Botany - B.Ye. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter IV. Plant Reproduction

Reproduction is the ability of organisms to produce offspring, accompanied by an increase in the number of individuals. Reproduction is an essential property of living organisms, ensuring the survival of a species across time and space, as well as the transmission of hereditary traits from generation to generation. Species that lose their ability to reproduce eventually die out.

There are two MAIN TYPES OF plant reproduction: Selection/8.html">Asexual and sexual. Asexual reproduction is conventionally divided into Vegetative Reproduction and reproduction via spores.

Vegetative reproduction. Vegetative reproduction is The formation of new plants from vegetative Organs or parts thereof. It relies on the remarkable regenerative capacity of plants—their biological ability to regenerate a complete Organism from a fragment of the body. Unlike animals, plants retain the capacity for vegetative reproduction across all Levels of biological Organization.

In unicellular Algae, this process simply involves the division of the parent Cell. In multicellular algae, it may occur through the fragmentation of the thallus or the formation of specialized structures such as buds or bulbils. The offspring resulting from vegetative reproduction are referred to as a clone.

In lower spore-producing plants, vegetative propagation mostly occurs via fragments of the rhizome, whereas flowering plants exhibit a vast diversity of vegetative reproduction Methods. This type of reproduction is categorized into natural and artificial. Natural vegetative reproduction occurs spontaneously in nature, whereas artificial propagation is driven by human agricultural and horticultural practices.

Plant metamophoses play a crucial role in natural vegetative reproduction. Many plants propagate through rhizomes, runners, stolons, bulbs, ROOT and stem sprouts, or layers.

Most perennial grasses (such as couch grass, yarrow, lily of the valley, and irises) propagate via rhizomes. Renewal buds develop on these rhizomes, eventually giving rise to aerial shoots that mature into independent plants.

Monocotyledonous plants belonging to the lily, onion, and amaryllis families reproduce via bulbs. In nature, plants like peonies and cyclamens propagate through tubers, while thistles, acacias, sea buckthorn, and raspberries spread via root sprouts. Strawberries typically propagate using runners.

Certain plants are termed "viviparous" due to their ability to produce brood buds. Brood buds are rudimentary shoots that form either in the leaf axils (such as toothwort and lily), along the leaf margins (Bryophyllum), or within inflorescences (bulbous bluegrass). Upon detaching from the parent plant, these buds develop into new individuals.

Layers are the lower branches of certain shrubs that, when covered with soil, develop adventitious roots. In time, as they lose their connection to the parent plant, they become independent individuals. In nature, hazel and currant naturally form layers, especially when growing on the slopes of ravines.

Artificial vegetative propagation is widely utilized in human agriculture and horticulture. For instance, irises, mint, lemon balm, and lilies of the valley are propagated by rhizomes; strawberries by runners; currants and gooseberries by layers; raspberries and lilacs by root sprouts; potatoes, dahlias, and Jerusalem artichokes by tubers; and tulips, lilies, narcissuses, amaryllis, hyacinths, onions, and garlic by bulbs. When necessary, certain plants—such as primroses, phlox, and chrysanthemums—are propagated by dividing the bush.

In addition to these methods, human agricultural practices have introduced other techniques of artificial vegetative reproduction, namely: cutting, grafting, and tissue culture.

A cutting is a portion of a vegetative organ used for propagation. Cuttings can be stem, root, or leaf cuttings. Stem cuttings are further divided into autumn and summer cuttings. Autumn cuttings consist of a segment of a young SHOOT up to 50 cm long and 1 cm thick; they are harvested at the end of the growing season and stored in a cool place until rooting. In spring, they are rooted and planted in their permanent Location. This method is used to propagate grapes, currants, poplars, and willows. Summer cuttings are harvested during the summer, must contain at least two nodes, and are rooted in specialized greenhouses before being transplanted. This technique is used for various ornamental plants. Indoor decorative plants—such as Saintpaulia, begonia, and gloxinia—are typically propagated using leaf cuttings.

Tissue and organ culture, also known as micropropagation, is employed to obtain healthy, virus-free planting material of cultivated crops. The Starting Material for this technique includes the Meristems of apical and axillary shoot buds, young leaves, leaf petioles, parts of inflorescences and flowers, as well as bulbs and tubers. Meristem fragments are placed into a specialized nutrient medium to grow plants, which are subsequently multiplied via cuttings to yield disease-free planting stock.

Grafting, or transplantation, involves joining the cut cuttings or buds of the plant being propagated (the scion) with another rooted plant (the rootstock). Over 100 grafting methods are known, the most common being approach grafting (inarching), whip grafting (using a cutting), and budding (shield budding). Grafting is particularly widespread in horticulture and floriculture, serving as an essential tool for preserving cultivars and breeding new plant varieties.

The KEY FEATURES OF vegetative reproduction are as follows:

- the offspring precisely and fully replicate the traits of the parent organism, making them well-adapted to specific environmental conditions;

- the offspring develop at the expense of the parent plant, rendering them highly viable and competitive;

- the offspring rapidly enter the fruiting phase;

- the offspring quickly colonize territories, particularly in environments unfavorable for seed-based reproduction.

The primary drawback of this reproductive mode is that prolonged vegetative propagation leads to the accumulation of deleterious genes in the offspring, which may ultimately result in species degeneration.

Asexual reproduction. Asexual reproduction is carried out via specialized propagules such as spores or zoospores. Most commonly, a spore is a single cell containing a specific reserve of nutrients. In lower plants, a spore develops directly into an organism similar to the parent. Spores may be non-motile (aplanospores), which are dispersed by air currents or, less frequently, by Water; or motile (zoospores), which possess flagella and are transported by water. Consequently, spore walls vary in Structure: spores and aplanospores feature a thick, rigid Cell wall, whereas zoospores have a delicate, thin membrane.

