Growth and Reproduction
Today we are going to discuss another interesting topic and that is growth and reproduction in plants and animals. We will go through the different influencing factors and internal factors that play a role in growth and reproduction is detail below and we will start with the definition of growth first….
Definition: Growth is a permanent and irreversible increase in size, form or volume of a living organism and is always attend by an increase in weight.
Method of growth- Growth can be achieve by three principal methods.
- Increase in number of cells produce through cell division.
- Proliferation of the intercellular spaces.
- Increase in volume of cell.
Rate of growth: When the amount of growth is expressed per unit time is called rate of growth it is generally slower in plant than the animals.
If the amount of growth is graphically plotted against time, a sigmoid curve is obtained. If this growth rate curve is examined three phases are found.
Lag period: At first the rate of growth is very slow and this is lag period.
Grand period: During this period the growth rate accelerates and obtains a peak. According to some biologist the whole period of growth is referred to a grand period.
Receding period: During this period the growth rate retard and comes to a stand still.
The duration of sunlight has the profound influence on the rate of growth in plant this relation is known as photoperiodism. Thus the plants are grouped in two types.
- Short day plants-those that grow well during the winter example cabbage.
- Long day plants- those that grow well during the summer example lady’s finger.
Through selective breeding high yielding varieties of paddy have been produced, which are day neutral.
Factors influencing growth
- Sunglight- Growth requires food, which is produced through photosynthesis, which occurs in presence of sunlight.
- Oxygen- During growth metabolism activities occurs at a fast rate and that demands high amount of energy, which can be released through respiration that requires oxygen.
- Temperature- Upto a certain limit higher the temperature higher is the rate of growth. The optimum temperature of a growth ranges between (25-30)° C
- Carbondioxide- It is essential for the production of the food, which is a must for growth to occur.
- Water- It is probably important factor for the growth. Water is needed for the translocation of food to the growing regions from the storage organs. Water is needed also as a raw material for the manufacture of the food through photosynthesis.
- Stored food- The amount of stored food is directly proportional to the rate of growth.
- Hormones- Hormones are the growth regulating substances in the living body.
- Enzymes- During growth metabolic activities rapidly take place, all metabolic activities are controlled by enzymes.
Growth in plant- Growth in plant is divided into types:
- Primary growth: It is the growth in the length or height in the plant body. It is restricted to stem tip and root tip carrying apical meristametic tissue. It is accomplished by apical meristem and intercalary meristem.
- Secondary growth: It is the growth in thickness or the width in plant, it is achieved generally by the cells of the cambium hence it is normally absent in the monocot plants. But the dicot plants grow in thickness. It is accomplished by lateral meristem.
As a result of a secondary growth concentric rings are formed from the center towards the periphery in the main trunk of a plant. These ring like structures are called annual rings. By counting the number of ring the age of a plant can be determined.
Phases of growth in plant
Plant growth is achieved through three successive phases:
- Phase of formation: During this phase the cells of meristemetic tissue divide mitotically to form new cell.
- Phase of elongation: The cells formed during the former phase gradually enalarge in volume resulting in the visible growth.
- Phase of maturation: During this phase the cells attain maximum maturity get lignified and loose the power of cell division.
Measurement of plant growth:
As the rate of growth in plants is very slow it is very difficult to measure the amount of growth for a shorter period thus the amount of growth is some how magnified for the purpose of measurement. The instrument used for this purpose is Arc indicator / auxanometer.
Growth in animal
In animal the rate of growth is faster than that in plants, animals, growth maintain a fixed proportion among the various organs of an animal body.
Animals growth can be described under two headings:
- Embryonic development:- The growth that occurs during the embryonic condition of an animal is called embryonic growth. The first condition in the embryonic life is the zygote formed through the gametic union. The zygote begins to divide mitotically to pass on to the next stage called blastula when the new cells formed get arranged in a single layer around a central cavity called blastocoel. The blastula passes on to the next stage called gastrula, which are two layered and there is cavity inside it called gastrocoel. The gastrula passes through several stages to form the adult embryo.
