It consists of a whorl of stamens (Filament + Anther). Each lobe of anther has 2 thecae (dithecous).
Anther consists of 4 microsporangia. They develop to pollen sacs.
Structure of a microsporangium: 4 layers: epidermis, endothecium, middle layers & tapetum (nourishes developing pollen grains). Outer 3 layers give protection. Young anther bears sporogenous tissue. It consists of pollen mother cells (PMC).
Microsporogenesis: Formation of microspore tetrads (cluster of 4 microspores) from a pollen mother cell. Microspores develop into pollen grains.
Pollen grain (male gametophyte): It has two layers:
o
Exine: Outer layer made up of sporopollenin. It can withstand high temperature and strong acids and alkali.
o
Intine: Inner wall made up of cellulose and pectin.
A mature pollen grain has 2 cells: Vegetative cell (food reserve) & generative cell.
Economic importance of pollen grains:
o
Rich nutrients. Pollen tablets are used as food supplements.
o
Pollen grains are stored in liquid N2(pollen banks).
o
Pollen grains of some plants (e.g. Parthenium) are allergic.
Gynoecium (female reproductive part)
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It is monocarpellary (single pistil) or multicarpellary (> one pistil).
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In multicarpellary, pistils may be fused (syncarpous) or free (apocarpous).
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Each pistil has 3 parts: Stigma, Style & Ovary (consists of ovules).
Structure of Megasporangium (Ovule):
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Ovule is attached to placenta by a stalk (funicle).
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Hilum: Junction between ovule & funicle.
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One or two integuments encircle the ovule except at the micropyle (a small opening).
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Opposite the micropylar end is the chalaza (basal part).
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Enclosed within the integuments, there is nucellus. Its cells contain reserve food materials.
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Located in the nucellus is the embryo sac (female gametophyte).
Megasporogenesis: Formation of 4 megaspores from a megaspore mother cell (MMC).
Formation of Female gametophyte (embryo sac):
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One megaspore is functional and other 3 degenerates.
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Embryo sac formation from a single megaspore is called monosporic development.
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Nucleus of megaspore → mitosis → 2 nuclei → move to opposite poles → 2-nucleate embryo sac → 4-nucleate → 8-nucleate → cell walls are laid down → organization of female gametophyte.
Distribution of cells in embryo sac:8-nucleate & 7-celled.
Transfer of pollen grains from the anther to the stigma of a pistil. 3 types:
§
Autogamy (self-pollination): Pollination from the anther to stigma of the same flower. Plants like Viola, Oxalis & Commelina produce 2 types of flowers:
o
Chasmogamous flowers: Exposed anthers & stigma.
o
Cleistogamous flowers: They do not open at all. Anthers & stigma lie close to each other. Cleistogamy leads to inbreeding depression.
§
Geitonogamy: Pollination from anther to stigma of another flower of the same plant. It is functionally cross-pollination but genetically like autogamy.
§
Xenogamy: Pollination from anther to stigma of a different plant. It brings genetically different pollen grains to the stigma.
Agents of Pollination
1. Abiotic agents (wind & water)
Pollination by wind (anemophily): More common abiotic agent. Common in grasses.
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Such flowers have a single ovule in each ovary. Numerous flowers packed into an inflorescence. E.g. Corncob.
Pollination by water (hydrophily): Rare. Mostly monocots. E.g. Vallisneria & Hydrilla, Zostera (marine).
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Distribution of some bryophytes & pteridophytes is limited because they need water for the transport of male gametes and fertilisation.
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In Vallisneria, the female flower reaches the surface of water by the long stalk and the male flowers or pollen grains are released on to the surface of water.
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Wind & water pollinated flowers not colourful, no nectar.
2. Biotic agents (animals)
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E.g. Bees, butterflies, flies, beetles, moths, birds, bats etc.
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Pollination by insects (Entomophily) is more common.
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Features of insect-pollinated flowers: Large, colourful, fragrant. Rich nectar. The flowers secrete foul odours to attract flies and beetles. Sticky pollen grains.
Nectar & pollen grains are floral rewards for pollination.
Pollen/nectar robbers: Insects that consume pollen or nectar without bringing about pollination.
Outbreeding Devices
Continued self-pollination results in inbreeding depression. To avoid this, there are some devices in plants:
a.
Avoid synchronization of release of pollen and receptivity of stigma.
b.
Arrangement of anther & stigma at different positions.
c.
