What Is Flower ?
• Flowers are morphologically and embryo logically marvels and the sites of sexual reproduction in angiosperm .
A typical angiosperm flower consists of whorls of floral appendages attached on the receptacle:
• 1.Calyx – consist of sepals – non-essential
• 2.Coralla – consist of petals — non-essential
• 3.Androecium – consist of stamen – essential
• 4.Gynoecium – consist of carpel – essential
Pre-Fertilization- Structure And Events
MALE PART- ANDROECIUM
FEMALE PART — GYNOECIUM
Stamen ,Microsporangium ,Pollen Grains
A typical stamen consist of two parts:
1.Anther —it is a terminal bilobed structure.
2. Filament — it is a long and slender stalk. Its proximal end remains attached to thalamus or the petal of the flower.
Structure Of Typical Anther
1. A typical anther is bilobed. The two anther lobes are separated by a groove in front and are attached to each other by a band of vascular sterile tissue called connective.
2. Each anther is four sided tetragonal located at corners, two in each lobe ,hence a mature anther is tetrasporangium.
3. Microsporangium forms pollen sacs which on maturity become filled with pollen grains.
Anther wall consist of four layers:-
1. Epidermis — Outermost ,single layered and protective in fuction.
2. Endothecium — Cells of this layer have a- cellulosic fibrous bands arising from inner tangential wall which help in dehiscence of anther due to their hygroscopic nature.
3. Middle layer — It is 1-3 layered . It degenerates at maturity .
4. Tapetum — it is the inner most layer of anther wall which surrounds the sporogenous tissue.
it is the process of formation of microspore from PMC (Pollen Mother Cells). As development occurs in the anther, the sporogenous tissue undergoes meiosis to form microspore tetrad.
Each cell of sporogenous tissue has capacity to give rise to a tetrad. Hence, each cell is a potential pollen or PMC. As the anther matures, the microspores get detached from each other and develop into pollen grains.
when the anther is young ,a group of compactly arranged homogenous cells called the sporogenous tissue occupies the center of each microsporangium.
• Represent the male gamete and are spherical, having a two-layered wall:
• Exine (outer) — Hard layer made of sporopollenin, which is extremely resistant and can withstand high temperatures, acidic and alkaline conditions, and enzymes
• Intine (inner) — Thin and continuous layer made up of cellulose and pectin
• Mature pollen grain contains two cells:
• Vegetative cell — Large with irregular nucleus, contains food reserves
• Generative cell — Small and floats in the cytoplasm of the vegetative cell
• In 60% of the angiosperms, pollen grains are shed at 2-celled stage while in others generative cell undergoes mitosis to form two male gametes (3-celled stage).
• The viability of pollen grains after they are shed depends upon temperature and humidity. It ranges from 30 minutes to few months.
Pollen is one of the most common causes of allergies in the United States. Pollen is a very fine powder produced by trees, flowers, grasses, and weeds in order to fertilize other plants of the same species. Many people have an adverse immune response when they breathe in pollen.
The immune system normally defends the body against harmful invaders, such as viruses and bacteria, to ward off illnesses. In people with pollen allergies, the immune system mistakenly identifies the harmless pollen as a dangerous intruder and begins to produce chemicals to fight against the pollen. This is known as an allergic reaction, and the specific type of pollen that causes it is known as an allergen. The reaction leads to numerous irritating symptoms, such as sneezing, stuffy nose, and watery eyes.
The use of bee pollen increased during the late 19705, following testimonials by athletes that supplementation with this product increased stamina and improved athletic ability. Bee pollen has been used in certain cultures for thousands of years, but has become increasingly popular over recent years and recognized for its potential health benefits. It has been traditionally used for a variety of purposes, including relief of constipation; treatment of prostatic conditions, such as prostatitis, benign prostatic hyperplasia, and prostate cancer; wound healing; and for its proposed antioxidant action.
Pollen viability refers to the ability of the pollen to perform its function of delivering male gametes to the embryo sac. Pollen viability varies between minutes and years, and which primarily depends on the taxonomic status of the plant and on the abiotic environmental conditions. In order to maintain the viability and fertilizing ability of the pollen for a long period of time special storage conditions are needed.
Pistil\Carple, MegaSporanGium (Ovule) ,Embryo Sac
GYNOECIUM– It is the female reproductive part of plant . It consist of single pistil (monocarpellary) or may have more than one pistil (multicarpellary).
A Multicarpellary Pistil May Be
Carpel Free From Each Other
Carpel Fused Together
Parts Of Carpel
Stigma- Serves as a landing platform for pollen grains after pollination.
Stlyle- Elongated slender part beneath the stigma.
Ovary- The basal bulged part of the pistil.
Ovary has an ovarian cavity with one or more chambers (locule).
The placenta is located inside the ovarian cavity .Arising from the placenta are the megasporangia commonly called ovules
Structure Of Ovule/ Megasporangium
-The stalk of ovule by which it remains attached to placenta.
– It is the point of attachment of funiculus to the body of ovule.
– It is the protective envelop surrounding the body of ovule.
-It is a small opening near the tip of integument
-It is a basal part of ovule.
-it provide nourishment to embryo sac
– it is located in the nucellus
-called female gametophyte
The process of formation of megaspore from megaspore mother cell (MMC) is called megasporogenesis.
•Ovules generally differentiate a single megaspore mother cell (MMC) .
•The MMC undergoes meiosis and forms a linear tetrad of four haploid megaspore. Out of which one remain functional (chalazal end ) and three degenerate(micropylar end).
•One of the functional megaspore develops into female gamete.
Embryo Sac Formation: Monosporic Development
The process of formation of embryo sac from the single functional megaspore is termed as monosporic development.
•:.Basal megaspore is functional and it develops to form embryo sac.
