Reproduction in lower and Higher Animals (Part 2) Menstrual Cycle : The menstrual cycle is a characteristic of primates. It starts at the time of puberty and the period is called menarche. The menstrual cycle in humans lasts for 28 days. The days are numbered from the first day of blood flow in the menstrual period. A series of events occur regularly in females after every 26 to 30 years. The menstrual cycle includes four phases:- I) Menstrual ovulation phase/Menstrual cycle phases : It is also called as bleeding phase this phase extends from the 1st to the 4th day of the menstrual cycle. When the ovum is not fertilized, the high-level progesterone inhibits the secretion of luteinizing hormone, and the LH level decreases in the level of progesterone from the corpus leuteum. After 14 days of ovulation, if the ovum is not fertilized, the lining of the uterus degenerates, and menstruation begins. This phase lasts about 4 days. The days when bleedi...
Reproduction in lower and higher plants
Reproduction:
- Reproduction is the production of young ones like a parent.
- Reproduction is an essential process as it leads to the continuation of species as well as maintain the continuity of life.
- Each organism has its own particular method of reproduction.
- All these methods generally fall into two categories:
1) Asexual reproduction
2) Sexual reproduction
1) Asexual Reproduction:
- It does not involve the fusion of two compatible gametes or sex cells.
- It is a process resulting in the production of genetically identical progeny from a single organism and inherits the genes of parents.
- Such morphologically and genetically identical individuals are called clones.
- Organism choose to reproduce asexually by different modes or ways :
1) Fragmentation :
- Multicellular organisms can break into fragments due to one or other reasons.
- These fragments grow into new individuals.
- Eg = spirogyra.
2) Budding :
- Budding is the most common method of asexual reproduction.
- It takes place during favourable conditions by producing one or more outgrowth or buds.
- Eg = Yeast.
3) Spore formation :
- In Chlamydomonas asexual reproduction occurs by flagellated motile zoospores which can grow into a new individual.
- Eg = Chlamydomonas.
2) Sexual Reproduction :
- It involves the fusion of two compatible gametes or sex cells.
- It is an elaborate complex and slow process.
- Because of the fusion of male and female gametes sexual reproduction results in offspring that are not identical to their parent.
- It is also called as amphimixis or syngenesis.
- All organism grows to certain maturity in their life before they start reproducing sexually.
- Period of growth between their birth up to their reproductive maturity is called juvenile phase.
- In plants period of growth between seed germination up to initiation of flowering is called vegetative phase.
- When the organism starts reproducing sexually is called reproductive phase.
3) Vegetative Reproduction :
- Plants reproduce asexually through their vegetative parts.
- Hence, the new plants formed are genetically identical to their parents.
- There are also few methods which would not occur naturally in plants.
- Agriculture and Horticulture exploit vegetative reproduction to multiply the fresh stocks of plants.
- According to the human requirements the artificial methods are conducted as follows :
(A) Cutting :
- The small piece of any vegetative parts of a plant having one or more buds is used for propagation.
- Stem cutting: Rose, Leaf-cutting: Sansevieria, Root cutting: Blackberry.
(B) Grafting :
- Two parts are joined in such a way that they grow as one plant.
- In this method, part of the stem containing one or more bud (scion) is joined onto a rooted plant called stock.
- Eg = Apple, Pear, Rose, etc.
(C) Tissue culture :
- It is a method by which a small amount of plant tissue is carefully grown to give many plantlets.
- Micropropagation method is also used nowadays.
4) Structure of Anther :
- An immature stage of the anther is represented by a group of parenchymatous tissue surrounded by a single-layered epidermis.
- Anther is generally dithecous and tetrasporangiate.
- Each monothecous anther contains two pollen sacs.
- In dithecous anther four pollen sacs are present, Therefore, it is called tetrasporangiate.
5) T. S. of Anther :
- The archesporial cell divides into an inner sporogenous and outer primary parietal cell.
- The sporogenous cell forms sporogenous tissue.
- Each cell of sporogenous tissue is capable of giving rise to a microspore tetrad.
- Parietal cell undergoes division to form the anther wall layer :
a) Epidermis :
- The outermost protective layer made up of tubular cells.
b) Endothecium :
- The sub-epidermal layer made up of radially elongated with fibrous thickenings.
c) Middle layer :
- Inner to endoethcium is a middle layer made up of thin-walled cells.
d) Tapetum :
- The innermost nutritive layer of anther wall.
