Reproduction in Plants:
Reproduction (or procreation or breeding) is the biological process by which new individual organisms – “offspring” – are produced from their “parents”. Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. Reproduction is the sine-qua-non of ‘Continuity of Life’ and without it, all life will cease to exist. This article is based on reproduction in plants.
There are two forms of Reproduction: Asexual and Sexual. In asexual reproduction, an organism can reproduce without the involvement of another organism; while Sexual reproduction typically requires the sexual interaction of two specialized organisms, with typically a male fertilizing a female of the same species to create offspring organisms whose genetic characteristics are derived from those of the two parental organisms.
Reproduction in plants is no different. It takes place via both Sexual and Asexual mechanisms and differs according to the plant species. We will learn here in depth about the various modes of reproduction in plants:
Asexual Reproduction 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 a vegetative piece of the original plant such as root, stem or leaf is involved in producing an offspring, it is know as ‘Vegetative Reproduction in Plants.’ It is often known as a process of ‘Survival’ and expansion of Biomass.
The broad advantages and disadvantages of Vegetative Reproduction in Plants are listed in the table below:
|Advantages of Asexual Vegetative Reproduction in Plants||Disadvantages of Asexual Vegetative Reproduction in Plants|
|Enables perennial survival of plants||Plants can reproduce only in limited distance|
|Facilitates expansion in size||Pathogens are transmitted from Parent to Offspring.|
The various types of Asexual Vegetative Reproduction in Plants are:
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. 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.
Image source: Google Images
Fragmentation in plants is a form of asexual reproduction or cloning in which an organism is split into fragments. Each of these fragments develop into mature, fully grown individuals that are clones of the original organism. Fragmentation is the mode of reproduction in Algae such as Spirogyra.
Image source: Google Images
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 favorable, 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.
Image source: Google Images
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.
Bryophyllum is a plant which reproduces through the vegetative propagation through leaves. Plants such as Sweet Potato and Dahlia reproduce through vegetative propagation by the roots.
It is a replacement of the normal sexual reproduction by asexual reproduction without fertilization. Seeds generated by Apomixis are a means of asexual reproduction, involving the formation and dispersal of seeds that do not originate from the fertilization of the embryos. The Offspring is genetically similar to parents. For example: Dandelion.
Image Source: Google Images
Sexual Reproduction In Plants:
For sexual reproduction of plants, interaction between the male and female species is a prerequisite. The offsprings’ genetic structure is not identical but derived from the combination of parent plants.
The sexual reproduction in plants takes place in two phases:
Through meiosis, the genes of the organism are rearranged and the number of chromosomes is reduced to half i.e Haploid. The plant Produces Gametophytes through the process of meiosis. The Gametophyte then produces the Male or Female Gametes by cell division or mitosis.
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.
History of Sexual Reproduction In Plants:
- First generation plants were aquatic and released the ‘Sperm’ or male gametes directly into water; where it would meet with other plants’ female counterparts.
- As taller and more complex plants evolved, reproduction was through ‘Spores’ which would travel along with the wind.
- The seed plants evolved later, which had ‘Pollen’ a protective covering for the male gamete; which enabled to preserve the viability of the Sperm for greater distances.
Parts of the Reproductive System in Plants:
The Flower is the basic reproductive organ in plants.
A flower is called Unisexual if it has only either the male or the female reproductive parts in it; example: flowers of Corn, Papaya, Cucumber.
A flower is called bisexual if it has both the male and the female reproductive parts in it; example: flowers of Mustard, Rose, Petunia.
Structure of the Flower:
The flowers are made up of the vegetative parts i.e. the calyx and the corolla and the reproductive parts i.e. the androecium and the gynoecium which are arranged in layers or ‘whorls’
The male reproductive part or the Androecium is made up of units called ‘Stamen‘. Each Stamen consists of two parts: A stalk called the ‘Filament‘ which is topped by ‘Anther.‘ ‘Pollen Grains‘ which contain the male gametes or sperms are produced in the Anther.
The female reproductive part or the Gynoecium is the immermost whorl of the flower. It is made up of units called ‘Carpel.’ Multiple fused carpels form the ‘Ovary‘ where ‘Ovules’ containing female gametes are produced. Pistil is a structure comprising of fused carpels and has a sticky tip called the Stigma which acts as a receptor of pollen. The long Stalk acts as a supporting structure and aids development of Pollen tubes from the stigma downwards.
Image Source: Google Images.
