BOTANY WITH FUNDAMENTALS OF HYDROBOTANY - 2010

4. PLANT REPRODUCTION

All organisms reproduce their own kind, ensuring the survival of the species in space and time. The production of offspring is an essential property of living organisms, just like growth, development, Nutrition, etc. When the capacity for self-reproduction is lost, species go extinct, which has repeatedly occurred in the course of plant evolution. Two main concepts related to the production of offspring must be distinguished: reproduction and propagation.

Reproduction is the general capacity of organisms to produce offspring similar to themselves.

Propagation is the general property of organisms to produce offspring accompanied by an increase in the number of individuals of a given species.

However, the production of offspring does not always lead to propagation. Propagation does not occur if the number of daughter individuals equals or is less than the number of parent individuals; in this case, the offspring, replacing the parents, practically do not coexist with them in time. Moreover, propagation is often unaccompanied by reproduction. For instance, in the male fern (Dryopteris filix-mas), spores germinate into prothallia that do not resemble the spore-producing parent plant.

In some species, an increase in the number of individuals is observed over time. They propagate rapidly, occupying new areas. These are progressive species. Others are characterized by a constant number of individuals and range. These are stable species. In still others, a reduction in the number of individuals and their range is observed. These are regressive, endangered species.

Reproduction as a property of living matter has existed since the earliest stages of its development. Plant evolution was accompanied by the evolution of their modes of reproduction. At the same time, the evolution of reproduction became a major driving force in plant evolution, leading to The Emergence of new specialized Organs.

There are two main modes of reproduction: Selection/8.html">Asexual and sexual.

Asexual reproduction is a mode of reproduction in which new individuals are formed from parent organisms without a preceding sexual process. This is an older mode of reproduction. It is subdivided into Vegetative Reproduction and asexual reproduction in the narrow sense, or proper asexual reproduction.

4.1. Vegetative reproduction is reproduction by parts of a plant Organism or by rudiments of daughter individuals (buds) capable of developing into an independent plant. This method is a biological phenomenon, an adaptation inherent to plants themselves. Vegetative reproduction is carried out thanks to the ability of plants to regenerate, i.e., the property of restoring a whole organism, similar to the parent, from a part of the body. Each separated part, as a rule, forms new organs (roots on a SHOOT, shoots on a ROOT). Often, all necessary organs are formed in the future independent plant even before Separation from the parent, for example, new rosette shoots with adventitious roots at the ends of strawberry runners.

Unlike animals, the capacity for vegetative reproduction in plants is characteristic of all Levels of Organization.

In unicellular plants, vegetative reproduction occurs by Cell Division. Multicellular and large non-cellular lower plants—Algae, Fungi, and Lichens—can break apart into pieces, each of which can become an independent organism. This is the least perfected method of vegetative reproduction.

Offspring formed As a result of vegetative reproduction from a single parent individual are called a clone.

Forms of vegetative reproduction are especially diverse in fungi. They have developed special unicellular adaptations for vegetative reproduction: conidia, oidia, and chlamydospores.

Conidia are outgrowth-like hyphae or specialized Branches of the mycelium that disarticulate into Cells covered with a thin wall.

Oidia are outgrowths of the mycelium that form cells arranged in chains.

Chlamydospores are thick-walled secondary fungal spores formed from hyphae through their transverse division into short cells.

Each major natural group of plants is often characterized by forms of vegetative reproduction unique to it: blue-green algae by hormogonia, fungi by conidia, oidia, chlamydospores, lichens by isidia and soredia, etc.

Modes of vegetative reproduction are particularly diverse in higher plants, especially angiosperms. In these plants, buds are primarily the source of regeneration. The potential regenerative capacities of plants are inexhaustible. New plants can be obtained from a flower stalk, a stamen, and even a single cell.

A distinction is made between natural and artificial vegetative reproduction.

Natural vegetative reproduction of plants is reproduction occurring in nature without human intervention. Among seed plants, only annuals and biennials do not reproduce vegetatively under natural conditions. Among perennials, almost all herbaceous and many woody plants are capable of vegetative reproduction. For example, in duckweed, this occurs through the separation of a shoot from the parent plant, which develops into a new plant.

Vegetative reproduction by rhizomes, above-ground creeping shoots, bulbs, and roots on which adventitious buds form is most widespread in seed plants.

Above-ground creeping shoots (runners, stolons) form adventitious roots at the nodes and buds in the leaf axils that give rise to vertical, leafy shoots. The internodes of the creeping shoots die off, and the new plants lose their connection with the parent plant (strawberry, cinquefoil).

