweed; f) Spirogyra has a ring-shaped, open chromatophore; g) algae reproduce by vegetative, asexual and sexual methods.
2. Algae absorb water and minerals:
a) rhizoids; b) leaves; c) roots; d) the whole body.
3. In chromatophores in the light the following is formed:
a) chlorophyll; b) sugar; c) agar-agar; d) iodine.
4. Asexual reproduction of unicellular algae occurs:
a) fusion of gametes; b) disputes; c) parts of the body; d) all of the above methods.
a) duckweed; b) elodea; c) pleurococcus; d) water lily.
6. Which phase in the life of Ulothrix is diploid?
a) green thread of algae; b) zoospores; c) zygote;
d) gametes.
7. What is the lichen thallus formed by?
8. What is the symbiosis of fungus and algae in lichen?
Charovaya algae, or rays (Charophyceae) - a class of a once large group of ancient plants that combine the characteristics of algae and higher plants. The name comes from ancient Greek χᾰρά - joy, beauty. In total, no more than 700 species of characeae are known.
The development cycle of charophyte algae is dominated by the bisexual (in some species dioecious) gametophyte (n). The body of the gametophytes is a multicellular thallus 20–30 (up to 100–200) cm high. The thallus has a metameric structure, consisting of an axial part on which branches (lateral shoots) are located in whorls. The thallus is attached to the ground by special outgrowths – rhizoids. The growth of the axial part of the thallus is unlimited, and the lateral shoots have maximum growth. Each internode of the metamer is one multinucleated giant, up to several centimeters long, elongated cell, incapable of division (in some charophytes it is also covered with a bark - a second layer of long cells), while each node consists of several small mononuclear cells collected in a disk, differentiating in the process of division and forming lateral shoots of the thallus of the first - third orders. Cell membranes are sometimes calcified. Chloroplasts are green and contain chlorophylls a And b, from additional pigments - lycopene. The reserve substance is starch.
Rice. 39. Life cycle of green algae hara
Charophyte algae are characterized by vegetative and sexual reproduction. Vegetative propagation is carried out through special nodules on rhizoids or star-shaped clusters of cells (nodules) on the lower nodes, which give rise to a new thallus.
During sexual reproduction of the hara, genital organs are formed in the metameric nodes. The female organ is the oogonium, oval, up to 1 mm long, consisting of an egg, which is spirally surrounded on the outside by five narrow cells. It is attached to the oogonia thallus by a single-celled stalk and has a crown of five or ten short cells on top. Antheridia are male gametangia, located in the nodes under the female oogonia. They have a spherical shape with a diameter of up to 0.5 mm, formed by eight flat cells fastened at the edges with processes extending inwards, on which many male reproductive cells arise in a complex way. After formation, sperm enter the water, swim up to the crown and are screwed into the oogonium. As a result of karyogamy, a resting zygote (oospore) is formed. Mature zygotes can become covered with a layer of lime and accumulate reserve substances (starch, lipids). After a while, the zygotes separate from the maternal thallus and fall onto the ground. After a period of rest, the zygote nucleus divides by meiosis to form four haploid nuclei. One of the haploid nuclei is separated from the rest, in the zygote shell (at the top of the oospore), by a cell wall, forming a central lens-shaped cell. The trinuclear oospore cell performs a feeding function. The central cell divides, forming a large shoot cell and a small rhizoid cell, which by dividing develop, respectively, into the shoot and rhizoid of the new thallus.
MUSHROOMS (Fungi or Mycota) is the kingdom of living nature, uniting eukaryotic organisms that combine some of the characteristics of both plants and animals. Science studies mushrooms mycology, which is considered a branch of botany, since mushrooms were previously classified as part of the plant kingdom.
Many fungal cells have a cell wall, only the zoospore and vegetative cells of some primitive fungi. 80–90% of it consists of nitrogenous and nitrogen-free polysaccharides; for the majority, the main polysaccharide is chitin, in oomycetes – cellulose. The cell wall also includes proteins, lipids and polyphosphates. Inside there is a protoplast surrounded by a cytoplasmic membrane. The protoplast has a structure typical of eukaryotes. There are storage vacuoles containing volutin, lipids, glycogen, fatty acids (mostly unsaturated) and other substances. One or more nuclei. Different groups have different predominant ploidy stages.
The basis of the body of mushrooms is the mycelium (mycelium) - a system of thin branching threads - hyphae. The mycelium usually has a large total surface area, since food is absorbed through it osmotically. In lower fungi, the mycelium does not have cellular partitions, that is, it is a syncytium. The hyphae grow apically and branch profusely. During the formation of sporulation organs, and sometimes vegetative structures, they are tightly intertwined, forming a false tissue called plectenchyma, sometimes it can differentiate into layers with different functions, usually resembles parenchyma, but unlike it, it is formed not by cell division, but by the interweaving of hyphae. A parallel plexus of hyphae forms mycelial cords, sometimes reaching large sizes and then called rhizomorphs(honey mushroom, house mushroom). Special modifications of the mycelium that serve to withstand difficult conditions are called sclerotia, from them new mycelium or fruiting organs develop.
All fungi are heterotrophic organisms. Fungi absorb minerals and organic substances from the environment. Depending on the need for substances, fungi colonize certain substrates. Fungi are characterized by external digestion, that is, first, enzymes are released into the environment containing organic substances, breaking down polymers outside the body into easily digestible monomers, which are absorbed into the cytoplasm. Some fungi are capable of secreting all the main types of digestive enzymes - proteases that break down proteins; lipases that break down fats; carbohydrases that break down polysaccharides, so they are able to settle on almost any substrate. Other fungi secrete only certain classes of enzymes and colonize the substrate containing the corresponding substances.
Mushrooms reproduce: vegetatively – in parts of mycelium; asexually - endogenous spores (formed in lower fungi in sporangia) and exogenous spores - conidia (segments of special hyphae - conidiophores); sexually - by zygotes (typical of lower fungi). Most fungi are characterized by a sexual process.
Domain Eukaryotes, or Nuclear ( Eucaryota)
Kingdom Mushrooms ( Fungi or Mycota)
Subkingdom Fungi-like organisms
Department Myxomycetes ( Myxomycota)
Division Plasmodiophorans ( Plasmodiophoromycota)
Subkingdom Lower mushrooms
Division Oomycetes ( Oomycota)
Division Labyrinthulidae ( Labyrinthulomycota)
Division Hyphochytriaceae ( Hyphochytriomycota)
Division Chytridiomycetes ( Chytridiomycota)
Division Zygomycetes ( Zygomycota)
Sub-kingdom Higher mushrooms
Division Ascomycetes ( Ascomycota)
Division Basidiomycetes( Basidiomycota)
Division Deuteromycetes ( Deuteromycota)
Zygomycetes(Zygomycota) is a department of fungi that unites 10 orders, 27 families, about 170 genera and more than 1000 species. They are distinguished by developed coenocytic mycelium of variable thickness, in which septa are formed only for the separation of reproductive organs.
Life cycle of development of zygomycetes using the example of the mucor mold
The mold fungus mucor germinates from heterothallic (heterogeneous) spores on substrates enriched with carbohydrates, forming unicellular haploid multinuclear (without septa) mycelia of the “+” and “–” strains (fungal mycelia are not distinguishable by gender). Mucor reproduces vegetatively by the growth of mycelium. Asexual reproduction occurs through two types of spores: conidia And zygospores, which, spreading with air currents on the substrate under favorable conditions, germinate into new mycelia.
