Reproduction is a fundamental feature of all known life including reproduction in plants. Each individual organism exists as a result of reproduction. Reproduction is essential for ‘Continuity of Life’ and without it, all life will cease to exist. This unit is based on reproduction in plants.
Reproduction (or procreation or breeding) is the biological process by which new individual organisms “offspring” are produced from their “parents”.
There are two forms of reproduction in plants, asexual reproduction and sexual reproduction. In asexual reproduction, an organism can reproduce without the involvement of another organism; while sexual reproduction typically requires sexual interaction of two specialized organisms, which typically involves a male fertilizing a female of the same species to create offspring whose genetic characteristics are derived from those of the two parental organisms.
Plant reproduction is the process by which plants generate new individuals, or offspring. Reproduction is either sexual or asexual. Sexual reproduction is the formation of offspring by the fusion of gametes.
Asexual reproduction is the formation of offspring without the fusion of gametes. Sexual reproduction results in offspring that is genetically different from the parents. Asexual offspring are genetically identical except for mutation.
In higher plants, offspring are packaged in a protective seed, which can be long lived and can disperse the offspring some distance from the parents. In flowering plants (angiosperms), the seed itself is contained inside a fruit, which may protect the developing seeds and aid in their dispersal.
1a. Sexual Reproduction in Plants – Seed Development
Sexual reproduction in plants results from fertilization, the union, of gametes (cells that have undergone meiosis) from two genetically different plants. Meiosis reduces the number of chromosomes in half producing gametes.
Fertilization recombines gametes from different individuals, producing a zygote, which develops into either spores or seeds.
The main purpose of sexual reproduction is the rearrangement of genes in the next generation.
This serves to increase the genetic diversity of the population, enhancing the evolutionary viability of the species in the face of changing environmental factors.
All plants have a life cycle that consists of two distinct forms that differ in size and the number of chromosomes per cell.
In flowering plants, the large, familiar form that consists of roots, shoots, leaves, and reproductive structures (flowers and fruit) is diploid and is called the sporophyte.
The sporophyte produces haploid microscopic gametophytes that are dependent on tissues produced by the flower.
The reproductive cycle of a flowering plant is the regular, usually seasonal, cycling back and forth from sporophyte to gametophyte. The flower produces two kinds of gametophytes, male and female.
The female gametophyte arises from a cell within the ovule, a small structure within the ovary of the flower. The ovary is a larger structure within the flower that contains and protects usually many ovules.
Flowering plants are unique in that their ovules are entirely enclosed in the ovary. The ovary itself is part of a larger structure called the carpel, which consists of the stigma, style, and ovary.
Each ovule is attached to ovary tissue by a stalk called the funicle. The point of attachment of the funicle to the ovary is called the placenta.
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The male gametophyte is the mature pollen grain. Pollen is produced in the anthers, which are attached at the distal end of filaments. The filament and anther together constitute the stamen, the male sex organ.
Through chemical recognition, the pollen tube changes its direction of growth and penetrates through the placenta to the ovule.
Here the tube reaches the embryo sac lying close to the micropyle, and sexual fertilization takes place.
It involves union of both male and female ‘gametes’ and leads to restoration of the number of chromosomes to form a Diploid Zygote, which then develops into the offspring. The resulting genetic composition has characteristics of both the parent plants from which it is derived.
After fertilization, the diploid zygote develops into the embryo, and the triploid endosperm cells multiply and provide nutrition. The testa usually shows a scar called the hilum where the ovule was originally attached to the funicle.
In some seeds a ridge along the testa called the raphe shows where the funicle originally was pressed against the ovule.
The micropyle of the ovule usually survives as a small pore in the seed coat that allows passage of water during germination of the seed.
The embryo consists of the cotyledon(s), epicotyl, and hypocotyl. The cotyledons resemble small leaves, and are usually the first photosynthetic organs of the plant.
The portion of the embryo above the cotyledons is the epicotyl, and the portion below is the hypocotyl.
The epicotyl is an apical meristem that produces the shoot of the growing plant and the first true leaves after germination.
The hypocotyl develops into the root. Often the tip of the hypocotyl, the radicle, is the first indication of germination as it breaks out of the seed.
Flowering plants are classified as monocotyledons or dicotyledons (most are now called eudicots) based on the number of cotyledons produced in the embryo.
Common monocotyledons include grasses, sedges, lilies, irises, and orchids; common dicotyledons include sunflowers, roses, legumes, and all non-coniferous trees.
The endosperm may be consumed by the embryo, as in many legumes, which use the cotyledons as a food source during germination. In other species the endosperm persists until germination, when it is used as a food.
1b. Sexual Reproduction – Fruit Development
The fruit of a flowering plant is the mature ovary. As seeds mature, the surrounding ovary wall forms a protective structure that may aid in dispersal. The surrounding ovary tissue is called the pericarp and consists of three layers.
From the outside to inside, these layers are the exocarp, mesocarp, and endocarp. The exocarp is usually tough and skinlike.
The mesocarp is often thick, succulent, and sweet. The endocarp, which surrounds the seeds, may be hard and stony, as in most species with fleshy fruit, such as mango.
