Reproduction
- 1. 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 various types of Asexual Vegetative Reproduction in Plants are: 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. 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 develop into mature, fully grown individuals that are clones of the original organism. Fragmentation is the mode of reproduction in Algae such as Spirogyra.
- 2. 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. 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.
- 3. Apomixis: 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. 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: Meiosis: 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. Fertilization: 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. Parts of the Reproductive Systemin Plants: The Flower is the basic reproductive organ in plants.
- 4. 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 Stigmawhich acts as a receptor of pollen. The long Stalk acts as a supporting structure and aids development of Pollen tubes from the stigma downwards. ProcessofSexual Reproductionin 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)
- 5. ProcessofFertilization: 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.
- 6. Reproduction in animals Most of animals, even the simplest ones, have a sexed reproduction, which allows to increase the genetic variability of individuals and organisms’ diversity. Their internal organs, which are different in males and females, are called gonads and specifically produce gametes (sexed cells: sperms and egg-cells). They are bound to join and form a single cell called zygote or fertilized egg, from which the embryo, that is the new organism, will develop. Some animals, even invertebrates, are hermaphrodites, as they are able to produce both sperms and egg-cells. The chance to be able to behave as male and female individuals is an advantage for animals like earthworms, snails and slugs, very slow animals that in this way double their possibility to meet a mate. In this case both individuals will be able to produce new offspring. Where does fertilization occur? Animals that live in a water habitat release a high number of gametes into the water, where they will meet to form the zygote. This is called external fecundation. Those organisms that live on the land had to solve a serious problem: providing gametes with a liquid environment that allows their survival and fertilization. Some amphibians have adopted a simple solution: male and female exchange particular signs and go back into the water to release the gametes at the same time. Almost all terrestrial animals have adopted another solution: the male introduces the sperms into the female body, where the fertilization occurs (internal fecundation). This solution is advantageous as it protects the offspring during the first growth stages. The male individuals of insects, vertebrates and many animals that have a specialized organ (sexual organ) to insert the sperms inside the female organ and in particular into a hollow organ (vagina or cloaca). Other organisms have found particular solutions. For example some male arthropoda like mites and scorpions or some male amphibians like tritons build “bags” of sperms (spermatophore) and it’s the females who insert them into their own bodies. As for some spiders and squid, the male collects the bag he has formed and, by helping himself with his forelegs and tentacles, he inserts them into the female body. Embryo protection/ The care for the new individual during his growth From the zygote, the cell that forms after the insemination, the embryo develops, that is the new organism that at the beginning needs to be protected. In particular the embryo will have to be fed, oxygen will be needed for its breathing, and carbon dioxide will have to be kept away. Animals have solved this problem in two ways: • Insects, reptiles, birds and mammals monotremata like echidna (oviparous animals) lay the eggs in the same environment as they live. The embryo grows inside the egg that contains all the nutritional substances it needs. The eggshell protect it from dehydration, but allows oxygen to enter and prevents carbon dioxide from disperse outside. Some parents, however, take care of the eggs by building a nest and hatching them. Once the new organism has developed and gets free from the egg protection, he will need more care. • Mammals are viviparous animals since the females keep the embryo inside their body until a determined stage in its development. Marsupialia (kangaroo and opossum) deliver the babies when they have not completely developed yet. Then they are kept inside a
- 7. special pouch until the growth has been completed. Mammals other than marsupialia and monotremata are called placentalia, as they can count on a structure (placenta) that allows the embryo to grow completely inside the female body, guaranteeing an efficient supply of nutrients. Growth and development in plants Development is the progression from earlier to later stages in maturation, e.g. a fertilized egg develops into a mature tree. It is the process whereby tissues, organs, and whole plants are produced. It involves: growth, morphogenesis (the acquisition of form and structure), and differentiation. The interactions of the environment and the genetic instructions inherited by the cells determine how the plant develops. Growth is the irreversible change in size of cells and plant organs due to both cell division and enlargement. Enlargement necessitates a change in the elasticity of the cell walls together with an increase in the size and water content of the vacuole. Growth can be determinate—when an organ or part or whole organism reaches a certain size and then stops growing—or indeterminate— when cells continue to divide indefinitely. Plants in general have indeterminate growth. Plants differ from animals in their manner of growth. As young animals mature, all parts of their bodies grow until they reach a genetically determined size for each species. Plant growth, on the other hand, continues throughout the life span of the plant and is restricted to certain meristematic tissue regions only. This continuous growth results in: • Two general groups of tissues, primary and secondary. • Two body types, primary and secondary. • Apical and lateral meristems. Apical meristems, or zones of cell division, occur in the tips of both roots and stems of all plants and are responsible for increases in the length of the primary plant body as the primary tissues differentiate from the meristems. The plant may continue to grow in length, but no longer does it grow in girth. Herbaceous plants with only primary tissues are thus limited to a relatively small size. Woody plants, on the other hand, can grow to enormous size because of the strengthening and protective secondary tissues produced by lateral meristems, which develop around the periphery of their roots and stems. These tissues constitute the secondary plant body. Growth and development in animals Animal development, the processes that lead eventually to the formation of a new animal starting from cells derived from one or more parent individuals. Development thus occurs following the process by which a new generation of organisms is produced by the parent generation. Reproduction and development