Conidia - vegetative spores formed by septation of nuclei at the tops of mycelial outgrowths - conidiophores.
Zygospores are formed after the sexual process. For this purpose, in zygomycetes, the hyphae of the mycelium of the “+” and “–” strains grow towards each other. Upon their contact, the nuclei located in the apical (apical) part of both hyphae are isolated by a septum from the rest of the hyphae, forming cells - gametangia. After the formation of the gametangia of the “+” and “–” strains, they merge with each other to form a zygote. The process of plasma fusion of gametangial cells in a zygote is called plasmogamy, the process of fusion of heterosexual haploid nuclei into diploid nuclei is called karyogamy and represents the sexual process. From the formed zygote, a sporangiophore grows onto the surface of the substrate. During its germination, the diploid nuclei in the sporangiophore divide by meiosis, forming genetically recombined (altered) haploid nuclei. After reaching the surface of the substrate, a zygosporangium is formed at the top of the sporangiophore, in which the haploid nuclei “+” and “–” are isolated by septa in the zygospore cells. After the zygospores ripen, the zygosporangium opens and the zygospores scatter.
Rice. 40. Life cycle of development of the mucor mushroom
Ascomycetes or marsupial mushrooms ( Ascomycota) - a department in the kingdom of fungi, uniting organisms with septate (divided into parts) mycelium and specific organs of sexual sporulation - bags (asci), most often containing 8 ascospores. They also have asexual sporulation, and in many cases the sexual process is lost, and these types of fungi are classified as imperfect fungi ( Deuteromycota).
Ascomycetes include up to 2,000 genera and 30,000 species. Among them are yeast (class Saccharomycetes) are secondary unicellular organisms. Other well-known representatives of ascomycetes include morels, parmelia, stitches and truffles.
Life cycle of development of ascomycetes using the example of the Sarcoscipha mushroom
The development cycle of ascomycete fungi begins with the germination of heterothallic (heterogeneous) spores on the substrate into haploid multicellular septate mycelia. The mycelium grows in the substrate by dividing the apical cells by hyphae by mitosis. To carry out asexual reproduction by spores, outgrowths from the mycelium of the fungus grow onto the surface of the substrate - conidiophores, on the tops of which conidia - vegetative spores - are formed from septate cells. To carry out the sexual process, the hyphae of the male and female mycelium grow towards each other. At the tips of these hyphae multinucleate cells are formed - gametangia, male - antheridium, female – askogon. A tube grows from the ascogon into the antheridium, the trichogyne, through which nuclei from the antheridium move into the ascogon. Male and female nuclei in askogon are distributed in pairs, but do not merge with each other. After this, hyphae of the secondary mycelium begin to grow from the askogon, into which, as they develop, pairs of nuclei migrate. In the hyphae of the secondary mycelium and ascogon, synchronous mitotic divisions take place, which lead to the formation Dikarions– cells with two haploid nuclei (n+n). On the surface of the substrate (in some mushrooms and inside the substrate), a fruiting body is formed from the primary hyphae of male and female mycelia, as well as secondary dikaryonic mycelium. ascocarp. The fruiting bodies of ascomycete fungi can be of the following types:
- apothecia– open, in the form of a bowl;
- cleistothecia– closed, rounded, from which ascospores emerge after rotting or destruction of the ascocarp;
- perithecia– flask-shaped, with a small hole for the exit of ascospores.
On the inner surface of the ascocarp hymenophore, from the apical (apical) cells of the hyphae of the secondary (ascogenous) dikaryonic mycelium – hymenia, are formed aski– sporogenic cells. Occurs in asci cells karyogamy dikaryon nuclei (n+n), forming a diploid nucleus (2n), which then divides by meiosis and each of the four haploid nuclei divides again by mitosis, forming eight ascospores (n) inside the ascus. In some ascomycetes, mitotic nuclear division does not occur, so four ascospores are formed in the ascus. After maturation, the ascus bursts and shoots spores into the air, at a distance of 2–30 cm. Under favorable conditions, ascospores germinate into the substrate as mycelium.
Rice. 41. Life cycle of development of the fungus Ascomycete Sarcoscipha
Basidiomycetes (Basidiomycota) is a division of the fungal kingdom that includes species that produce spores in club-shaped structures called basidia. Together with ascomycetes they belong to the subkingdom of higher fungi ( Dikarya).
Life cycle of development of basidiomycetes using the example of the dung cap mushroom
The development cycle of basidiomycete fungi begins with the germination of heterothallic (different strain) spores (n) on the substrate into haploid primary multicellular septate mycelia of the plus strain “+” and the minus strain “–”. When the mycelial hyphae “+” and “–” meet, plasmogamy– fusion of two primary mycelia (n) into a secondary dikaryonic mycelium (n+n), each cell of which now contains two nuclei “+” and “–”. Primary and secondary mycelia grow in the substrate by dividing the apical cells by mitosis.
To carry out asexual reproduction by spores, the secondary mycelium of the fungus, in places enriched with nutrients, forms a fruiting body - basidiocarp, formed from tightly intertwined mycelium hyphae. The fruiting body of cap mushrooms itself consists of a stem and a cap. On the inside of the cap there is hymenophore- surface covered hymenium– a layer of spore-forming cells. The apical spore-forming cells of basidiomycetes are called basidia. Occurs in basidia karyogamy– fusion of two “+” and “–” nuclei. After karyogamy, the diploid nucleus (2n) is divided by meiosis into four haploid nuclei (n). On the basidium, four outgrowths are formed into which one nucleus enters. Outgrowths with nuclei are separated from the basidium by septa (constrictions), thereby turning into basidiospores(n). After ripening, they are separated from the basidia and carried by air currents. Under favorable conditions on the substrate, haploid basidiospores germinate into primary heterothallic mycelia.
Rice. 42. Life cycle of development of the basidiomycete fungus
LICHENS- symbiotic organisms consisting of the mycelium of ascomycete fungi and unicellular algae or cyanobacteria. Algae or cyanobacteria in lichens are represented by species that are also found in a free-living state, while lichen fungi, as a rule, exist only in symbiosis with them. The mycelium of the fungus absorbs water and minerals from the environment and feeds them to the algae cells. Photosynthesis occurs in algae cells, producing organic substances that enter the mycelium of the fungus.
The body of the lichen is called slang (Fig.). On the outside, the slan is formed by tightly intertwined gelatinized mycelium hyphae, forming the upper and lower cortex. The lower bark of the slan is thinner than the upper and has special outgrowths for attachment to the substrate - rhizins. Inside the slan there is a core - a thick layer of loose, colorless, weakly gelatinized hyphae (this layer, which makes up 2/3 of the thickness of the slan, is storage and is formed by hyphae with large cells). If the algae in the loose mycelium are evenly distributed, then the algae is called homeomeric, if they are located in one pronounced algal ( gonidial) layer, then – heteromeric. The shapes of lichens are:
- scale– flat, tightly adjacent to the substrate;
- leafy– adjacent to the substrate, but having rising edges in the form of leaf blades;
- bushy– voluminous, tall lichens in the form of small branching bushes.
Lichens reproduce vegetatively isidia or soredia– glomeruli in which the hyphae of the fungal mycelium entwine several algae. Isidia form on the surface of the slate as swellings, after which they break off and are transported by water or air currents. Soredia are formed inside the bark and are forced out through pores (openings) in the bark. Carried by wind or water and exposed to favorable conditions, isidia or soredia grow into new lichens.