- Postembryonic development:- It may be of two types:
- i) Direct development: When the embryo resemble the parents just from the time of birth, it is called the direct development eg. Bird.
- ii) Indirect development: Here the larva after being hatched from the egg instead of resembling the parents it looks different. Then series of changes occur, this process is known as metamorphosis. The young one ultimately resembles the parents eg. toad, insects, etc.
|1. It is indefinite in nature.
2. Growth is restricted to the meristematic region alone.
3. Rate of growth is slow.
|1. It is definite or determinate in nature.
2. Growth occurs in all growing region without any region restriction.
Definition: It is a process of multiplication in number by living organism through the production of offsprings that resemble the parent.
Aim/objective: The principle aim of reproduction is the perpetuation of the race.
Relation between growth and reproduction: Reproductive maturity is achieved by an individual after the attainment of a specific amount of growth. The zygote produced through reproduction becomes adult through the process of growth thus growth and reproduction are supplementary to one another.
- Vegetative reproduction:- Here reproduction is achieved through any vegetative part of the body, which is not destined for reproduction.
- Natural process:
- i) Fragmentation: In some filamentous algae new individual is formed out of any fragment of parent body. Spirogyra, Ulothrix.
- ii) Budding: In yeast any part of the cell at first bulges out. The cytoplasm pushes into the bulged out portion of the cell, due to this the bulged out portion gradually swells. In the mean time the mother nucleus divides into two nuclei one of which migrates inside the bulged out portion, which is now called a bud. The bud ultimately gets detached from the mother cell and develops to a new yeast. The rate of reproduction in yeast is so fast that it is often seen to carry chains of bud. Yeast.
iii) By various body parts:
- Sweet potato, dahlia produces new plants from their roots.
- The underground modified stems of potato, onion, ginger.
- The subaerial modified stems of water hyacinth, B. thankuni etc.
- The aerial modified stem (bulbil) yam etc. reproduces through stem.
- iv) Bulbil: In some plant the axillary bud get swelled up as a result of deposition of stored food. Such a modified bud is called bulbil. When it matures it gets detached from mother plant and coming in contact with a soil forms a new plant. Yam.
- v) Leaves: The leaves of Bryophyllum (B. patharkunchi), Begonia etc produce new plants.
- Induced or Artificial method-
With a view to maintaining parental characters totally unchanged to off springs, plants are often induced to reproduce in vegetative method.
- i) Cutting: The branches of certain plants are cut off the cut surface is then put under soil when roots develops from this area. To ensure rooting hormone (seradix) is often applied on the cut surface. drumstick, dahlia.
- ii) Gootee: The epidermis from an area of a healthy branch is peeled off this area then plastered with a mixture of mud and cow dung the plaster is then secured and covered by a piece of plastic sheet. Within a fortnight or so roots develop from this area. Rose, chinarose etc.
iii) Layering: The branch of plant is forcibly bent so that a part of it comes in contact with the soil. The epidermis of that part is peeled off and this area is watered. Roots start developing from this area within a fortnight. Eg. Lemon, jasmine.
- iv) Grafting: Plant which is induced to this type of reproduction is called the cion, to which it is grafted on another plant is called the stock belonging to the same species. The epidermis from a part of a branch of a cion and from a part of stem of the stock is peeled off. This peeled off area of both the plants is tightly secured with jute fibers after watering for a month or so an organic connection is established along the said area. The basal part of the cion and the terminal part of the stock from this area are cut off and the new plant formed carries all the traits of the cion. Eg. Mango, litchi.
- Asexual reproduction- It is the type which is achieved with the help of spores in the plants. A spore is the unit of asexual reproduction and is a singled cell microscopic structure. Spores are generally formed with in the sporangium of the non-flowering plants. The spores are generally formed in the sporophytic generation of plants. When spores come in contact with favorable nutritive medium each spore germinates to a new plant. Spores are generally of two types.
i) Zoospore-These are motile spores and can locomote from one place to another with the help of cilia. Eg. Moss, fern etc.