- 8. A common feature of all forms of asexual reproduction is that the cells—always a substantial number of cells, never only one cell—taking part in the formation of the new individual are not essentially different from other body, or somatic, cells. The number of chromosomes (bodies carrying the hereditary material) in the cells participating in the formation of a blastema is the same as in the other somatic cells of the parent, constituting a normal, double, or diploid (2n), set. In sexual reproduction, a new individual is produced not by somatic cells of the parent but by sex cells, or gametes, which differ essentially from somatic cells in having undergone meiosis, a process in which the number of chromosomes is reduced to one-half of the diploid (2n) number found in somatic cells; cells containing one set of chromosomes are said to be haploid (n). The resulting sex cells thus receive only half the number of chromosomes present in the somatic cell. Furthermore, the sex cells are generally capable of developing into a new individual only after two have united in a process called fertilization. Each type of reproduction—asexual and sexual—has advantages for the species. Asexual reproduction is, at least in some cases, the faster process, leading most rapidly to the development of large numbers of individuals. Males and females are independently capable of producing offspring. The large size of the original mass of living matter and its high degree of organization—the new individual inherits parts of the body of the parent: a part of the alimentary canal, for instance—make subsequent development more simple, and the attainment of a stage capable of self-support easier. New individuals produced by asexual reproduction have the same genetic constitution (genotype) as their parent and constitute what is called a clone. Though asexual reproduction is advantageous in that, if the parent animal is well adapted to its environment and the latter is stable, then all offspring will benefit, it is disadvantageous in that the fixed genotype not only makes any change in offspring impossible, should the environment change, but also prevents the acquisition of new characteristics, as part of an evolutionary process. Sexual reproduction, on the other hand, provides possibilities for variation among offspring and thus assists evolution by allowing new pairs of genes to combine in offspring. Since all body cells are derived from the fertilized egg cell, a mutation, or change, occurring in the sex cells of the parents immediately provides a new genotype in each cell of the offspring. In the course of evolution, sexual reproduction has been selected for, and established in, all main lines of organisms; asexual reproduction is found only in special cases and restricted groups of organisms. Plants movement The movement of higher plants are chiefly in the form of bending, twisting, and elongation of certain plant parts or organs. Spontaneous movement: There are other plant movements which take place spontaneously, without any external stimuli. These movements are described spontaneous or autonomic movements. Induced movement: Some plant movements are caused in response to certain to certain stimuli and they are said to be induced or plant movement which take place spontaneously, without any irritability and sensitivity of protoplasm. There are the following three types of autonomic movements:-
- 9. 1. Movements and locomotion 2. Growth and curvature movement 3. Variation movements Similarly, paratonic movements also of three kinds:- 1. Tropic movements 2. Tactic movements 3. Nastic movements 1. TROPIC MOVEMENTS : Growth movements, which occur in response to unidirectional external stimuli & result in positioning of the plant part in the direction of the stimulus, are said to be tropic movements. Depending upon the nature of stimuli, these movements are of following types:- A) Phototropism B) Geotropism C) Hydrotropism D) Chemotropism 2. TACTIC MOVEMENTS Tactic movements are movements of locomotion, which are induced by some unidirectional external stimuli. Their direction is controlled by the direction of the stimulus. Depending upon the nature of stimuli, these movements are of following types:- A) Phototactic B) Chemotactic C) Thermotactic 3. NASTIC MOVEMENTS The movement can be due to changes in turgor or changes in growth Depending upon the nature of stimuli, these movements are of following types:- o A) Nyctinasty o B) Chemonasty o C) Seismonasty
- 10. Plant hormones Auxin This hormone is present in the seed embryo, young leaves, and apical buds' meristem. Functions of Auxins Stimulation of cell elongation, cell division in cambium, differentiation of phloem and xylem, root initiation on stem cuttings, lateral root development in tissue culture Cytokinin They are synthesized in roots and then transported to other parts of the plant. Functions of Cytokinins Stimulation of cell division, growth of lateral buds, and apical dominance Ethylene Ethylene is present in the tissues of ripening fruits, nodes of stems, senescent leaves, and flowers. Functions of Ethylene Leads to release of dormancy state Stimulates shoot and root growth along with differentiation Abscisic Acid Abscisic acid is found mostly near leaves, stems, and unripe fruit. Functions of Abscisic Acid Stimulation of closing of stomata Gibberellin Gibberellins are present in the meristems of apical buds and roots, young leaves, and embryo. Functions of Gibberellins Stimulates stem elongation