Structure of maize grain: A-Seed; B-Same cut length wise; C- Embryo enlarged
A fruit is termed simple if it is produced by a single ripened ovary in a single flower (apples, oranges). An aggregated fruit is a cluster of mature ovaries produced by a single flower (blackberries, strawberries).
A multiple fruit is a cluster of many ripened ovaries on separate flowers growing together in the same inflorescence (pineapple). A simple fruit may be fleshy or dry.
A fleshy simple fruit is classified as a berry (grape, tomato, papaya), pepo (cucumber, watermelon, pumpkin), hesperidium (orange), drupe (apricot), or pome (apple).
Dry simple fruits have a dry pericarp at maturity. They may or may not dehisce, or split, along a seam to release the seeds. A dehiscent dry fruit is classified as legume or pod (pea, bean), silique or silicle (mustard), capsule (poppy, lily), or follicle (milkweed, larkspur, columbine).
An indehiscent dry fruit that does not split to release seeds is classified as an achene (sunflower, buttercup, sycamore), grain or caryopsis (grasses such as corn, wheat, rice, barley), schizocarp (carrot, celery, fennel), winged samara (maple, ash, elm), nut (acorn, chestnut, hazelnut), or utricle (duckweed family).
Some fruiting bodies contain non-ovary tissue and are sometimes called pseudocarps. The sweet flesh of apples and pears, for example, is composed not of the pericarp but the receptacle, or upper portion, of the flowering shoot to which petals and other floral organs are attached.
Protection can be physical (hard coverings) or chemical (repellents of seed predators). Sweet, fleshy fruits are attractive food for birds and mammals that consume seeds along with the fruit and pass the seeds intact in their fecal matter, which can act as a fertilizer.
Dry fruits are usually adapted for wind dispersal of seeds, as for example with the assistance of wing-like structures or a fluffy pappus that provides buoyancy.
The diversity of fruiting bodies reflects in part the diversity of dispersal agents in the environment, which select for different fruit size, shape, and chemistry.
2. Asexual Reproduction in Plants
The ability to produce new individuals asexually is common in plants. In asexual reproduction, new plants are produced that are genetically identical clones of the parent plant, and without the contribution of genetic material from another plant.
Asexual reproduction in plants can be further divided into two: Vegetative reproduction and apomixis.
Vegetative Reproduction in Plants
When a vegetative piece of the original plant such as root, stem or leaf is involved in producing an offspring, it is known as ‘Vegetative Reproduction in Plants.’
It is often known as a process of ‘Survival’ and expansion of biomass. Below are examples of vegetative reproduction.
1. Budding
Budding is a form of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site.
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The new organism remains attached as it grows, separating from the parent organism only when it is mature, leaving behind scar tissue. For example: Yeast is a single-celled organism which reproduces by this mode.
2. Fragmentation
Fragmentation in plants is a form of asexual reproduction or cloning in which an organism is split into fragments.
Each of these fragments develops into mature, fully grown individuals that are clones of the original organism. Fragmentation is the mode of reproduction in Algae such as Spirogyra.
3. Spore Formation
The term ‘Sporogenesis’ is used to refer to the process of reproduction in plant via spores. Reproduction via spores involves the spreading of the spores by water or air. Reproductive spores grow into multi-cellular haploid individuals or sporelings.
Each spore is covered by a hard protective outer covering to withstand unfavorable conditions such as high temperature and low humidity, thus enabling survival for a long time.
Once conditions are favourable, a spore germinates and develops into a new individual. The plant organisms which reproduce by this method are fungi on bread, certain types of moss and ferns, etc.
4. Vegetative Propagation
Vegetative Propagation is a type of asexual reproduction in which new plants are produced from roots, stems, leaves and buds.
Since reproduction is through the vegetative parts of the plant, it is known as vegetative propagation.
Vegetative propagation using cuttings- productivity is dependent on environment
Differences Between Sexual and Asexual Reproduction
The most important differences between the mechanisms of sexual and asexual reproduction in plants are tabulated below:
s/nSexual reproductionAsexual reproduction1Both male and female parent plants are requiredReproduction occurs through a single parent plant2It can occur in only bisexual plantsIt occurs in unisexual plants3Morphologically developed plants use sexual mechanismMorphologically underdeveloped plants use asexual mechanism4Fully developed reproductive parts such as anther and pistil are presentReproductive parts are not present5The original parent continues to exist after process of reproductionIn most methods, original parent ceases to exist after process of reproduction6The characteristics of the offsprings are derived from both the parent plants.The characteristics of the offsprings are identical to parent plant7Seeds are required to produce new plantsNo requirement of seeds
In summary, reproduction in plants occurs through either asexual or sexual mechanisms. A study of these mechanisms is of utmost importance in the science of Agriculture, Horticulture and Floriculture.
Healthy crops can be cultivated on the basis of this knowledge, and with scientific agricultural practices they will lead to higher yields.
Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. There are two forms of reproduction: Asexual and Sexual.
The productivity of crops arising from sexual reproductive method is usually influenced by both the environment and the genetic composition.
Fruits and seeds are the main products of sexual reproduction while in asexual reproduction, vegetative reproduction and apomixis are common products.
Vegetatively propagated plants are clones and therefore their differences in growth can only be ascribed to environmental effects instead of genetic differences.
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Frequently Asked Questions
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