Most perennial herbs reproduce by rhizomes. Buds form on the rhizomes, from which above-ground shoots develop that transform into independent plants. This is how horsetail, couch grass, yarrow, ground elder, irises, and lily of the valley reproduce.

Many herbaceous monocotyledonous plants from the Liliaceae and Amaryllidaceae families (such as onions, tulips, and others) propagate by means of bulbs.

Potatoes, dahlias, and cyclamens propagate by tubers.

Thistles, rose hips, and lilacs propagate by root suckers. In this process, new shoots develop from adventitious buds that form on the roots. Once the roots connecting the daughter plants to the parent plant wither away, the new plants become independent.

Brood buds (or bulbils) are tiny embryonic shoots that form in the leaf axils or inflorescences. Upon dropping from the parent plant, they take root (as seen in sundews and *Bryophyllum*). Sometimes, brood buds can transform into small bulbs (such as toothworts, lilies, and bulbous bluegrass) or bulbils (such as alpine bistort).

Cacti, *Elodea*, and willows propagate by parts of stems (stem cuttings).

Artificial propagation is a form of vegetative reproduction that does not occur naturally and involves the surgical separation of plant parts. It is most commonly used when a plant fails to produce seeds under certain conditions, when the seeds do not retain the traits of the variety, or for the accelerated propagation of a specific plant or cultivar.

There are numerous Methods of artificial vegetative propagation.

Propagation by root suckers. This method is used to propagate raspberries, cherries, plums, blackberries, bird cherry, sea buckthorn, and others.

Division of the bush is used to propagate certain shrubby plants (such as currants) and ornamental herbaceous perennials (such as primroses, daisies, delphiniums, phlox), as well as vegetable crops (such as sorrel and rhubarb).

Propagation by layering is carried out by bending lateral shoots of the parent plant and burying them in soil. Once rooted, they are separated from the parent plant and transplanted to a new Location. This method is used to propagate gooseberries, ficuses, carnations, roses, oleanders, azaleas, grapes, and others.

Cuttage (cuttings). A cutting is a portion of a vegetative organ used for propagation.

Stem cuttings are used to propagate grapes, roses, currants, willows, indoor geraniums, and the like. Stem cuttings are divided into winter (leafless) cuttings and summer (green) cuttings with leaves. Winter cuttings are used to propagate currants, grapes, willows, and poplars, while summer cuttings are used for roses, thuyas, cucumbers, melons, tomatoes, eggplants, peppers, and potatoes.

Root cuttings are used to propagate horseradish, rose hips, roses, plums, cherries, raspberries, apples, hazelnuts, cranberries, and others.

Leaf cuttings are used to propagate begonias, gloxinias, purslane, coleus, hyacinths, and tomatoes.

When planted in the soil, a cutting takes root and produces a shoot. On stem cuttings, the shoot develops from axillary buds, whereas on leaf and root cuttings, it forms from adventitious buds. Adventitious roots form endogenously from the callus.

Cuttage is widely used in Tissue and organ culture to obtain virus-free varieties of potatoes, tomatoes, and other crops. For this purpose, shoots are grown from the bud apex on an artificial nutrient medium. These shoots are subsequently propagated via cuttings to yield a specific quantity of virus-free plants. These plants are then used to grow seed material suitable for commercial agricultural production.

Grafting (transplantation) involves fusing cut scions or buds of one plant to be propagated (the scion) with another rooted plant (the rootstock).

Although about 100 different grafting techniques are known, they can be reduced to three fundamentally different methods: approach grafting (or inarching); whip grafting (or grafting with a scion); and bud grafting (or budding).

Fruit crops are primarily propagated by grafting. The KEY FEATURES OF vegetative reproduction are as follows:

- the offspring faithfully and accurately reproduce the traits and CHARACTERISTICS OF THE parent plant, making them highly adapted to specific environmental conditions;

- the offspring develop directly from the parent organism and are therefore more viable;

- upon separating from the parent plant, the offspring are better equipped for competitive survival;

- the offspring enter the fruiting stage earlier;

- vegetative reproduction can take place under conditions less favorable than those required for seed propagation;

- vegetative reproduction allows individual specimens to rapidly achieve dominance within a phytocoenosis, such as the spread of aquatic and wetland plants along shorelines.

4.2. Asexual reproduction. Asexual reproduction is carried out by specialized propagules formed by the organism, such as zoospores or spores.