Rice. 43. Cross section of heterothaloma lichen Lobaria verrucosa
ANIMALS- the kingdom of multicellular heterotrophic organisms capable of actively moving. They reproduce vegetatively and sexually.
PLANTS- the kingdom of multicellular organisms, predominantly photoautotrophic mode of nutrition. Includes: mosses, ferns, horsetails, mosses, gymnosperms and flowering plants. Often all algae or some of their groups are also classified as plants. Plants are characterized by life forms: trees, shrubs, subshrubs, herbs. Plants reproduce vegetatively (parts of vegetative organs), asexually (spores) and sexually (seeds).
Mosses or bryophytes– department of higher plants, grouped into classes: liver mosses(6,000–8,000 species), anthocerote mosses(100–200 species) and leafy mosses(10,000 species). Mosses are, as a rule, small plants, the length of which rarely exceeds 50 mm. They differ from other higher plants in that in their life cycle adult plants are represented by heterosexuals gametophytes, A sporophyte develops on the female gametophyte. They reproduce asexually - by spores and vegetatively by parts of the thallus.
The development cycle of liver moss using the example of Marchantia variablea
In the development cycle, adult (perennial) Marchantia polymorpha plants are represented by heterosexual (dioecious) male and female gametophytes. The vegetative body of a plant is lamellar thallus, has the appearance of a fleshy, lobed, dichotomously branched thallus up to 10-12 cm long and up to 3 cm wide, formed by many cell layers (about 30). The thallus does not have specialized conducting tissues. Two types of outgrowths form on the lower epidermis:
- rhizoids- colorless simple and reed unicellular outgrowths that perform the function of fixing the plant in the substrate. Marchantia is able to absorb water throughout the body,
- amphigastria– colored (dark purple) single-layer, multicellular scales (reduced “leaves” - phyloids), which are capable of retaining water for a long time during drought.
It is possible to distinguish between male and female gametophytes during the development of special organs (supports for gametangia) on the surface of the thalli - male antheridiephores and women's archegonyephorans.
The antheridiophore is a stalk with an octagonal disk (cap) located on top. On the upper side of the cap, in the antheridial chambers, there are male gametangia - antheridia. Male biflagellate gametes are formed in the antheridia. spermatozoa.
Archegonyephores develop on female gametophytes in the form of a nine-rayed shield (asterisk) on a stalk. During the development of archegonyphora, archegonia(female gametangia) are formed from the morphologically upper side of the support. Due to the uneven growth of both sides of the base of the stand (shield), they are displaced between the rays of the shield to its lower side, where they are located in groups. A special protective wrapper is formed around each group of archegonia. Archegonia are pitcher-like chambers directed with the neck down, in the abdomen of which one egg.
The sexual process in the development cycle of marchantia occurs through water, mainly during rain. Raindrops knock out mature sperm from the antheridia and, together with them, are reflected on the female caps of archegonyphores. Running between the rays of the female stands, drops of water along with sperm hang at the location of the archegonia, covering them. Through water, male gametes penetrate through the neck of the archegonium into the abdomen - calyptra, where the egg is fertilized, forming zygote(the first diploid cell of the sporophyte). After fertilization, an individual cup-shaped protective wrapper - 4-5 lobed pseudoperiantium - begins to form around each archegonium from its stalk. At this time the zygote divides mitosis, forming embryo sporophyte, which is attached to the mother’s body (the scutellum of the archegonyephorus) with a sucker - haustoria, and receives food from there. As the size of the embryo increases, the calyptra (abdomen of the archegonium) stretches (increases in size).
The adult sporophyte, developing from the embryo, includes: foot(suction cup) – securing part, leg And sporangium(a box of sporogon). During its growth, the sporophyte breaks the calyptra and carries the sporangium out. Diploid cells form in sporangia sporogenous tissue which receive nutrition through elongated cells with spirally thickened cell walls - elaters. As a result of the division of sporogenic cells meiosis haploid heterothallics are formed disputes. When the spores mature, the elaters dry out and curl into a spiral, loosening the mass of spores in the sporangium. When the spores mature, the single-layer capsule of the sporangium easily opens with eight backward-curved teeth, and the elater springs contribute to the uniform dispersion of the spores. Heterothallic (heterogeneous) spores of Marchantia germinate in favorable conditions protonemata(pre-adults) are small, pigment-free filamentous outgrowths, from the apical cell of which thalli of the male and female gametophyte of the new generation are formed, respectively.
Vegetative propagation marching occurs with the help brood buds, which are formed in special outgrowths - brood baskets, on the upper side of both female and male thalli. As a result of water getting on the thallus, the brood buds are washed out (sprayed) and sprout on the soil into new thalli of the corresponding sex. Marchantia is common in moist places on the banks of reservoirs, under forest cover.
Rice. 44. Scheme of the life cycle of marchantia moss
Rice. 45. Life cycle of marchantia moss
The development cycle of leafy moss using the example of cuckoo flax moss
In the development cycle, adult moss plants are represented by dioecious gametophytes. Cuckoo flax plants have a vertical stem, up to 40 cm long, with reduced spirally arranged in three rows leaves, fixed in the soil rhizoids. In the center of the stem there is a concentric conducting cord, the middle part of which is occupied by water cells - hydroids(upstream), and peripheral - liptoids– conducting cells of organic compounds (downward flow). At the top of male plants are antheridia, in which the formation occurs spermatozoa, on the tops of women's - archegonia With eggs. Fertilization occurs with the help of drops of water. Knocking sperm out of the antheridia, raindrops are reflected on the archegonia of neighboring plants. Through the water, sperm penetrate into the abdomen of the archegonia, where they fertilize the eggs. Develops from a fertilized egg sporophyte, which is his foot grows into the gametophyte. Leg The sporophyte in the process of growth elongates in length by 15-20 cm, at the top of which a box with sporangia, closed lid. The edge of the boll often has a jagged edge called peristome. The maturation of the sporophyte lasts up to 6 months (in other mosses up to 18). In the sporangia of the sporophyte, spores are formed from cells of sporogenic tissue by dividing the latter meiosis. Once the spores have matured, the lid of the capsule opens and the spores are dispersed through the peristome. Under favorable conditions, spores germinate into protonemata– thread-like chlorophyll outgrowths (thallus). Gametophytes of the next generation of the corresponding sex grow from the axillary kidney-shaped structures of the protonemata.
Rice. 46. Diagram of the life cycle of cuckoo flax moss
Rice. 47. Life cycle of cuckoo flax moss
Horsetails – a division of higher spore plants, currently classified in the subdivision Horsetails of the Fern division and has 15 species. Horsetails are characterized by the presence shoots, consisting of clearly defined metamers (internodes And nodes with whorled arrangement leaves). Sporophytes in their anatomical structure resemble the structure of angiosperm cereal plants. Asexual reproduction is carried out by heterosexuals disputes, vegetative – rhizomes(underground shoots).