- It immediately encloses the sporogenous tissue (MMC = Microspore mother cell)
6) Microsporogenesis :
- It is a meiotic formation of haploid megaspore from the diploid microspore mother cell.
- The arrangement of a megaspore in a tetrad is generally tetrahedral.
- All the four microspores of a spore tetrad are functional.
- Microsporogenesis is found inside the microsporangium.
7) Structure of Microspore :
- Pollen grain is a non-motile, haploid, unicellular body with a single nucleus.
- It is surrounded by a two-layered wall called sporoderm.
- The outer layer exine is thick and made up of a complex, non-biodegradable substance called sporopollenin.
- It may be a smooth or sculptured pattern.
- The inner wall layer, intine consist of cellulose and pectin.
8) Development of Male Gametophyte :
- Pollen grain marks the beginning of male gametophyte.
- The outer layer of the pollen grain is called Exine.
-The inner layer of the pollen grain is called Intine.
- Pollen grain contains a single nucleus with a large amount of cytoplasm.
- It undergoes mitotic division to produce bigger naked vegetative cell and a small thin-walled generative cell.
- Generative cell is attached to the intine.
- Vegetative cell is freely floating in the cytoplasm.
- The vegetative cell is rich in food.
- The generative cell floats in the cytoplasm of the vegetative cell.
- The second mitotic division is concerned with generative cell only and gives rise to two non-motile male gametes.
- The mitotic division of generative cell takes place either in pollen grain or pollen tube.
- The pollen grain is shed from the anther at this two-celled stage in most of the angiosperm takes place.
9) Structure of Anatropous ovule :
- Each ovule develops inside the ovary and is attached to the placenta by a small stalk called Funiculus.
- The place of attachment of funiculus with the main body of ovule is called Hilum.
- In angiosperms, the most common type of ovule is Anatropous.
- The ovule consists of central parenchymatous tissue, and the nucleus is surrounded usually by two protective coverings called Integuments.
- A narrow opening at the apex of the ovule is called Micropyle.
- Chalaza is the base of ovule directly opposite to micropyle.
10) Megasporogenesis :
- It is a meiotic formation of haploid megaspore from the diploid megaspore mother cell.
- The arrangement of megaspore in a tetrad is commonly linear.
- Only one megaspore of a spore tetrad is functional.
- It is found inside the megasporangium.
11) Development of female gametophyte :
- Megaspore mother cell undergoes meiosis to form a linear tetrad of haploid cells.
- Gynoecium is the female part of the flower.
- Gynoecium consists of three parts stigma, style and ovary.
- Megaspore mother cell is formed from the archesporial cell.
- Archesporial cell is hypodermal in origin.
- The primary archesporial cell divides preclinically to form two cell i.e outer cell and inner cell.
- Outer cell is called a primary parietal cell and the inner cell is called a primary sporogenous cell.
- Primary parietal cell may or may not divide further.
- On the other hand, The primary sporogenous cell does not divide further and hence forms MMC (Megaspore mother cell).
12) Pollination :
- The transfer of pollen grains from the anther to stigma is nothing but pollination.
- It is the pre-requisites of fertilization.
- Pollination is divided into three types :
A) Autogamy : (self-pollination)
- It is a type of pollination in which the bisexual flower is pollinated by its own pollen grains.
- Offsprings are genetically identical to their parent.
- It is also called as self-pollination.
- Eg = Pea plant.
B) Geitonogamy :
- Transfer of pollen grains from the anther to the stigma of another flower of the same plant.
- It is functionally similar to cross-pollination.
- It cannot bring genetic variations.
- Eg = Cucurbita maxima.
C) Xenogamy :
- It generates genetically different offspring.
- The transfer of pollen grains from the anther to the stigma of a different plant of the same species.
13) Abiotic Agents :
- The agents or things that cannot move from one place to another is called Abiotic agents.
- Eg = Water and wind.
- Abiotic agents are classified into two types :
A) Anemophily [ Anemo - wind; phily - pollination ]
- It requires that these pollen grains are light and non-sticky.