•:. Upper megaspore are non —functional or degenerate.
Development Of Monosporic Embryo Sac
•Firstly, nucleus undergoes mitotic division.
•2 nucleate embryo sac is formed.
•Now again , they undergo mitotic division.
•Due to which , 4nucleate embryo sac is formed.
•Nuclei undergo 3 mitotic division and form 8 nucleate embryo sac.
•Cell wall formation takes place and embryosac is formed.
– It is located at mycropylor end.
– having 1 egg cell , Synergids
-It has 2 poller nuclei.
-It is located at chalazal end. -has 3 cell at the basal end
❖ The transfer of pollen grains(male gamete ) to stigma is known as pollination.
❖ The transfer of pollen grains are occurred by pollinating agents like wind ,water ,insects and through birds.
Types of pollination ( on the basis of source of pollen)
❖Autogamy– The transfer of pollen grains from the anther to the stigma of the same flower. seen in those plants which produce chasmogamous and cleistogamous flowers.
❖Cleistogamous flowers — In such flowers anthers and stigma lies close to each other.
They are closed flowers.
❖Chasmogamous flowers– Anthers and stigma are exposed. Autogamy is difficult . pollinating agents are needed for pollination.
In such flowers , the anther and stigma lies close to each other. Some plants like viii12, oxalis , and commelina produce b th types of flowers ( chasmogamous) and closed flower (cleistogamous) .
Adaptations seen in plant to ensure self pollination
❖ Bisexuality — Presence of both the essential whorls in the same flower.
❖ HOMOgaMy— Maturation of both androecium and gynoecium at the same time i.e there should be synchromy in release of pollen and stigma maturation.
❖ CieiStOgaMy— A condition in which flower does not open .
❖ ADVANTAGES OF CLEISTOGAMY
- It ensures seed formation even in the absence of any pollinating agent .
- It is a cheaper for plant as there is no coastly nector or fragnance which plant has to produce for
❖ DISADVANTAGES OF CLEISTOGAMY
1. The offspring produced have limited genetic diversity.
•The transfer of pollen grains from anther to stigma of another flower of the same plant.
•This transfer involves an agents of pollination hence functionally it is known as cross pollination but pollen grain come from the same plant so genetically it is self pollination.
•The transfer of pollen grains from anther to the stigma of another flower of different plant of the same species.
•This is the only type of pollination which brings genetically different types of pollen grains to stigma.
•So it is fuctionally as well as genetically cross pollination.
Agents which carry pollen grains from anthers to stigma of same or different flower. There are two types:-
-Living organisms which act as agents of pollination. – insects, birds are examples.
-non-living organism which act as agents of pollination. -water and wind are examples.
Pollination By Wind (ANEMOPHILY)
The process of the transfer of pollen from one individual plant to another, whereby the pollen is carried by air currents.
Characteristics of wind pollination
1.Stigma is large, branched and feathery
2.Pollen grains are produced in large amounts and are often smooth and light so that they can be carried away by the gentlest of wind currents..
3.Pollen grains are light.
4.Non-sticky pollen grains
Pollination By Water (Hydrophily)
A rare form of pollination in which pollen is carried to a flower by water. It occurs by one of two methods.
1.In Canadian pondweed (Elodea canadensis) the male flowers break off and float downstream until they contact the female flowers.
2.In Zostera, a marine species, the filamentous pollen grains are themselves carried in the water.
Zoophily is a form of pollination whereby pollen is transferred by animals.
• Flower should be colourful
• Have fragnance/H fl,v.I.B
• sticky pollen grains
• odoured flowers if pollinated by flies and beetles
Floral reward for insects:-
1.Safe place to lay eggs
2.Pollen or nector
3. Yucca and pronuba moth
Pronuba Moth And Yucca Plant
Steps taken as follows :-
•so, firstly pronuba moth lay its egg in locule of the ovary of yucca plant.
•In turn flower get pollinated.
•both of them can’t complete their life cycle without each other.
•the larvae of the moth come out of the eggs .
•After hatching egg larvae feed on yucca seeds.
•Larvae then burrow out of the fruit.
•then it moves under the soil & make their cocoons.
Pollen/Nector Robbers- Insects which consume pollen or nector without bringing about pollination. example —queen of bombus affinis
Example —queen of bombus affinis
Out Breeding Devices
Devices or features of plants which discourage self pollination and encourage cross pollination are called as out breeding devices
It prevent inbreeding depression which may be a result of continued self—pollination.
1.Unisexuality —Formation of unisexual flowers two conditions are there —
a)Monoecious plants — male and female flowers are produced on the same plant .This prevent autogamy but not geitnogamy example- castor, maize
b)Dioecious plants — male and female flowers are produced on separate plants. It prevents autogamy as well as geitnogamy .
example- papaya, vallisneria
2.Dichogamy- Different maturation time of androecium and gynoecium in the same flower. Either the pollen is released before the stigma becomes receptive much before the release of pollen.
3.Self- Incompatibility- The failure of the pollen grains from fertilizing the ovule by habiting pollen germination or pollen tube growth in the pistil. It is a genetically controlled mechanism.
4.In some species , the anther and stigma are placed at different position, so that pollen cannot come in contact with t stigma of the same flower.
❖ Pollination does not guarantee the transfer of the right type of pollen on stigma.
All pollinations do not lead to a successful fertilization because for a successful fertilization, pistil of a flower ha to recognize the pollen of same species. Therefore, interaction between pollen grains and stigma needs to be understood properly. Once compatible pollen is accepted by pistil, events for fertilization proceed, whereas incompatible pollens are rejected. This interaction where a pistil is capable of recognizing its pollen is the result of long term pollen-pistil interaction and chemicals released by pollen.