- ii) Aplanospores- These are non-motile spores that are carried from one place to another by air, water etc. mucor, yeast etc.
iii) Bacteria produce special types of spores called endospores. The slime layer of a bacterium gets solidified to produce a very tough coat with in which the protoplasm shrink and thus an endospore is formed. The endospore can withstand a very high temperature and pressure under favourable condition each endospore gets changed into bacterium.
Definition: It is the type of reproduction which is achieved through union of gametes with the formation of a zygote that forms a new individual hence gametes are the units of sexual reproduction. It may be of two types
Conjugation- When two similar gametes (isogametes) unite to form the zygote called zygospore. It is generally found to occur in case of lower forms of organism eg. In plant Spirogyra, Mucor etc. In animals Paracoecium and Monocystis.
Fertilization- When union occurs between two dissimilar gametes (anisogametes) with the formation of zygote called oospore it is found to occur in higher organism like flowering plants and vertebrates is called fertilization.
In animals fertilization is of two types:
1) Internal fertilization. Here the female gamete is fertilized by the male gamete inside the body of the female animal. The males here posses the copulatory organ which is introduced in the body of the female and the male gametes or the sperms are liberated through this organ. Like mammals.
External fertilization- Here the male gametes come out from the male body and female gametes come out from the female body. The union between these two types of gametes occurs outside the body of the parents, like toad, bony fishes.
Sexual reproduction in plants – Conjugation – In Spirogyra conjugation takes place in two ways 1) Scalariform conjugation- Here conjugation occurs between two filaments the isogametes of one filaments migrate through conjugation tubes into the other filaments and conjugate the gamete here in zygospores are formed each of which develops a new Spirogyra. 2) Lateral conjugation- Here union between isogametes occur between two adjacent cells.
Fertilization in plants
Male parts: (Stamen) pollen grains or microspores are formed within the anther through the meiotic divisions of diploid microspore mother cell. Each pollen grain is binucleated unicellular structure it remains covered by a thin elastic membrane called intine, which is again externally protected by a tough wrinkled coat called exine. At some places the exine is absent and the intine remains naked there such a pore is called germ pore. Out of the two haploid nuclei one is called tube nucleus and the other is generative nucleus.
Female part: There are ovules inside the ovary of a flower each ovule carries an elliptical sac called embryo sac. This sac remains covered by two integuments that leave a fine gap one end of the embryo sac. This opening is called micropyle. In a matured ovule out of the three haploid nuclei placed inside the embryo sac towards the micropyle the middle one is the ovum or the megaspore form through the mitotic division of diploid megaspore mother cell. Along the central part of the embryo sac there remains a diploid nucleus called definitive nucleus, which is formed through fusion of two haploid polar nuclei.
Process: The pollen grains are transfer and to the stigma by the process of pollination. A pollen grain absorbs the stigmatic fluid, it swells up and as a result the intine pushes out through the germ pore to form the pollen tube the tube nucleus migrates at the tip of the pollen tube with generative nucleus following it. The pollen tube moves chemotropically towards the micropyle of the ovule via the style. In the mean time the generative nucleus gets mitotically divided into two haploid nuclei called the sperm nuclei, the male gametes. As the tip of the pollen tube touches the embryo sac the point of contact dissolves, tube nucleus disorganized and through the path thus described two sperm nuclei enter the embryo sac one of them fertilizes the ovum to form the diploid oospore.
Sperm nuclei (n) + ovum (n) = oospore (2n)
In gymnosperm the remaining sperm nucleus perishes, in angiosperm the remaining sperm nucleus unite with the definitive nucleus to form a triploid endosperm nucleus.