- Eg = Wheat, Rice, Corn, Barley, etc
ADAPTATIONS :
- The flowers are small, colourless without nectar and fragrance.
- Pollen grains are light, dry and produced in large quantity.
- Anther is versatile in nature.
B) Hydrophily : [ Hydro - water ; Phily - pollination ]
- Found in 30 genera of aquatic monocots.
ADAPTATIONS :
- Flowers are small and inconspicuous.
- Floral parts are unwettable.
- Pollen grains are long and unwettable.
- Nectar and fragrance are lacking in flowers.
Hydrophily is classified into two types :
1) Hypo-hydrophily :
- Pollination occurs below the surface of the water.
- Pollen grains are heavier than the water.
- Eg = Zostera (seagrass).
2) Epi-hydrophily :
- Pollination occurs at the surface of the water.
- Gravity of pollen grain is equal to that of water.
- Some aquatic plants are anemophilous Eg = Halogaries.
- Some aquatic plants are entomophilous Eg = Lotus.
14) Biotic Agents :
- The agents which can move from one place to another is called Biotic agents.
- Biotic agents are classified into three types :
A) Entomophily : [ Entomos - Insect ]
- It occurs in rose, jasmine and Cestrum.
ADAPTATIONS :
- Flowers are large, showy, bright in colour.
- Flowers produced sweet odour and nectar glands also.
- Stigma is rough and sticky due to mucilaginous secretion.
B) Ornithophily : [ Ornitho - Birds ]
- Few types of birds are specialised for pollination.
- They usually have small size and long peaks.
- Eg = Sunbirds and Hummingbirds.
ADAPTATIONS :
- Flowers are bright coloured.
- They secrete profuse, dilute nectar.
- Pollen grains are sticky and spiny.
C) Chiropteryphily : [ Chiropteros - Bats ]
- Bats can transfer pollens over a long distance, sometimes several kilometres.
ADAPTATIONS :
- Flowers are dull coloured with a strong fragrance.
- They secrete abundant nectar.
- Flowers produces a large number of edible pollen grains.
15) Out-breeding devices (Contrivances) :
- Plants have a mechanism that discourages to do self-pollination.
- Genetically diversities are increasing as plants always try to do cross-pollination.
- Self-pollination results in inbreeding depression.
A) Uni-sexuality :
- In unisexuality dioesicm occurs.
- Dioecism = The plant bears either male or female flowers.
- Hence, self-pollination is not possible.
- Eg = Papaya.
B) Dichogamy :
- Anther and stigma mature at different times in a bisexual flower.
- Dichogamy is divided into two types:- 1) Protandry and 2) Protogyny.
- Protandry: Androceium matures earlier than the gynoecium.
- Eg = Sunflower.
- Protogyny: Gynoceium matures earlier than the androecium.
- Eg = Gloriosa.
C) Prepotency :
- Pollen grains of other flowers germinates rapidly over the stigma.
- Eg = Apple.
D) Heterostyly :
- Anther (heteroanthy) and stigma (heterostyly) is placed at a different place.
E) Self-sterility :
- This is a genetic mechanism due to which the germination of pollen on the stigma of the same flower.
- Eg = Tobacco.
16) Pollen-Pistil Interaction :
- It is the interaction of pollen grains with sporophytic tissue.
- It begins with pollination and ends with fertilization.
- Pollination does not guarantee the transfer of the right type of pollen often wrong type also land on stigma.
- The pistil has the ability to recognise and accept the right type or compatible pollen of the same species.
- Thus, the wrong type of pollen is discarded by the pollen-pistil.
- Compatibility and incompatibility of the pollen-pistil is determined by special proteins.
- The astigmatic surface of flower refuse other wrong type or incompatible pollen grain.
- The compatible pollen grain absorbs water and nutrient from the surface of the stigma, germinates and produces pollen tube.
Artificial hybridization :
- The major approaches used in crop improvement.
- The desired pollen grains are hand pollinated and used for fertilization.
- This is accomplished through emasculation and bagging procedure.
17) Double Fertilization :
- Double fertilization is the fusion of two male gametes brought by a pollen tube to two different cells of the same female gametophyte to produce two different structures.
- This process is discovered by Nawaschin in 1898 in Lilium plant.