Process of Sexual Reproduction in Plants:
The sexual reproduction ins plants is carried out majorly by the process of Pollination. It is a process through which the pollen grain from an anther (male gamete) lands on to the stigma and gradually mates with the ovule (female gamete)
Image Source: Google Images
Pollination can be of two types:
|Self Pollination||Cross Pollination|
|Pollen grains are transferred to the stigma of the same flower||Pollen grains are carried to stigma of another flower of the same plant type|
|Occurs in bisexual plants having anther and stigma maturing at same time||Occurs in unisexual flowers or in bisexual flowers having anther and stigma maturing at different times|
|Example: Plants like Wheat, Peas||Example: Plants like Lady Finger, Tomato, Brinjal|
For majority of plants, insects or animals act as ‘Pollinators‘ or carriers of pollen from one plant to another. Flowers have numerous mechanisms to attract the pollinators such as color, scent, heat, nectar glands, edible pollen and flower shape.
Process of Fertilization:
The formation of a Diploid Zygote by the union of two Haploid male and female gametes is known as ‘Fertilization’
The process begins after the pollen grain sticks to the stigma and begins sending down the pollen tube through which the male gametes pass and unite with the female gametes in the ovary.
Fertilization in flowering plants is unique among all known organisms, in that not one but two cells are fertilized, in a process called Double Fertilization. One sperm nucleus in the pollen tube fuses with the egg cell in the embryo sac, and the other sperm nucleus fuses with the diploid endosperm nucleus. The fertilized egg cell is a zygote that develops into the diploid embryo of the sporophyte. The fertilized endosperm nucleus develops into the triploid endosperm, a nutritive tissue that sustains the embryo and seedling.
Fruit and Seed Formation:
The matured and fertilized ovules form the seeds whereas the the fruits are formed from the mature ovaries.
The fruit formed by a single ripened ovary is called as a ‘Simple Fruit’ For example: Apples, Oranges, Apricots
The fruit formed by a cluster of matured ovaries, of a single flower is termed ‘Aggregated Fruit’ for example: Blackberry, Raspberry, Strawberry.
A cluster of many ripened ovaries on separate flowers growing together in the same inflorescence is called a ‘Multiple Fruit’ for example: Pineapple, Mulberry, Fig.
Difference between Asexual and Sexual Reproduction in Plants:
The most important differences between the mechanisms of sexual and asexual reproduction in plants are tabulated below:
|Asexual Reproduction||Sexual Reproduction|
|Reproduction occurs through a single parent plant||Both male and female parent plants are required|
|It occurs in unisexual plants||It can occur in only bisexual plants|
|Morphologically underdeveloped plants use asexual mechanism||Morphologically developed plants use sexual mechanism|
|Reproductive parts are not present||Fully developed reproductive parts such as anther and pistil are present|
|In most methods, original parent ceases to exist after process of reproduction||The original parent continues to exist after process of reproduction|
|The characteristics of the offsprings are identical to parent plant||The characteristics of the offsprings are derived from both the parent plants.|
|No requirement of seeds||Seeds are required to produce new plants|
Applied Biology of Reproduction Of Plants:
Thus to summarize, 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 also the scientific agricultural practices lead to a higher yield.
Many new techniques of cultivation such as Grafting, Cutting, Budding, etc are in vogue nowadays to produce hybrid crops. These technique of grafting, cutting, etc are based on the knowledge of asexual vegetative reproduction and serve as enhancements of the natural process of plant cultivation.
In horticulture, a “cutting” is a branch that has been cut off from a mother plant below an internode and then rooted, often with the help of a rooting liquid or powder containing hormones. When a full root has formed and leaves begin to sprout anew, the clone is a self-sufficient plant, genetically identical to the mother plant. Examples include cuttings from the stems of blackberries, African violets, verbenas,etc. to produce new plants.
A related use of cuttings is grafting, where a stem or bud is joined onto a different stem. Nurseries offer for sale trees with grafted stems that can produce four or more varieties of related fruits, including apples. The most common usage of grafting is the propagation of cultivars onto already rooted plants, sometimes the rootstock is used to dwarf the plants or protect them from root damaging pathogens.
Since vegetatively propagated plants are clones, they are important tools in plant research. When a clone is grown in various conditions, differences in growth can be ascribed to environmental effects instead of genetic differences.
Some Important Terms to Keep in Mind:
- Apomixis- A replacement of Sexual Reproduction wherein the seeds are not fertilised
- Meiosis- The division of nuclei to form haploid cells
- Mitosis- Multiplication of cells giving rise to new cells with similar chromosomal structure
- Haploid- A single set of chromosomes
- Diploid- A set of pair of chromosomes
- Androecium- Male reproductive parts of flower
- Gynoecium- Female reproductive parts of flower
- Zygote- Diploid cell formed by union of two haploid male and female gametes
- Endosperm- A triploid chromosomal tissue that sustains the fertilized embryo and seedling.
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