BOTANY. PLANT MORPHOLOGY - O. A. Shevchuk - 2014

PLANT REPRODUCTION AND PROPAGATION

Reproduction refers to the ability of organisms to produce offspring similar to themselves.

Propagation is the process whereby The production of offspring leads to an increase in the number of individuals of a given species. Propagation ensures the long-term survival of a species in space and time. The loss of the capacity for both reproduction and propagation leads to the extinction of a species. In general, plants have a very high potential for propagation. Typically, they produce numerous seeds and spores from which new generations of plants develop, thereby continuing the existence of the species. The number of new individuals may exceed, equal, or be fewer than the parent generation. The population size of the offspring depends largely on environmental conditions.

When plants propagate through parts of their body (e.g., strawberry runners or willow cuttings), both propagation and reproduction occur simultaneously. Offspring produced in this manner are entirely identical to the parent plants. In contrast, the sexual process gives rise to new individuals that differ from the parental forms, while their number generally increases. It should be noted that The formation of offspring does not always lead to an increase in species population size, i.e., propagation. Specifically, if the number of offspring equals or is less than the number of parental forms, the process represents the replacement of old individuals by new ones rather than propagation. For example, after the Fertilization of an egg Cell on a fern prothallus, a single seedling (sporophyte) develops, while the gametophyte (prothallus) itself quickly dies. In this case, there is neither propagation (an increase in the number of individuals) nor reproduction (the offspring are unlike the parents); nevertheless, the survival of the species is well ensured.

Types of Propagation

New individuals can arise from parental forms either asexually (without the participation of Gametes and the sexual process) or sexually (As a result of the sexual process). Accordingly, a distinction is made between asexual and sexual propagation. In a broad sense, during asexual propagation, the parent form divides into parts either through the division of unicellular organisms (e.g., Algae) or the vegetative body or individual Organs (stems, roots, leaves) into cuttings, or through the formation of vegetative propagules (buds, tubers, bulbs). This form of asexual propagation is called vegetative propagation. Spore propagation—utilizing specialized Cells called spores—is also a form of asexual propagation. This is the most ancient method of propagation in unicellular organisms (algae), though it also occurs in multicellular forms. In higher plants, spore formation involves a reduction in the chromosome number, meaning that the offspring and parent individuals are not identical and differ from one another to varying degrees.

In the ontogeny of plants, propagation can be single or repeated, and organisms are accordingly referred to as monocyclic or polycyclic.

The same Organism can propagate both asexually and sexually. Sexual propagation is defined as the type of propagation in which new individuals are formed as a result of the sexual process, namely the fusion (copulation) of two sex cells (gametes) and the formation of a zygote. The zygote Nucleus contains a double set of Chromosomes derived from two genetically distinct parents. This leads to increased genetic Variability in the offspring and creates favorable conditions for natural Selection.

Vegetative Propagation. Vegetative propagation—the formation of new individuals from PARTS OF THE parent form—is based on regeneration, whereby a viable part of the vegetative body, separated from the parent organism, develops all necessary organs and lives as an independent organism. Vegetative propagation occurs in plants across various Levels of Organization, from algae to angiosperms, and in some species (e.g., *Elodea*) it serves as the sole method of propagation or alternates with other Methods. The offspring produced from a single parent individual via vegetative means is called a clone. This type of propagation is of great importance in plant breeding because it preserves varietal traits without deviation (ensuring faithful reproduction). However, in some cases, prolonged vegetative propagation may reduce plant hardiness, age the enzymatic systems, or shorten the overall lifespan of the plants.

A distinction is made between natural and artificial vegetative propagation, though drawing a strict boundary between them is often difficult.