- Double fertilization is a complex mechanism in flowering plants.
- The growth of a pollen tube is guided by the chemicals secreted by the synergids.
- It usually enters the ovule through the micropyle. It is termed as porogamy
- But in some cases, it is found to enter through chalaza known as chalazogamy.
- In some pants, by piercing the integuments called mesogamy.
- Finally, it penetrates embryo sac of ovule through its micropylar end.
18) Development of Endosperm :
- The triploid primary endosperm nucleus repeatedly divides mitotically to form nutritive tissue called Endosperm.
- In post-fertilization changes within the ovule, the embryo and endosperm are seen to develop simultaneously.
- The other cell of the embryo sac disorganised sooner or later.
- The formation of triploid endosperm nucellus triggers cell division, which leads to the formation of the endosperm.
- There are three types of development of Endosperm :
A) Nuclear Type:-
- It is the most common type found in 161 angiospermic families.
- Free nuclear division.
- No wall formation.
B) Cellular Type:-
- Division of primary endosperm nucleus (PEN) is immediately followed by wall formation.
- It is mostly observed in 72 families of a dicot.
C) Helobial Type:-
- First division of primary endosperm nucellus (PEN) followed by a transverse wall.
- Which divides cells unequally.
- The smaller cell is called the chalazal cell.
- The larger cell is called the micropylar cell.
19) Development of Embryo:-
- The process of development of the zygote into an embryo is called embryogenesis.
- The embryo is developed at the micropylar end of the embryo sac.
- The growth of embryo triggers only a certain amount of endosperm is formed.
- After fertilization, the embryonic development begins.
- The zygote divides to form two-celled proembryo.
- The larger cell towards the micropyle is called basal or suspensor initial cell.
- Smaller cell towards chalaza is called terminal or embryonal initial cell.
20) Seed and Fruit development :
- One of the ways that can produce offsprings is by forming seeds.
- The flowers must be pollinated to produce seeds and fruit.
- Seeds development is initiated by fertilization.
- The integuments of the fertilized ovule persist and get transformed into seed coat of the mature seed.
- A typical outer seed coat called Testa.
- The inner thin, membranous called Tegmen.
- The nucellus in the ovule may persist in some genera like black pepper and beat as a thin, papery layer called Perisperm.
- The cotyledons in some non-endospermic seeds act as food storage.
21) Dormancy:-
- Structural or physiological adaptive mechanism for survival is called as Dormancy.
22) Apomixis :
- Phenomenon of formation of an embryo through asexual methods.
- Gametophytic cell produces embryo without fertilization is called as Apogamy.
- Sporophytic cells produce embryo without fertilization is called Apospory.
- Apomixis is classified into three types :
A) Recurrent Apomixis :
- In this type, the embryo sac generally rise either from an archesporial cell or from some other part of the nucellus.
- In diplospory, the unreduced embryo sac is derived from the diploid megaspore mother cell.
- In apospory, the nucellular cells give rise to optamatic embryo sac.
B) non-recurrent Apomixis :
- In this type, megaspore mother cell undergoes usual meiotic division and a haploid embryo sac is formed.
- Here, the embryo arises either from the egg by parthenogenesis or from some haploid cells of gametophyte through apogamy.
C) Adventive Embryony :
- In this type, embryos may develop from somatic nucellus or integuments along with normal zygotic embryo.
- It is a common method in Mango, orange, lemon, etc.
- It gives rise to a condition called Polyembryony.
23) Parthenocarpy :
- This term is coined by Noll.
-It is the condition in which fruit is developed without the process of fertilization.
- It occurs naturally in some varieties of pineapple, Banana, Papaya, etc.
- In these plants, it seems that the placental tissue in the unfertilized ovary products auxin IAA (Indole-3 Acetic Acid) which is responsible for the enlargement of the ovary into a fruit.
24) Polyembryony :
- The development of more than one embryos, inside the seed and the condition is described as polyembryony.
- It was first noticed by Leeuwebhoek in 1719.
- For eg = seeds like Citrus genus.
- It is the occurrence of more than one embryo is a seed which consequently results in the emergence of multiple seedlings.
- The polyembryony may be true or false depending upon whether many embryos arise in the same embryo sacs or in different embryo sacs in the same ovule.
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