The development cycle of horsetails using the example of horsetail
The development cycle of horsetail is dominated by sporophyte- an adult, perennial plant consisting of rhizomes, fixed in the soil adventitious roots. In spring, it grows from the buds of the rhizome onto the soil surface. spore-bearing, chlorophyll-free vertical the escape(stem) with a whorled arrangement of reduced (small) leaves, ending in a spore-bearing spikelet ( strobile). The spore-bearing spikelet itself in its structure has axis, on which umbrella-shaped spore-bearing leaves (scutellums on stalks) are located - sporophylls. On the lower side of the sporophylls, facing the strobilus axis, there are from 5 to 10 sporangia. In sporangia as a result of reduction division meiosis cells sporogenous tissue(2n) haploid, morphologically identical, but different sexes are formed disputes(male and bisexual). The spore shells have special outgrowths - elaters, which are twisted spirally around the spores when wet, and unfold when dry. This allows the spores to cling to each other and spread in groups. After the spores ripen, the spore-bearing leaves of the strobile open, the sporangia burst and the spores are carried by the wind. Due to the content of chloroplasts in the spores, they quickly (within 3 weeks) lose their germination capacity. Once on moist muddy soil, groups of spores germinate into chlorophyll-bearing gametophytes in the form of lobed plates fixed in the substrate rhizoids. Gametophytes reach sexual maturity 3-5 weeks after germination. On male gametophytes, smaller in size, are formed antheridia- male gametangia, in which multiflagellates are formed spermatozoa. On bisexual gametophytes, more dissected in shape, archegonia(female gametangia) develop before antheridia, which increases the likelihood of cross-fertilization. For sperm to reach the eggs located in the archegonia, it is necessary water. On one gametophyte, several eggs can be fertilized at once, from which they further develop embryos, there are young sporophytes. The embryos are attached with their feet to the abdomen of the archegonium and receive from the gametophyte the necessary nutrients for development, forming a rudimentary root, stem and bud. After formation, the embryonic root begins to grow, is fixed in the soil, and the young sporophyte is detached from the gametophyte, which dies after some time. After sporulation spring(spore-bearing) shoots die and green rhizomes grow from the buds assimilation shoots. Assimilation shoots have a vertical stem with a whorled arrangement on it lateral branches And leaves underneath them. They perform the function of forming organic compounds during photosynthesis and accumulating them in the rhizome. At the end of the growing season, the assimilation shoots die off, leaving a rhizome that overwinters in the soil.
Rice. 48. Scheme of the life cycle of horsetail
Rice. 49. Life cycle of horsetail
Moss - department of higher spore plants, represented by 1200 species. They are characterized by the presence of a developed conducting system, a root-shoot type of structure of sporophytes and a thallus structure of gametophytes, asexual reproduction by spores and vegetative reproduction by above-ground shoots.
The development cycle of lycophytic plants using the example of the club moss
Sporophyte Moss club moss is a perennial evergreen plant that dominates the development cycle. It contains creeping dichotomously branched stem, covered with spirally arranged small lanceolate-linear leaflets ( microphylls) and fixed in the soil by dichotomously branched roots. Stem shoots end with apical shoots kidneys or spore-bearing spikelets ( strobes). Sporiferous spikelets on long stalks consist of axes, on which spore-bearing leaves are arranged spirally ( sporophylls) with sporangia on the upper (dorsal) side. After reduction (meiotic) division of maternal diploid cells, haploid cells are formed disputes. The spores are covered with a double shell (endo- and exosporium) and contain up to 50% oil. After maturation, the spores disperse and germinate under favorable conditions. Gametophyte development ( outgrowth) from spores occurs slowly over 12-20 years. It forms rhizoids, with which it is fixed in the soil and absorbs water, and also enters symbiosis with the mycelium of the fungus, which is located in its crustal part. The shoot develops in the soil without access to light, and therefore does not have chloroplasts, but if it comes to the surface, they are formed. Gametophyte bisexual and resembles an onion, later grows to 2-3 cm in diameter, and has a saucer-shaped shape. Antheridia And archegonia placed side by side on the upper side and immersed in parenchymal tissue. Antheridia are oval, archegonia are flask-shaped. The abdominal part of the archegonia contains egg and the abdominal tubular cell, in the neck - cervical tubular cells. For moving spermatozoa water is needed from the antheridia to the archegonium eggs. Swimming into the neck of the archegonium, the sperm moves into the abdomen of the archegonium, where it merges with the egg, forming zygote– the first diploid cell of the sporophyte. Germ- a young sporophyte that slowly develops on the gametophyte, receiving the necessary nutrients from it. When the roots of the embryo grow into the soil, it detaches from the gametophyte and grows into an adult plant. With the gradual maturation of archegonia on the gametophyte, several sporophytes of different ages can form and develop in them simultaneously.
Rice. 50. Scheme of the life cycle of the club moss
Rice. 51. Life cycle of club moss
Ferns- a department of higher spore plants, uniting about 11,000 species. Fern-like plants are characterized by a root shoot structure of the body of sporophytes and a thallus structure of the body of gametophytes. They reproduce asexually - by spores and vegetatively - by shoots (rhizomes).
growth of lateral branches, one gets the impression of dichotomous branching. The thallus develops from a cylindrical rhizome attached to the substrate by rhizoids. The inner layer of the thallus consists of large medullary cells, colorless with few chromatophores. On both sides there is a layer of “cortical” cells, smaller ones with numerous chromatophores.
Life cycle. On the diploid sporophyte dictyota, spherical sporangia develop from the surface cells of the “bark”, where four immobile large tetraspores are formed through reduction division. Tetraspores germinate into a haploid gametophyte. Dictyota is a dioecious plant. Despite the homogeneity of tetraspores, male gametophytes with antheridial sori and female gametophytes with oogonial sori (groups of oogonium) are formed from them. Antheridia have the form of multi-chambered containers, in each chamber of which one single-flagellate spermatozoon (antherozoid) is formed. Antheridia are collected in groups (antheridial sori), which are covered with a sheath. Unilocular oogonia (also collected in sori, but without a covering) form one egg cell in each. It falls out of the gametangium and is fertilized in water by an antherozoid. The resulting zygote is covered with a membrane, and without a rest period it grows into a sporophyte.
The genus Laminaria or seaweed has a parenchymatous type of thallus, where real tissues appear; in the life cycle there is a heteromorphic alternation of generations.
Life cycle. The sporophyte is a leaf-shaped thallus with a dense stem-like petiole, attached to the substrate by powerful claw-shaped rhizoids. The leaf part is shed annually and grows again due to the activity of meristematic cells located between the petiole and the blade. On the surface of the leaf-shaped plate, sori are formed, consisting of paraphyses and zoosporangia. The shell of the paraphysis at the apex is strongly mucused, forming a kind of thick mucous cap. The mucous caps of adjacent paraphyses close together, resulting in a continuous thick layer of mucus that protects the sorus. In zoosporangia, depending on the species, 16-128 identical zoospores develop. The first nuclear division is reduction (Figure 18).