Most lower plants (algae, fungi) and almost all higher plants, with the exception of seed plants, reproduce via spores. Typically, a spore is a single, or less commonly two, microscopic cells that, upon separating from the parent plant, are capable of growing into a new individual. The spore wall features uneven thickenings, and the spore germinates through these unthickened regions in the wall. The spore Cytoplasm is rich in nutrients and contains a nucleus, Mitochondria, Plastids, or proplastids.

Spores fall into two categories: zoospores and aplanospores.

Zoospores are spores that generally possess flagella, enabling them to move about in an aquatic environment. They lack a rigid polysaccharide Cell wall and are typical of lower aquatic plants.

Non-motile spores (aplanospores) lack locomotor organs. They are passively dispersed by Water or wind and are protected from desiccation by a tough cell wall. Such spores are characteristic of terrestrial lower plants and all higher spore-producing plants. In the latter, the spore wall is typically double-layered, consisting of an intine and an exine.

Spores are formed within specialized structures known as sporangia, whereas zoospores develop inside zoosporangia. These are referred to as endospores. However, spores can also form On the surface of a plant organism, in which case they are called exospores (for example, conidiospores in certain fungi).

In lower plants, such as algae and fungi, a sporangium consists of a single cell; the spores are formed by the division of its contents and are released into the environment when the sporangial cell wall ruptures.

In higher spore-producing plants, the sporangium is a multicellular organ featuring a single- or multi-layered wall. Inside the young sporangium, sporogenous tissue develops, the cells of which undergo meiotic division to produce spores.

In unicellular plants (such as Chlamydomonas and Chlorella), the entire plant Functions as the sporangium.

Spores differ in their origin and are classified as either asexual or sexual.

Asexual spores are formed via mitosis. They germinate into a new individual that closely resembles the parent plant.

Sexual spores develop following a sexual process, with the cells of such spores being produced through Meiosis. Most commonly, they germinate into offspring that differ from the parent plant.

In many higher plants (certain clubmosses, ferns, and all seed plants), spores vary in size, being divided into smaller microspores and larger mega- (or macro-) spores. This phenomenon is known as heterospory. Microspores develop into male gametophytes, whereas megaspores develop into female gametophytes.

Asexual reproduction is characterized by several key features: a very high reproduction rate; rapid dispersal of the species as a result of this high reproductive intensity; high offspring uniformity, retaining the traits and Properties of the parent plant; and the preservation of species population numbers thanks to the superior ability of spores to withstand adverse conditions.

4.3. Sexual reproduction. Sexual reproduction refers to the type of reproduction in which new individuals are formed as a result of a sexual process, The Essence of which is the mutual assimilation (fusion) of two physiologically distinct cells. This fusion produces a new cell known as a zygote. During this process, The Cell cytoplasms and nuclei merge, and the Chromosomes combine within the zygote nucleus; consequently, the zygote nucleus contains twice the number (set) of chromosomes found in the fusing gametic cells. Each chromosome in the zygote nucleus retains its individuality. The zygote combines the genetic material of two genetically distinct parents—this is the Biological essence of the sexual process, ensuring the emergence of genetically more diverse offspring exhibiting traits from both the maternal and paternal organisms. As a result, offspring produced through sexual reproduction tend to be more vigorous and better adapted to varying environmental conditions. Following a brief quiescent period, the zygote cell divides and gives rise to a new individual.

The diverse forms of the sexual process can be divided into two main groups: gametogamy and agametogamy. The former, which is more advanced, involves the fusion of specialized propagules called Gametes, whereas the latter is characterized by the fusion of haploid somatic cells.

There are three recognized types of advanced Sexual reproduction in the form of gametogamy: isogamy, heterogamy, and oogamy.

Isogamy is the simplest type of sexual process, in which morphological sexual differentiation has not yet evolved. The gametes are identical, both being motile due to the presence of flagella, and matching in shape and size. By fusing in pairs, they form a zygote that becomes enclosed in a protective wall. After a period of dormancy, the zygote germinates into a new individual. Isogamy is characteristic of certain algae and fungi.

Heterogamy represents the next evolutionary stage of the sexual process. In heterogamy, both gametes are motile and flagellated, but they differ in size: the smaller one is the male gamete, while the larger, often less motile one is the female gamete. Gametes are produced within gametangia. This type of sexual process is typical of chytrid fungi, green algae, and brown algae.

