Plant Reproduction

Types of reproduction 

Asexual reproduction

Plants exhibit two primary types of asexual reproduction that result in new plants genetically identical to the parent. The first method, vegetative reproduction, involves producing new plants from vegetative parts of the parent plant such as stems, roots, or leaves, rather than from seeds. This form of reproduction typically involves structural modifications of the plant's stems, roots, or sometimes leaves. Vegetative reproduction allows plants to expand and survive from one season to the next by creating clonal colonies, with each new plant, or ramet, being genetically identical to the parent. Examples of vegetative reproduction include the use of rhizomes, which are modified underground stems capable of growing into new plants, as seen in species like Polypody, Iris, Couch Grass, and Nettles. Stolons, or prostrate aerial stems, spread horizontally and are important in species such as strawberries and many grasses. Adventitious buds can form on roots or damaged stems and develop into new stems and leaves. Bulb formation is another form of vegetative reproduction found in plants like onions, hyacinths, and tulips, while tubers, such as those produced by potatoes and dahlias, and corms, seen in gladioli and crocus, are also methods of vegetative reproduction.


The second method, apomixis, is a form of asexual reproduction where seeds are produced without fertilisation. This process bypasses the need for sexual reproduction, allowing plants to produce seeds that are genetically identical to the parent. In apomixis, seeds form without the typical process of fertilisation, although in some cases, pollination is required to initiate embryo development. This process is observed in species like Hieracium, Taraxacum, some Citrus varieties, and Poa pratensis. One type of apomixis, known as pseudogamy, involves pollination to initiate embryo development, but the pollen does not contribute genetic material. Other forms of apomixis include the generation of plantlets or bulbils in place of seeds.


How people use asexual reproduction

Humans utilise various asexual reproduction methods to propagate plants, often enhancing natural processes. For instance, cuttings involve taking a segment of a plant, usually a stem or leaf, from the parent plant and rooting it to grow a new plant. This technique is used for species such as Rubus occidentalis and Saintpaulia. Grafting and budding involve joining a stem or bud from one plant onto another, a method commonly employed in fruit tree propagation to create trees with multiple fruit varieties or to propagate plants with specific desirable traits. Layering is another technique where a stem is bent to the ground and covered with soil to encourage rooting, which can be performed through air or ground layering. Division involves splitting a plant into sections, each with roots and shoots, to grow new plants. Micro-propagation is a laboratory-based technique that uses tissue cultures to produce large numbers of plants from a single parent. Techniques such as using stolons or runners, and employing storage organs like bulbs, corms, tubers, and rhizomes, are also methods of asexual reproduction. Striking or cuttings and twin scaling are additional techniques used to propagate plants. These methods are especially valuable for propagating cultivars with specific desirable traits and for conducting plant research by isolating environmental effects on growth without the influence of genetic variability.

Sexual reproduction 

Sexual reproduction in plants involves two key processes: meiosis and gamete fusion. Meiosis rearranges genes and reduces the number of chromosomes in cells, while the fusion of gametes restores the chromosome number to a complete diploid set. In plants and algae that exhibit alternation of generations, a haploid multicellular structure called the gametophyte plays a crucial role. The gametophyte, which has a single set of chromosomes, produces male and female gametes through mitosis. When these gametes fuse, they form a diploid zygote that develops into a multicellular sporophyte. The sporophyte, being the result of gamete fusion, contains two sets of chromosomes. It then produces haploid spores through meiosis, which develop into new gametophytes.


In land plants such as ferns, mosses, and liverworts, the gametophyte is relatively small. In flowering plants (angiosperms), the gametophyte is reduced to just a few cells: the female gametophyte, or embryo sac, is referred to as a mega-gametophyte, while the male gametophyte, or pollen, is called a micro-gametophyte.


Seed structure and anatomy

The seed is a crucial stage in a plant's life cycle, containing an immature plant, or embryo, from which a new plant will emerge under suitable conditions. The embryo consists of several key parts: in monocotyledons, it has a single cotyledon or seed leaf; in most dicotyledons, there are two cotyledons; and in gymnosperms, it may have two or more cotyledons. The radicle represents the embryonic root, while the plumule is the embryonic shoot. The epicotyl is the part of the embryonic stem located above the cotyledon attachment, and the hypocotyl is the portion below this attachment.