Zoospores emerge from the zoosporangium, swim for a short time, stop and germinate into microscopic dioecious filamentous gametophytes (thallusts). Male - branching with unicellular antheridia, in each of them one sperm matures
du. The female prothallus consists of several cells forming a short thread; it is formed in more favorable conditions. The sexual process is oogamous. The egg cell that matures in the oogonia is released and attached to its upper end. In this position, fertilization occurs, after which the zygote grows into a sporophyte without a resting period. Thus, the female gametophyte provides an attachment site for the future sporophyte.
sporophyte | zoosporangium |
Asexual
REPRODUCTION
paraphyses | |||||||||||||
young sporophyte | slimy | ||||||||||||
shell paraphysis | |||||||||||||
sperm | |||||||||||||
egg | |||||||||||||
antheridium | |||||||||||||
gametophyte | |||||||||||||
REPRODUCTION | |||||||||||||
gametophyte | |||||||||||||
zoospores
Figure 18 – Scheme of the life cycle of representatives of the genus Laminaria (Laminaria)
Brown algae, which do not have a change of generations and are characterized only by a change in nuclear phases, include representatives of the genus
fucus (Fucus). Their thalli are belt-shaped, dichotomously branched, up to 1 m long and up to 5 cm wide (Figure 19).
antheridium
sporophyte |
paraphyses
male scafidium
female scafidium
Figure 19 – Scheme of the life cycle of representatives of the genus Fucus (Fucus)
A thick midrib runs through the middle of the thallus, which in the lower part turns into a short stem with a cushion-shaped expanded base (basal disk), with the help of which attachment to the substrate occurs. In the upper part of many species, on the sides of the veins there are air bubbles that hold the thalli in an upright position.
Life cycle. Normal reproduction of fucus is possible only through sexual intercourse. At the ends of the branches, swellings (receptacles) are formed, in which scafidia are formed - receptacles for the genital organs. Scaphidia can be male or female. Between the oogonia in the female scaphidia there are paraphyses, which often extend beyond its boundaries. Each oogonia produces 8 eggs. In men's
scaphidia antheridia are located at the ends of special single-row branches growing from the wall of scaphidia; paraphyses are much shorter. It should be noted that the entire scaphidium is formed from one cell (prospore), but reduction division occurs immediately before the formation of gametes.
There are other points of view regarding the peculiarities of the change of nuclear phases in this representative, however, the presented diagram of the Fucus life cycle is classical.
The types of life cycles considered do not cover all of their diversity in brown algae. For example, there are also species with heteromorphic alternation of generations and a predominance of macroscopic gametaphyte, sporophyte of somewhat smaller size (Cutleria). In a large number of brown algae, a significant variety of changes in developmental forms is added by variations in life cycles depending on environmental conditions and the irregularity of generational changes.
Materials and equipment. Herbarium of algae, microscopes MBR - 1E, permanent preparations, dissecting needles, Petri dishes, tweezers, slides and cover glasses, bottles of water, pipettes, filter paper, tables.
Goal: To become familiar with the diversity of life cycles of brown algae using the example of specific representatives
1 Familiarize yourself with the systematic position of the objects of study
dovaniya. Write down the taxonomy:
Superkingdom of eukaryotes – Eucaryota Kingdom of tubulocristates – Tubulocristates Division of brown algae – Phaeophyta Class pheophyceae – Phaeophyceae Order ectocarpal – Ectocarpales
Genus Ectocarpus – Ectocarpus sp. Order dictyotales –Dictyotales
Genus of dictyota – Dictyota sp. Order laminaria – Laminariales
Genus of kelp – Laminaria sp. Order fucal (fucus) -Fucales
Fucus genus – Fucus sp.
2 Examine the general appearance of the ectocarpus on a herbarium specimen.
Sketch: 1) the appearance of the algae; 2) zoosporangium with zoospores; 3) life cycle diagram.
3 Consider the general appearance of the dictyota on a herbarium specimen. Behind-
draw: 1) longitudinal section of the thallus with tetrasporangia; 2) sections of thalli with female and male gametangia; 3) diagram of the life cycle of a dictyota.
4 Examine and sketch the general appearance of the kelp. On finished preparations, examine longitudinal and transverse sections of the petiole.
Draw: 1) the anatomical structure of the sporophyte thallus; 2) diagram of the life cycle of kelp.
5 Examine on herbarium specimens and sketch the appearance of the fucus, noting the midrib, air cavities, receptacles, and basal disc. Also sketch: sections of female and male scaphidia, indicating oogonia, antheridia and paraphyses.
Draw a diagram of the life cycle of Fucus.
Questions for self-control
1 Give a general description of brown algae, indicate their differences from other algae.
2 What types of reproduction are typical for the department? Phaeophyta?
3 What principles underlie the classification of the department?
4 Which brown algae are characterized by the greatest anatomical and morphological division of the thallus?
5 How is the change in nuclear phases and forms of development carried out in various representatives of brown algae?
6 Continue filling out table 1 “General characteristics of algal divisions” ( division Phaeophyta).
Lesson 4. Section Diatoms
(Bacillariophyta)
1 General characteristics of the diatoms department
2 Characteristics of the class Coscinodiscophyceae ( Coscinodiscophyceae), the main representatives of the class
3 Characteristics of the class Fragilariaceae, or Seamless, ( Fragilariophyceae), the main representatives of the class
4 Characteristics of the class Bacillaryophyceae, or Sutural, ( Bacillariophyceae), the main representatives of the class
1 General characteristics of the Diatoms department
The Diatoms department (Bacillariophyta) has more than 20 thousand species. These are photoautotrophic tubulocrysts of microscopic size, with an exclusively cocoid thallus, having covers in the form of a silica shell. The shell is tightly adjacent to the plasmalemma and consists of two parts: epitheca and hypotheca. The larger part (epithecium) slides its edges over the hypotheca like a lid onto a box (Figure 20).
epicingulum | hypocingulum |
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finite | |||||||
(polar) | |||||||
hypovalva |
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lamellar |
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axial field | |||||||
wall-mounted |
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chromatophore |
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epivalva | |||||||
vacuole behind |
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central | chromatophore |
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cytoplasm | cytoplasm |
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drops of oil | hypotheca |
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lamellar | drops of oil |
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wall-mounted | |||||||
chromatophore | |||||||
shell structure | internal | shell structure | internal structure |
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cell growth |
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cell structure | |||||||
VIEW FROM THE CASE | VIEW FROM THE BELT |
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Figure 20 - The structure of diatoms using the example of pinnularia (Pinnularia)
The epitheca consists of a flat or convex valve (epivalva) with curved edges and a belt rim (epicingulum). The hypotheca has similar parts: a valve (hypovalva) with curved edges and a belt rim (hypocingulum). The belt rims fit tightly to each other, together making up the belt of the shell. In most diatoms, one or more insert bezels, which increase the volume of the cell and promote its growth. The shape of the shell is varied and is characterized, first of all, by the type of symmetry of the valve. Valves with multiple axes of symmetry are called
radially symmetrical or actinomorphic . Otherwise it is called zygomorphic . Zygomorphic valves are symmetrical in the longitudinal and transverse directions ( bisim-
metric), symmetrical along only one axis ( monosymmetrical), mirror symmetrical or asymmetrical.
The outer and inner patterns of the shell, observed in a light and electron microscope, are called shell structure. It is specific to different taxa and is formed by various structural elements, the most important of which are perforations - a system of holes of various structures located on the valves, through which the protoplast communicates with the external environment. There are small pores (areolas) and large elongated chambers covered with a perforated film (alveoli). The shell valves may have one or two mucus pores through which mucus is secreted, which serves to attach algae to the substrate and form colonies. The thickenings protruding above the outer or inner surface of the valve are called ribs; they provide strength to the shell. In many diatoms, protrusions, bristles, spines, and spines are formed on the outer surface of the shell, which increase its surface and serve to connect cells into a colony. In mobile diatoms, on the valve side of the shell there is a seam in the form of a pair of through slits, as well as nodules - two polar and one central (represent thickenings of the walls of the valve). This shell structure, along with a small volume of protoplast and numerous oil droplets, ensures that diatoms float in the water column. Through the seam, the cytoplasm is released and circulated, which ensures the reactive movement of the algae.