Spores are formed either inside the parent cell or within specialized structures called sporangia or zoosporangia. In lower plants, spores are produced via mitosis, whereas in higher plants, they are formed through Meiosis and are termed meiospores; these never develop directly into an organism resembling the parent plant.

Fungi are particularly well-adapted to spore-mediated reproduction. As a rule, fungal dispersal across space occurs via spores, while the sexual process is triggered by unfavorable environmental conditions. In fungi, spores and zoospores are formed not only internally within sporangia but also exogenously On the surface of the thallus (mycelium). These external spores are called conidia, and the structures bearing them are termed conidiophores.

The sporangia of lower plants develop from a single cell, whereas in higher plants, they are multicellular organs possessing a single- or multi-layered wall.

In many higher plants, spores can vary in size: smaller spores are called microspores, and larger ones are megaspores. They differ not only in size but also in physiological content.

Spore reproduction enables plants to spread rapidly across space.

Sexual reproduction, sexual process. Sexual reproduction is defined as reproduction in which new individuals are formed As a result of a sexual process, The Essence of which lies in the fusion of two physiologically dissimilar Cells—Gametes—resulting in the formation of a new cell, the zygote. The zygote combines the genetic material of two non-identical parental forms, which ensures The Emergence of genetically diverse offspring with traits from both parents.

The sexual process arose in the Cytology/cytology/16.html">Early stages of historical development and may have initially manifested as the engulfment of one cell by another; over time, these cells entered into a symbiotic relationship, giving rise to the sexual process. The refinement of the sexual process took quite a long time. Initially, unspecialized somatic cells fused—this type of sexual process is called agametogamy. Only later did gametogamy emerge as a more advanced type of sexual process, resulting in the formation of specialized sex cells—gametes.

Agametogamy has manifested in the following forms: hologamy, conjugation, zygogamy, and gametangiogamy.

Hologamy is the fusion of the protoplasts of two unicellular algae, occurring in Chlamydomonas, Closterium, and other green algae.

Conjugation is the fusion of the protoplasts of two morphologically identical, yet physiologically dissimilar individuals, characteristic of certain green algae, such as Spirogyra.

Zygogamy is a special type of sexual process characteristic of certain fungi, in which physiologically dissimilar Regions of the mycelium fuse. The sexual product is a zygospore.

Gametangiogamy is characteristic of ascomycetous fungi and consists of the fusion not of gametes, but of multinucleate unicellular sex organs—gametangia.

The following forms of the more advanced sexual process—gametangiogamy—are known: isogamy, heterogamy, and oogamy.

Isogamy, or equal mating, is the fusion of two motile, identical gametes. It is characteristic of certain algae and fungi.

Heterogamy is the fusion of two gametes, one of which is larger and less motile. It occurs in some fungi, green algae, and brown algae.

Oogamy is the most advanced type of sexual process, characterized by a clear differentiation of gametes: the female gamete—the egg cell—is larger and non-motile, while the male gamete—the sperm cell—is smaller and motile. As a rule, gametes are formed in specialized sex organs called gametangia. In lower plants, the female gametangium is called an oogonium, while in higher plants it is called an archegonium. The male sex organs in both lower and higher plants are called antheridia. For the most part, the gametangia of lower plants are unicellular, whereas in higher plants they are multicellular.

In the course of evolution, a phenomenon unique to plants arose: the alternation of two generations in The life cycle—the asexual and the sexual. The asexual generation (sporophyte) is a plant that develops from a zygote resulting from the sexual process. The sexual generation (gametophyte) is a plant that grows from a spore, upon which the sexual process takes place. The Morphology/12.html">ALTERNATION OF GENERATIONS is accompanied by A change in nuclear phases: sporangia form on the diploid sporophyte, within which haploid spores are produced via meiosis. A gametophyte grows from the spore, developing sex organs and gametes; the sexual process occurs, forming a zygote from which the embryo of the future sporophyte develops.

Thanks to the alternation of generations, two modes of reproduction are combined: asexual and sexual. Asexual reproduction ensures an increase in the number of individuals, while sexual reproduction enriches the offspring with new Gene combinations, increases their heterozygosity, promotes plant ADAPTATION TO ENVIRONMENTAL conditions, and drives the evolutionary process.

The alternation of generations and nuclear phases is characteristic of most plants; it originated in the aquatic environment and manifests in the following types:

- without alternation of generations: the plant spends its life cycle in the haplophase, with only the zygote being diploid, which typically forms under unfavorable conditions (observed in Chlamydomonas, Ulothrix); the plant exists in the diplophase, with haploid gametes (Fucus, a brown alga);

- isomorphic alternation of generations: gametophytes and sporophytes exist in parallel and are morphologically indistinguishable from one another (the brown alga Dictyota);

- heteromorphic alternation of generations: the gametophyte and sporophyte differ in size and lifespan; this is the most widespread type of development, characteristic of all Higher Plants and some red and brown algae. In this case, The Development of the sporophyte and gametophyte is unequal. Thus, in most higher plants, the sporophyte is long-lived and performs many diverse Functions, whereas the gametophyte is smaller, lives for 1–3 years, and its primary function is the sexual process. Only one group of higher spore-bearing plants—bryophytes—is characterized by the dominance of the gametophyte in the life cycle.

- alternation of three or more Haploid and Diploid generations (most red algae);

- with the alternation of three nuclear phases (haploid, diploid, and dikaryophase)—ascomycetes and Basidiomycetes.

- the appearance of a triploid phase in the life cycle—angiosperms.

Thus, sexual reproduction is considered the most advanced type of reproduction, as it ensures offspring heterozygosity, increases their resilience and environmental adaptability, and creates conditions for the evolutionary process.