Within the seed, there is typically a reserve of nutrients for the developing seedling. The nature of this stored nutrition varies by plant type. In angiosperms, this nutrient reserve initially forms as endosperm, a tissue generated through double fertilization. This triploid endosperm is rich in oil, starch, and protein. In gymnosperms, such as conifers, the food storage occurs in the female gametophyte, a haploid tissue. In some species, the embryo is surrounded by endosperm or female gametophyte tissue, which is used by the seedling upon germination. In other species, the endosperm is absorbed by the embryo, with the cotyledons filling with this stored food. Mature seeds of these species, known as exalbuminous seeds, lack endosperm and include varieties such as beans, peas, oaks, walnuts, squashes, sunflowers, and radishes. Seeds that retain endosperm at maturity are termed albuminous seeds, which include most monocots and many dicots, like castor beans, as well as all gymnosperms.


The seed coat, or testa, develops from the integument surrounding the ovule and varies from a thin layer to a thicker protective covering. This coat shields the embryo from mechanical damage and dehydration. Some seeds also feature appendages such as an aril, elaiosome, or hairs, and may display a hilum, a scar where the seed was attached to the ovary wall by the funiculus.


Seed Production

Seed production varies among plant groups, distinguishing angiosperms from gymnosperms. Angiosperm seeds are produced within a fruit, a hard or fleshy structure that encloses the seeds. Some fruits have layers of both hard and fleshy material. Gymnosperms, however, do not develop a special fruit structure; their seeds develop exposed on the bracts of cones, though they may become covered by cone scales in some conifers.


Seed production can fluctuate significantly in natural plant populations due to factors such as weather, pests, diseases, and internal plant cycles. For instance, over a 20-year period, forests of loblolly pine and shortleaf pine produced between 0 to nearly 5 million viable seeds per hectare, with variations in seed crops ranging from bumper to poor and adequate yields.


Types of seeds

Many structures commonly mistaken for seeds are actually dry fruits. For example, sunflower seeds are sold still encased in the hard fruit wall, which must be opened to access the seed. Stone fruits, like those from the genus Prunus, have a hardened fruit layer fused around the seed. Nuts, such as acorns and hazelnuts, are one-seeded fruits with a hard shell and an indehiscent seed.


Seed development

Seeds consist of an embryo with two growth points: one forming the stems and the other the roots, encased in a seed coat with nutrient reserves. In angiosperms, seeds have three genetically distinct parts: the embryo, derived from the zygote; the endosperm, usually triploid; and the seed coat, originating from maternal ovule tissue. Seed development begins with double fertilization, where the egg and sperm nuclei form a zygote, and the polar nuclei fuse with a second sperm cell to create the primary endosperm. After fertilization, the zygote is largely inactive, while the primary endosperm divides rapidly to form the endosperm tissue, providing nourishment to the developing plant until germination. The seed coat forms from the integuments of the ovule, with the inner integument becoming the tegmen and the outer the testa.


In gymnosperms, seed development involves a simpler process where one sperm nucleus fuses with the egg nucleus, while the other sperm cell is not used. Sometimes, each sperm fertilizes an egg cell, and one zygote may be aborted. Gymnosperm seeds include the embryo and maternal tissue, often forming a cone structure around the seed in conifers like pine and spruce.


Ovules develop into seeds post-fertilization, with main components including the funicle (attaching the ovule to the placenta), the nucellus (where the embryo sac develops), the micropyle (a small opening for pollen tube entry), and the chalaza (where integument and nucellus join). The shape of the ovules influences the final seed shape, with common types including anatropous (curved), orthotropous (straight), campylotropous (curved embryo sac), and amphitropous (partially inverted).


In flowering plants, the zygote's initial division sets the embryo's polarity, with the chalazal pole becoming the main growth area and the micropylar pole forming the suspensor, which supports nutrient absorption from the endosperm. The embryo comprises the epicotyl (forming the shoot), radicle (forming the root), hypocotyl (connecting epicotyl and radicle), cotyledons (seed leaves), and the seed coat (testa). Monocots, like corn, have additional structures such as the coleoptile (forming the first leaf) and the coleorhiza (protecting the root). Both monocots and dicots may have seed coats with various patterns, textures, or appendages.


Seed fispersal methods

Plants, unable to move to new locations, have evolved numerous seed dispersal strategies to ensure their offspring reach favorable conditions for growth. Dispersal methods vary depending on whether seeds are released from fruits in a dehiscent manner (e.g., capsules, follicles, legumes) or remain enclosed in indehiscent fruits (e.g., achenes, nuts). Some seeds are dispersed while still in their fruit or cone, which later opens to release them, while others are expelled or released from the fruit before dispersal. For instance, milkweeds produce follicles that split to release seeds, and iris capsules open into three valves to release their seeds.


Anemochory (By wind)


Hydrochory (By water)


Zoochory (By animal)