Cells of representatives of the department have a typical eukaryotic structure The cytoplasm in them forms a wall layer or accumulates at the poles or in the center of the cell, connecting by cytoplasmic bridges. The nucleus lies in the central mass of the cytoplasm or in the wall layer, closer to the hypotheca (ucentric diatoms), or
– in the cytoplasmic bridge in direct contact with the chloroplast, closer to the epitheca (in pennates) (For an explanation of the terms centric and pennate, see further in the text). Mitosis is open, but instead of centrioles, the function of organizing the division spindle is performed by
polar discs are visible . Located at the core Golgi complex.
Plastids (chloroplasts) are secondary symbiotic, rhodophytic type, four-membrane (two outer membranes are organized into
chloroplast endoplasmic reticulum , directly go-
extending into the core shell). Thylakoids, collected in threes and sometimes penetrating the pyrenoid, are present circumscribed lamella. Set of photosynthe-
chemical pigments: chlorophyll is, β - and ε -carotenes, xanthophylls (fucoxanthin, diatoxanthin, neoxanthins and diadinoxanthin), which determines the color of the thallus from light yellow, golden to greenish-brown. A cell may have several mitochondria with tubular cristae. Vacuoles - four types: with cell sap, free-
tin, with chrysolaminarin and oils (the last three components are products of assimilation diatoms).
Vegetative propagation most characteristic of diatoms, is carried out cell division in two. Before division, drops of oil accumulate in the protoplast, it increases in size, and moves the valves apart so that they touch only the edges of the girdle rims. The nucleus divides mitotically, and then the entire protoplast. Each new cell receives one shell flap, which is or becomes an epitheca, and the hypotheca is completed. Repeated vegetative divisions lead to a progressive decrease in cell size in the population, since cells that receive the hypotheca of the mother cell (it becomes an epitheca) constantly complete the construction of an even smaller valve (their own hypotheca). Restoration of the original cell sizes characteristic of a given species occurs during the germination of resting cells, as well as as a result sexual process, accompanied by the formation of mauxospores (growing spores). The auxospore grows to the maximum possible size for the species, then takes on a typical shape and forms a carapace. In a number of species of diatoms, auxospore formation occurs due to autogamy: after meiosis, two nuclei remain viable, which fuse inside their cell. Actually asexual reproduction This is not typical for diatoms; however, some species are capable of forming microspores, the nature and routes of formation of which have not yet been studied.
The sexual process is iso-, hetero- (aniso-) or oogamy. In the case of oogamy, the only flagellated stage for diatoms is formed - the spermatozoon. It has one flagellum, covered with retronemes, its axoneme lacks central microtubules (formula 9+0, instead of 9+2), the radicular system is reduced, where instead of triplets there are only doublets of microtubules, the basal body is pressed to the nucleus. In some diatoms, the sperm does not have a flagellum and moves using pseudopodia. In the case of iso- and heterogamy, gametes are gutless and flow from the shell of one mother cell to another.
The life cycle of all diatoms is diplophase with gametic re-
duction without generational change.
Under unfavorable conditions, diatoms go into a dormant state. In this case, the protoplast moves to one of the ends of the cell, loses cell sap and is strongly compressed. The vital activity of these cells is resumed when favorable conditions occur. Some planktonic lake species are able to wait out the winter period in this state at the bottom of reservoirs. In a number of species, the formation of silica cysts is observed.
Diatoms are widespread and inhabit various biotopes: fresh and salty, standing and flowing water bodies, wet rocks, soil and even arable land; able to live on snow and ice. The role in nature and practical significance of diatoms is very great: they participate in the creation of organic matter and the absorption of carbon from the World Ocean, are part of the trophic chains of aquatic ecosystems, participate in the silicon cycle and sedimentation, and are used in environmental monitoring and archaeological dating of sedimentary rocks.
The taxonomy of diatoms is based on the structure of the shell, primarily the symmetry of the valves, the presence and structure of the suture. The department is divided into three classes: Coscinodiscophyceae
(Centric) – Coscinodiscophyceae(Centrophyceae), Fragilario-
ficiaceae (Seamless) – Fragilariophyceae, Bacillariophyceae (Suture) – Bacillariophyceae. The last two classes are traditionally called pennate diatoms.
2 Characteristics of the class Coscinodiscophyceae (Coscinodiscophyceae), the main representatives of the class
The class Coscinodiscophyceae unites algae with radially symmetrical (actinomorphic) valves without a seam (Figure 21). In most cases, the valves are round, so they are often called centric diatoms. The sexual process is oogamy. The class includes 22 orders.
The most widespread order is Melosirales, of which Melosira is a typical representative (Figure 22). Melosira cells are cylindrical, connected in colonies by mucus ridges, spines or teeth. The carapace has high folded valves and a complex belt rim; the valves are round, with small areoles.
In the life cycle of Melosira there is observed oogamous sexual process. Sexual structures differentiate from vegetative ones
cells. The female reproductive cell (corresponding to the oogony) undergoes meiosis, followed by the degeneration of three nuclei, to produce one egg cell. In the male (corresponding to spermatogonia or antheridium) - it first forms a four-flagellated spermatogenic cell, which, after meiosis, buds off four single-flagellated spermatozoa. After fertilization, an auxospore is formed from the zygote.
antheridium
spermatozoa
egg
auxospore
vegetative cells in a filamentous colony
mother cell shell valve
VEGETATIVE REPRODUCTION | SEXUAL REPRODUCTION |
Figure 22 – Melosira life cycle diagram
Reproduction of their own kind in algae occurs through vegetative, asexual and sexual reproduction.
Vegetative propagation unicellular algae consists of dividing individuals in two. In multicellular algae, it occurs in several ways, including mechanical destruction of the thallus into parts (by waves, currents, as a result of gnawing by animals) or as a result of processes accompanied by the disintegration of threads into multicellular or unicellular parts. For example, the division of blue-green algae threads into parts is often preceded by the death of individual cells. Sometimes special formations are used for vegetative propagation. Buds grow on the thalli of sphacelaria (from brown algae), which fall off and grow into new thalli. Charal algae form unicellular or multicellular nodules that overwinter and produce new plants. In a number of filamentous algae (for example, in green ulothrix), individual cells become rounded, accumulate a large amount of reserve nutrients and pigments, and at the same time their shell thickens. Such cells are called akinetes. They are able to survive unfavorable conditions when ordinary vegetative cells die, which leads to the destruction of the thread. Filamentous blue-green algae have a similar type of akinetes, but they are sometimes called spores. Some red, brown, green and chara algae have creeping shoots on which new thalli grow.
Reproduction by parts of thalli does not always lead to the resumption of normal plants. Seaweed that grows exclusively on hard soils (stones and rocks) is often partially or completely destroyed by wave action. Torn parts or entire thalli are not able to re-attach themselves to solid soils, since this is hampered by the constant movement of water. In addition, the attachment organs are not formed again. Currents carry such thalli to the calmest places, usually with a muddy or sandy bottom, where they continue to grow, lying on the ground. Over time, the older parts die off and the branches extending from them turn into independent thalli. In this way, their constant vegetative propagation occurs. Moreover, due to their growth in quiet places, such algae are greatly modified: their branches become thinner, narrower and branch weaker. In such cases, we speak of unattached or free-living forms of the corresponding species. Sometimes they form large accumulations, for example, unattached forms of red algae: Phyllophora in the Black Sea, Furcellaria in the Baltic Sea, Ahnfeltia in the Far Eastern seas.