Natural vegetative propagation occurs without human intervention. Examples include cell or thallus division in lower plants and liverworts, the Separation of body parts in other higher plants, propagation by rhizomes (most perennial grasses), bulbs (onion, tulip, squill, etc.), tubers (corydalis, cyclamen, Jerusalem artichoke), ROOT suckers (many perennials such as horseradish, thistle, sow thistle, etc.), bushes and shrubs (lilac, blackthorn, raspberry, aspen, etc.), layering (linden, gooseberry, fir, etc.), and above-ground creeping shoots (stolons) (strawberry, creeping cinquefoil, dewberry, ground ivy, etc.).

In nature, plants frequently propagate using gemmae or brood buds. In lower plants and bryophytes, these are better termed brood bodies (*Marchantia*, *Tetraphis*, *Bryum*, etc.). In higher plants, brood buds form along leaf margins (*Kalanchoe*), on leaf Veins (*Asplenium*), in leaf axils (*Dentaria bulbifera*, *Lilium bulbiferum*, lesser celandine), and in inflorescences (*Allium*, *Polygonum*) (Fig. 90). Often, brood buds transform directly into shoots that root immediately upon detaching. This phenomenon is characteristic of plants in polar regions, steppes, and high altitudes (certain stonecrops, fescues, bulbous bluegrass, etc.). However, these plants should not be termed viviparous. True viviparity is characteristic of mangroves, where seeds germinate while still attached to the mother plant, and a fully formed seedling drops into the mud of the tidal zone (*Rhizophora*). A specific type of brood bud is the overwintering bud of aquatic plants, known as a hibernaculum. Such buds form in autumn in arrowhead, frogbit, Water soldier, bladderwort, and others, and develop into new individuals in the spring.

Certain plants propagate extremely intensively through vegetative means. For instance, duckweeds form a thick layer covering the entire surface of standing water bodies, while field horsetail and creeping couch grass proliferate aggressively in agricultural fields.

Artificial vegetative propagation. Humans artificially propagate plants using tubers, bulbs, rhizomes, division of turf-forming plants and bushes, root suckers, creeping stems, and layering.

Cuttings—shoots artificially separated from the mother plant and subsequently rooted—are very frequently used for vegetative propagation. Cuttings may be stem, root, or leaf cuttings. Winter stem cuttings (leafless), 20–30 cm long, are harvested during the winter dormancy period, stored in moist sand, and planted in a substrate in spring. For summer stem cuttings (2–4 cm long), leaves are shortened by half, and the cuttings are rooted in a tilted position within a humid chamber in a hotbed or greenhouse. Such cuttings form a callus at the lower end, from which adventitious roots develop due to the action of indoleacetic acid. Poplars, willows, currants, roses, fuchsias, and many other plants are propagated by cuttings. Begonias, coleus, Usambara violets (*Saintpaulia ionantha*), gloxinias, and others are propagated via leaf cuttings. Root cuttings, 10–20 cm long, are harvested in autumn, stored in sand, and planted in a substrate in spring. This method is used to propagate plums, cherries, raspberries, chicory, etc.

Grafting or transplantation is a typical form of artificial vegetative propagation. It does not occur in nature at all and involves transplanting parts of one plant onto another so that they subsequently fuse and grow together. Grafting is most effective when propagating a desired variety while maximally preserving all its quality traits. Most frequently, grafting is applied to propagate varieties of fruit trees, stone fruits, grapes, etc. For this purpose, the stem and root system of a wild rootstock, termed the rootstock, is utilized. A cutting of the varietal plant to be propagated (the scion) is grafted onto it. There are various methods of grafting: approach grafting or inarching (firmly pressing together two shoots of adjacent plants with matching cut strips); whip-and-Tongue grafting or copulation (fitting together obliquely cut scion and rootstock shoots of equal diameter); rind grafting (inserting a smaller-diameter scion beneath the readily slipping bark of the rootstock); cleft grafting; and budding or shield budding (placing a bud with a shield of bark, wood, and a portion of the leaf petiole into a T-shaped incision on the rootstock). Grafting is performed in late summer and spring.