Oogamy is a type of Sexual process in which the gametes are markedly dimorphic; the male gametes are small, flagellated, highly motile, and termed spermatozoa. Spermatozoa are produced within antheridia. The female gamete is large and non-motile, lacking flagella. Its cytoplasm contains abundant nutrient reserves that support The Development of the new plant during the early post-Fertilization stages. In lower plants, the egg cell develops within a unicellular gametangium called an oogonium, whereas in higher plants, it forms within a multicellular archegonium. Oogamy occurs in many lower plants and all higher plants.

In most seed plants, male gametes lost their flagella and independent mobility during the course of evolution, becoming known as sperm cells (spermatozoids). In seed plants, these non-motile, diminutive male gametes reach the similarly non-motile, large egg cell via a pollen tube. This specific mode of sexual process is termed siphonogamy.

Agametogamy is relatively rare and occurs exclusively in lower plants (algae, fungi). The main forms of agametogamy, representing a primitive or unspecialized sexual process, include hologamy, conjugation, zygogamy, and gametangiogamy.

Hologamy manifests as the fusion of the protoplasts of two whole individuals. It is found in the unicellular green algae Closterium and Dunaliella. Hologamy leads to a reduction in the total number of individuals.

Conjugation is a sexual process consisting of the fusion of protoplasts from two equivalent cells that are not differentiated into male and female types. During this process, the protoplast from one cell flows into the other. Conjugation is characteristic of conjugate green algae.

Zygogamy is a type of sexual process found in zygomycete fungi, in which two opposite-sex or physiologically specialized mycelial regions fuse.

Gametangiogamy is a type of sexual process in lower plants in which the contents of two multinucleate cells (gametangia) fuse without differentiating into specialized uninucleate sex cells (gametes). The nuclei (male and female) of the two copulating gametangia fuse in pairs. It is characteristic of ascomycetes and certain other fungi.

In The life cycle of all higher and many lower plants, sexual and asexual reproduction alternate.

Plants that produce organs of sexual reproduction are called the sexual generation, or gametophyte. Gametes are formed within the sex organs of gametophytes.

Plants that produce organs of asexual reproduction are called the asexual generation, or sporophyte. The sporophyte bears organs of asexual reproduction in which spores develop.

Concurrently with the Morphology/12.html">ALTERNATION OF GENERATIONS, A change in nuclear phases takes place. Haploid spores are formed on the diploid sporophyte via meiosis. These spores grow into a haploid gametophyte. The fusion of haploid gametes produces a diploid zygote, from which a diploid sporophyte develops, and so on.

In older, more primitive plants, the life cycle is dominated by the sexual generation—the gametophyte, which represents the haploid phase. Conversely, highly organized plants are characterized by the dominance of the asexual generation—the sporophyte, or diploid phase—during the alternation of generations.

The alternation of generations combines two modes of reproduction: asexual and sexual. Asexual reproduction increases the number of individuals, whereas sexual reproduction enriches the heredity of the offspring.

Let us trace the alternation of generations using the male fern as an example. The sporophyte of the male fern consists of a rhizome, adventitious roots, and leaves known as fronds. In summer, clusters of sporangia called sori develop on the underside of the fronds. Haploid spores are formed within the sporangia from diploid Cells of the sporogenic tissue through meiotic division. Upon landing on the soil under favorable conditions, these spores germinate into haploid bisexual plants called prothallia, or gametophytes. On the gametophyte, which appears as a Heart-shaped vegetative plate with rhizoids, sex organs called antheridia and archegonia are formed. Haploid spermatozoa are produced by mitosis from haploid cells of the spermatogenous tissue within the antheridia, while a haploid egg cell is formed within the archegonium. In the Presence of water (rain or dew), the spermatozoa swim toward the archegonium, penetrate its interior, and fertilize the egg cell. This produces a diploid zygote, which develops by mitotic division into a diploid sporophyte embryo. In due course, the sporophyte develops from the embryo on the gametophyte. Thus, the sporophyte and gametophyte represent different stages of plant development.

Review Questions and Tasks

1. What are the main modes of reproduction? Compare their characteristics.

2. How does vegetative propagation occur in plants?

3. What methods of artificial vegetative propagation of plants exist?

4. How does asexual reproduction occur in plants?

5. Describe the forms of vegetative reproduction in fungi. What adaptations for vegetative reproduction have they developed?

6. How does vegetative propagation differ from true asexual reproduction in plants?

7. Which type of reproduction is referred to as sexual?

8. What types of sexual processes in the form of gametogamy are known?

9. Describe the main forms of agametogamy as an imperfect sexual process.

10. How does the alternation of generations occur in plants? What are the advantages of the alternation of generations?