Unattached forms of bottom algae never form organs of sexual and asexual reproduction. Reproductive organs can be observed in them extremely rarely - on those scraps or thalli that were torn off after the formation of these organs. In these cases, their development and maturation are completed normally, but subsequently the reproductive organs no longer develop again.
Vegetative propagation is essentially a form of asexual reproduction carried out by vegetative parts.
Asexual reproduction the present is accompanied, firstly, by the division of the cell protoplast into parts and, secondly, by the release of division products from the membrane of the mother cell. Moreover, before the division of the protoplast, some not fully studied processes of physiological restructuring occur in it, leading to its rejuvenation. The release of division products from the shell of the mother cell is the most significant difference between true asexual reproduction and vegetative reproduction. It happens that one spore is formed in the cells, but, unlike akinetes, they leave the shell of the mother cell.
Asexual reproduction of algae occurs through spores or zoospores (spores with flagella). They are formed either in cells that do not differ in shape from other cells, or in special cells called sporangia, which often have different sizes and shapes than vegetative ones. The main difference between sporangia and other cells is that they arise as outgrowths of ordinary cells and perform only the function of forming spores. A distinctive feature of spores and zoospores is their simplified shape and small size compared to ordinary cells. They are spherical, ellipsoidal or ovoid, covered with or without a shell.
Blue-green algae, which are prokaryotes, have two types of spores - endospores and exospores. Endospores are formed several in cells as a result of fragmentation of the contents. Exospores arise as an outgrowth of a protoplast at the top of the cell (only in unicellular representatives of the order Chamesiphonidae); As it grows in length, constrictions appear, separating the spherical spores.
The formation of spores and zoospores in eukaryotic algae, both in sporangia and in vegetative cells, is preceded by nuclear division. In this case, depending on the characteristics of the development cycle, a reduction in the number of chromosomes (meiosis) may occur. Daughter nuclei are evenly distributed in the cytoplasm. At the same time, chloroplasts and other organelles divide; after they are grouped around individual nuclei, the cytoplasm divides and the final formation of spores or zoospores occurs. In some dinophytes, zoospores are formed by budding on the surface of the mother cell.
In most eukaryotic algae, asexual reproduction occurs through zoospores. In one cell or sporangium there can be from one (green edogonium) to several hundred (green cladophora). Zoospores can have different structures, which to a certain extent reflects differences in the structure of unicellular algae, which were the ancestors of the corresponding groups. Zoospores come with one, two, four or many flagella; in the latter case they are arranged with a rim at the end.
Several types of spores can be found in algae. Many of the green and yellow-green chlorococci have spores that cover themselves with a membrane inside the mother cell. Such disputes are called aplanospores. When a particularly thickened shell is formed, they are called hypnospores, since they are capable of remaining dormant for a long period of time. Hypnospores are formed one at a time per cell, but, unlike akinetes, the mother cell membrane does not participate in the formation of their shell. Sometimes aplanospores immediately in the mother cell acquire a shape similar to it. In such cases we talk about motorsport. There are also spores, the name of which reflects their number in the sporangium: tetraspores- 4 are formed (many are red and dictyot from brown), bispores- two spores in sporangia (some corallipid from red), monospores- one spore per sporangium (some red).
Spores and zoospores usually enter the water through a hole in the wall of the sporangium in a whole group, surrounded by a mucous membrane, which soon blurs. Upon exiting the sporangium, zoospores, while still in a common shell, begin to actively move and, after the shell ruptures, instantly spread out in different directions. Gametes are released in a similar way during sexual reproduction.
Sexual reproduction consists of the fusion of two cells (gametes), resulting in the formation zygote, growing into a new individual or producing zoospores. In algae, there are several types of sexual reproduction. In its simplest form, it represents the connection of the contents of two vegetative cells. In unicellular flagellated algae (some Volvoxaceae), the sexual process is reduced to the fusion of two individuals and is called hologamy. When the contents of two flagellated vegetative cells merge, the sexual process is called conjugation(Fig. 24). This is the only form of sexual reproduction in the green algal conjugate class. Much more often, sexual reproduction in algae, including unicellular flagellates, is associated with the fragmentation of cell contents and the formation of specialized germ cells inside them - gametes. In all algae, except conjugates and red algae, at least male gametes have flagella, but gametes of the opposite sex do not always have them. Gametes are formed in the same way as spores and zoospores. Special receptacles for gametes are called gametangia. The number of gametes in a cell or gametangium can vary from one to several hundred. In primitive algae, gametes are formed in vegetative cells.
Depending on the relative sizes of the gametes involved in the fusion, the following types of sexual process are distinguished (Fig. 24):
1) isogamy- gametes of the same size and shape;
2) heterogamy, or anisogamy, - one gamete (female) is larger than the other (male), but similar to it;
3) oogamy- the female gamete, called the egg, is devoid of flagella, motionless and much larger than the male one, which is called the sperm or antherozoid, it can be colorless; gametangia with eggs are called oogonia, and gametangia with male gametes are called spermatangia or antheridia;
4) autogamy- a special type of sexual process, common among some diatoms. It consists in the fact that the cell nucleus is first divided by meiosis into 4 nuclei, two of them are destroyed, and the remaining two nuclei merge, again forming a diploid nucleus. Autogamy is not accompanied by an increase in the number of individuals, but only by their rejuvenation.
With hetero- and oogamy, male and female gametes can develop on the same individual or colony ( bisexual or monoecious, species) or on different ( dioecious or dioecious, kinds). Among algae that are characterized by isogamy, there are homothallic species (they fuse gametes from one thallus or colony) and heterothallic(fusion is possible only between gametes from different individuals), which, due to the lack of morphological differences, are designated by the signs + and -, respectively, distinguish between + gametes and - gametes.
As a result of the fusion of gametes, a spherical zygote is formed, while the flagella disappear and a shell appears. The zygotes of some algae retain flagella for some time, then it turns out planozygote, which is capable of swimming from several days to three weeks. In the zygote, two gamete nuclei merge and it becomes diploid. Subsequently, zygotes of different algae behave differently. Some zygotes develop a thick shell (hypposigotes) and enter a period of rest that lasts up to several months. Other zygotes germinate without a dormant period. In some cases, new thalli directly grow from the zygotes. In others, the zygotes divide to undergo meiosis and form zoospores; such zygotes pre-grow, and depending on their size, 4-32 zoospores emerge.
Cases have been observed among algae parthenogenetic(without fertilization) development of female gametes. Outwardly they are similar to ordinary zygotes, and they are called azygotes or parthenospores.
In the same type of algae, depending on the time of year and external conditions, different forms of reproduction (asexual and sexual) are observed, with a change in nuclear phases (haploid and diploid). The exception is species that lack the sexual process. The changes undergone by individuals of a species between stages of the same name (moments of life) constitute its development cycle.
In some species, organs of asexual and sexual reproduction develop on different individuals; then the plants that form spores are called sporophytes, and plants that produce gametes - gametophytes. In other algae, spores and gametes are formed on the same plants; At the same time, such species may also have individuals that produce only spores, i.e., sporophytes (porphyra). Nowadays, plants capable of producing both spores (zoospores) and gametes are usually called gametophytes. However, to avoid confusion with true gametophytes, which produce only gametes, they are better called gametosporophytes.