Clonal micropropagation of plants. One of the most effective and economically profitable methods of vegetative propagation is clonal plant micropropagation in *in vitro* culture. Using tissue and cell culture methods (starting from a single cell or a group of cells) under sterile conditions, whole plants of economically valuable crops and virus-free forms of sugar beets, potatoes, grapes, ornamental and medicinal plants, and others are grown. Although this method is rather labor-intensive and requires strict adherence to technological sterility, it allows for the rapid production of mass quantities of virus-free, high-quality, uniform planting material, shortens the time required to obtain marketable products, and ensures high yields of cultivated plants. This method holds exceptional promise for biotechnology, plant breeding, and the conservation of biodiversity.

Spore propagation in plants. Spore propagation is characteristic of lower plants (algae), Fungi, and higher spore-bearing plants (bryophytes, clubmosses, horsetails, psilotophytes, ferns), in contrast to gymnosperms and angiosperms, which propagate by seeds and are therefore termed seed plants. The formation of spores on a plant is referred to as sporogenesis.

Spores are specialized cells that ensure both the propagation and dispersal of plants. They are haploid and arise through mitosis in certain fungi and algae (these are mitospores) or through Meiosis in higher plants (these are meiospores). In many plants, all spores are of equal size and possess identical physiological properties. Such spores are called isospores, and the plants bearing them are termed homosporous. Heterosporous plants produce spores of different sizes: smaller microspores (which give rise to male individuals upon germination) and larger megaspores (which give rise to female individuals upon germination).

Spores are formed within specialized organs of asexual reproduction called sporangia. In lower plants, sporangia are unicellular, whereas in higher plants they are multicellular, containing archesporial tissue within. The archesporial cells give rise to sporogenous tissue, which subsequently forms the spores. In lower plants, spores possess flagella and are motile zoospores. In higher plants, spores are non-motile, dispersed by wind, and possess a bilayered wall, in which the outer layer (exine) is thick and the inner layer (intine) is thin and colorless.

Sexual reproduction. Sexual propagation is defined as propagation in which new individuals are formed as a result of the sexual process, namely the fusion of two gametes—sex cells. In a typical case, two gametes of opposite sexes fuse: the male gamete—a motile spermatozoon or non-motile sperm cell—and the female gamete—the egg cell. The fusion of such gametes is termed fertilization or syngamy. This results in the formation of a zygote, which possesses a diploid set of chromosomes; genetic recombination within the zygote makes the offspring more diverse than the parental forms.

It is important to distinguish between two concepts: the sexual process and sexual propagation. A single sexual process results in the birth of only one offspring from two parents, whereas sexual propagation increases the number of individuals through the fusion of A large number of gametes produced by the parental organisms.

Sexual propagation is characteristic of all eukaryotic organisms that possess gametangia—organs responsible for gamete formation. The simplest type of sexual process, hologamy, occurs in certain unicellular algae. In *Dunaliella*, entire unicellular organisms fuse; these organisms do not differ in appearance and lack gametangia. The remaining lower plants possess unicellular gametangia, whereas higher plants have multicellular ones. Male gametangia in all plants are called antheridia. Female gametangia are termed oogonia in lower plants and archegonia in higher plants. Gametes are always naked, lacking a rigid Cell wall, and frequently bear flagella.

If fusing gametes are equal in size and shape and differ only physiologically, the process is called isogamy. It is characteristic of certain lower plants (particularly many algae) and fungi, with the gametes being motile in this case. If the fusing gametes differ in size (*Eudorina*, *Ectocarpus*), shape, and behavior, the sexual process is termed heterogamy, representing an intermediate form between isogamy and oogamy. Oogamy is the highest form of the sexual process, characteristic of all Higher Plants and most lower plants. The female gamete—the egg cell—is non-motile, lacks flagella, and contains a substantial reserve of nutrients. This process is better adapted to terrestrial conditions because the zygote formed by the fusion of the two gametes is protected. Higher plant forms do not require external water for fertilization. All of this indicates the significant evolutionary advancement of higher plants, particularly seed plants.