Sperm nucleus (n) + definitive nucleus (2n) = Endosperm nucleus (3n)
This act is known as double fertilization.
Fertilization: The endosperm nucleus through repeated division forms the special nutritive tissue in the seeds called endosperm or albumen.
Changes after fertilization:
- Ovary – fruit
- oospore – embryo
- endosperm nucleus – endosperm
- integument – seed coats
- ovules – seed
Alternation of gneration:
Sporogenic Meiosis Growth
Diploid generation (2n) Haploid generation (2n)
Growth Mitosis (n)
Zygote (2n) Fertilization Gametes
In the life history of certain plants the sporophytic generation with diploid set of chromosomes and gametophytic generation with haploid set of chromosome are found to alternate with one another. This phenomenon is called alternation of generation eg. Moss, fern etc.
Role of mitosis and meiosis: In this process the gametophytes remains in haploid condition, this plant produces the haploid male and female gametes. Hence the gametes are produced here through mitosis.
The haploid gametes unite to form the diploid zygote, which gives rise to diploid sporophyte. The diploid spore mother cell present in this sporangia of this sporophyte divide through meiosis to form spores, which germinate to gametophyte hence the sporophytic and gametophytic generations are brought about by the influence of mitosis and meiosis respectively.
Reproduction in animal
- Binary fission: This is the process of reproduction in Amoeba under favourable condition, an Amoeba mitotically split up into two. The nucleus of the mother Amoeba gets elongated dumbled shaped and ultimately split up into two nuclei the cytoplasm divides through cleavage and each half having a nucleus in it forms an infant Amoeba.
- Multiple fission: This is the method of reproduction in Amoeba in favourable conditions. An Amoeba withdraws all its pseudopodia and becomes more or less spherical. Three layered cysts are formed shell around it and thereby it can fight against the adverse natural conditions. The mother nucleus divides repeatedly to form many nuclei each of which gets encircled by a mass of cytoplasm. The structures thus formed are called pseudopodiospores. In this condition it lies hidden in the muddy bottom soil with the advent of rains the cysts dissolves the plasma membrane burst liberating the pseudopodiospores in water each develops to an infant Amoeba.
- Longitudinal fission: A hydra splits up into two individuals along the longitudinal axis of the body.
- Transverse fission: A hydra gets transversely slit up into two fragments each of which develops into a new hydra.
- Budding: Hydra reproduces by this method during the favourable condition the interstitial cells divide repeatedly as a result small swelling is formed on the body of the hydra. The swelling gradually enlarges to take a miniature shape of hydra. It is now called a bud. The bud matures and ultimately gets detached from the mother’s body to form a new hydra.
- Regenertation: If a hydra gets split up into several tiny fragments each fragment can grow to form hydra provided it has some interstitial cell in it. N.B. Regeneration also lead to growth of the tail of a wall lizard, the arms of a starfish, Octopus can regenerate.
- Sexual reproduction in animal:
- a) Hermaphrodite/Bisexual- When the male and female gonads are simultaneously present in the same individual the animal is called bisexual animals. Earthworm, hydra, tapeworm
N.B. The phenomenon by which the male and female animals can be distinguished externally is known as sexual dimorphism.
- b) Oviparous animals- There are the animals which lay eggs outside the body or those that develop from layed out eggs. Eg. Fishes, birds, amphibians.
- c) Viviparous animals- These are the animals directly give birth to the young ones. Mammals.
Parthenogenesis: It is the process in which the female gametes without being fertilized by the male gamete is directly converted into an embryo.
In plants Spirogyra, Mucor reproduces by this process where the female gamete is converted into an azygospore that gives rises to a new plant. In animals like honeybee, wasps, reproduce by this method. In flowering plants it results in the formation of seedless fruit and certain plants may be induced with hormone to adapt this process. Eg. Grapes.
In certain animal this process can also be induced thus the layed out eggs by certain amphibians when pricked with pin produce new individual eg. Frogs, Newts.
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