The development of reproductive organs of one type or another in gametosporophytes is determined by temperature. For example, the lamellar thallus of one of the porphyra species (Porphyra tenera) at temperatures below +15, + 17 ° C produce organs of sexual reproduction, and at higher temperatures - organs of asexual reproduction. And in other algae, gametes usually appear at a lower temperature than spores. At intermediate temperatures, the development of certain reproductive organs on gametosporophytes is determined by other factors - light intensity, day length, seasonal changes in the chemical composition of water or salinity (for seaweed). Gametosporophytes exist in Ulothrixaceae, Ulvacaceae and Cladophoraceae from green algae, in Ectocarpaceae, Chordariaceae, Sphacelariaceae and Punctariaceae from brown algae, and in Bangiaceae and some Nemaliaceae from red algae.
Sporophytes and gametophytes (gametosporophytes) can have the same or different structures, and accordingly, there are concepts of isomorphic (similar) and heteromorphic (different) changes in development forms (alternation of generations). For most algae, it is incorrect to talk about alternating generations of sporophytes and gametophytes (gametosporophytes), since they often exist simultaneously. Sometimes they can grow in slightly different environmental conditions. For example, the porphyry sporophyte has the appearance of branching threads from one row of cells, which are embedded in the calcareous substrate (mollusk shells, calcareous rocks) and prefer low light, penetrating to great depths. The porphyry gametosporophyte is lamellar and grows near the water's edge, including in the intertidal zone.
The difference in the structure of sporophytes and gametophytes (gametosporophytes) during a heteromorphic change in developmental forms can be very significant. The gametosporophyte or gametophyte can be multicellular, several centimeters high, while the sporophyte can be microscopic, unicellular (acrosiphony of greens). The opposite picture is also possible, when the gametophyte is microscopic and even unicellular, and the sporophyte reaches a length of 12 m (Japanese brown kelp). The gametophytes and sporophytes of the vast majority of algae are independent plants. In a number of algae, sporophytes grow on gametophytes (Phyllophora Brodie from the red ones) or gametophytes develop inside the thalli of sporophytes (Cyclosporaceae from the brown ones).
Since during the sexual process, as a result of the fusion of gametes and their nuclei, the set of chromosomes in the nucleus is doubled, then at some point in the development cycle, a reduction division of the nucleus (meiosis) occurs, as a result of which the daughter nuclei receive a single set of chromosomes. The sporophytes of many algae are diploid, and meiosis in their development cycle coincides with the formation of spores, from which haploid gametosporophytes or gametophytes develop. This meiosis is called sporic reduction(Fig. 25.1).
In sporophytes of primitive algae (cladophora, ectocarpus and many others), along with haploid spores, diploid spores can be formed, which again develop into sporophytes. Spores appearing on gametosporophytes serve to reproduce mother plants. Sporophytes and gametophytes of algae at the upper stages of evolution strictly alternate without self-renewal (kelp from brown, many floridae from red).
A number of algae have meiosis in the zygote, i.e. zygotic reduction(Fig. 25, 2). It is characteristic of a conjugate of green algae.
The zygotes of some freshwater green algae, such as volvox, ulothrix, etc., are unicellular sporophytes. They produce up to 32 zoospores, which is many times more in mass than a pair of parent gametes. Thus, these algae essentially exhibit sporic reduction.
Some groups of algae have gametic reduction, which is characteristic of the animal kingdom. Meiosis occurs during the formation of gametes, the remaining cells are always diploid (Fig. 25, 3). This change of nuclear phases is characteristic of diatoms and cyclosporous algae, as well as one of the species of Cladophora glomerata. It is interesting to note that diatoms predominate in number of species over other algae and are found in all habitats where algae can grow. In turn, Cyclosporaceae are among the most widespread seaweeds. Apparently, the development cycle with gametic reduction gives these algae some advantages.
In the green alga Prasiola stipitata, somatic reduction- meiosis occurs in the vegetative cells of the upper part of the diploid gametophyte, with areas of haploid cells appearing in which haploid gametes are subsequently formed (Fig. 25, 4).
In the development cycle of algae that lack sexual reproduction (blue-green, cryptophyte and euglena) or have it in rare cases (golden, yellow-green and dinophyte), only changes in body structure are observed. Therefore, in relation to such algae, it is customary to talk about cyclomorphosis. It may span several generations or be limited to the period of growth and development of one individual. In the most dramatic form, cyclomorphosis is expressed in Hyella caespitosa from blue-green algae and in Glenodinium borgei from dinophytes.
Both development cycles and cyclomorphoses in algae are characterized by great plasticity. Their passage is largely determined by environmental conditions. Therefore, they are not always accompanied by a strictly sequential manifestation of all stages. Depending on the growing conditions, individual stages and forms of development can disappear completely (for example, sporophyte or gametosporophyte and gametophyte) or, conversely, exist for several generations in order to give way to another form of development during the life of one generation. Strictly ordered development cycles exist in algae at the upper stages of evolution (Fig. 26).
The rudiments of algae in the form of spores, gametes and zygotes are not completely spontaneously dispersed by water. They have various types of taxis that determine the direction of their movement depending on external stimuli: light ( phototaxis), temperature ( thermotaxis), chemicals contained in water ( chemotaxis). Not only zoospores, but also spores without flagella have the ability to move. They exhibit an amoeboid movement, in which a protrusion is first formed, and then the contents of the entire spore are moved into it.
Each type of taxis can be positive or negative. With positive taxis, the algae primordia move in the direction of strengthening the active factor; with negative taxis - in the opposite direction. The nature of taxis is determined by the intensity of the factor and the physiological state of the moving cells. Too much light causes a change in positive phototaxis to negative. The phototaxis of zoospores of benthic (bottom) algae, initially positive, eventually changes to negative, which ensures their settling to the bottom. Negative phototaxis is also observed in zygotes of benthic algae. Male gametes have chemotaxis, allowing them to move towards unfertilized female gametes, which secrete special chemicals. It has been discovered that the spores of some benthic seaweeds, apparently by changing body volume and thus specific gravity, are concentrated in layers of water with a certain temperature and salinity. Depending on the direction of the current in these layers, the spores are carried to certain areas of the coast, where the development of thalli occurs.
Currents serve as the main means of transporting primordia over long distances. Zoospores remain viable for several days. A longer movement of algae primordia occurs by fruiting thalli or parts thereof, which remain alive until the end of the growing season.
The presence of flagella in zoospores and some gametes ensures their movement within only a few meters or tens of meters. The speed of movement of zoospores and gametes can be compared with the speed of single-celled organisms with flagella - it does not exceed 250 μm/sec, or 0.9 m/h. This low speed is important for choosing the most suitable water layers and places for direct attachment on the bottom. The fixation of benthic algae spores is influenced by the presence of other organisms and individuals or spores of a given species and their number per unit area.
For the germination of algae spores and zygotes, a set of conditions is required, including certain values of temperature, light, and the content of nutrients and biologically active substances. Otherwise they will not germinate. At the same time, the zygotes of some algae, for example fucus, which do not belong to hypnozygotes, remain viable for three to four months. The reproduction and preservation of some algae in unfavorable conditions is facilitated by the formation of cysts. They are known from golden, yellow-green, diatoms and dinophyte algae. One cyst is formed in each cell. The cell contents become rounded and a hard shell containing silica is developed around it. When cysts germinate, one individual is formed, rarely several.
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