Self-incompatibility: Prevention of self-pollen from fertilization.
d.
Production of unisexual flowers.
Pollen-pistil Interaction
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A process in which pistil recognizes compatible or incompatible pollen by producing some chemicals.
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Pollen grain germinates on stigma to produce a pollen tube → contents of pollen grain move into the pollen tube → Pollen tube grows through stigma & style → reaches the ovary → micropyle → ovule → enters one synergid via filiform apparatus. Filiform apparatus guides the entry of pollen tube.
Artificial hybridisation
A crop improvement programme in which desired pollen grains are used for pollination. The steps are:
o
Emasculation: Removal of anthers from the bisexual flower of female parent.
o
Bagging: Emasculated flowers are covered with a butter paper to prevent unwanted pollen.
o
Pollination: Dusting of pollen grains on the stigma.
o
Rebagging of the flowers. It is allowed to develop fruits.
DOUBLE FERTILISATION
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Pollen tube releases 2 male gametes into the cytoplasm of the synergid.
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One male gamete + egg cell nucleus (syngamy) → zygote (2n) → embryo.
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Other male gamete + two polar nuclei in the central cell → triploid primary endosperm nucleus (PEN). As it involves fusion of 3 haploid nuclei, it is called triple fusion. It becomes primary endosperm cell (PEC) and develops into the endosperm (3n).
Since 2 types of fusions (syngamy & triple fusion) take place in an embryo sac, it is called double fertilisation. It is an event unique to flowering plants.
POST-FERTILISATION: STRUCTURES & EVENTS
Post-fertilisation events: Endosperm & embryo development, ovule into seed & ovary into fruit.
Endosperm development
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Primary endosperm cell divides repeatedly to form a triploid endosperm tissue.
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Endosperm cells are filled with reserve food materials for nutrition of developing embryo.
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In endosperm development, PEN undergoes nuclear divisions to give free nuclei. This is called free-nuclear endosperm. E.g. Tender coconut water.
-
Endosperm becomes cellular due to cell wall formation. The white kernel of coconut is cellular endosperm.
Embryo development (Embryogeny)
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Embryo develops at micropylar end of the embryo sac.
Part of embryonal axis above the level of cotyledons is the epicotyl, which ends with the plumule (stem tip).
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Cylindrical portion below the level of cotyledons is hypocotyl that terminates with radicle (root tip).
-
Root tip is covered with a root cap.
Monocotyledonous embryo
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Only one cotyledon.
-
Cotyledon of the grass family is called scutellum.
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At lower end, embryonal axis has radicle and root cap enclosed in coleorhiza (an undifferentiated sheath).
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Portion of embryonal axis above the level of attachment of scutellum is the epicotyl. It has a shoot apex and a few leaf primordia enclosed in coleoptile.
Seed from Ovule
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Seed is the fertilized ovule. It is the final product of sexual reproduction.
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It consists of seed coat, cotyledon & an embryo axis.
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Mature seeds are 2 types:
o
Non-albuminous (Ex-albuminous) seeds: Have no residual endosperm. E.g. pea, groundnut, beans.
o
Albuminous seeds: Retain a part of endosperm. E.g. wheat, maize, barley, castor, coconut.
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In some seeds (black pepper, beet etc.) remnants of nucellus are also persistent. It is called perisperm.
Advantages of seeds:
·
Helps the species to colonize in other areas.
·
Have food reserves.
·
Seed coat protects the young embryo.
·
Generate new genetic combinations and variations.
·
They can be stored and used as food.
Viability of seeds after dispersal:
-
The oldest viable seed is that of a lupine (Lupinus arcticus). It germinated after 10,000 years of dormancy.
-
2000 years old viable seed is of the date palm (Phoenix dactylifera).
Fruit from Ovary
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The ovary develops into a fruit.
-
The wall of ovary develops into pericarp (wall of fruit).
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Fruits are 2 types:
o
True fruits: Fruit develops only from the ovary. E.g. most plants.
o
False fruits: In this, the thalamus also contributes to fruit formation. E.g. apple, strawberry, cashew etc.
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In some species, fruits develop without fertilisation. Such fruits are called parthenocarpic fruits. E.g. Banana.
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Parthenocarpy can be induced through the application of growth hormones. Such fruits are seedless.
APOMIXIS AND POLYEMBRYONY
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Apomixis is the production of seeds without fertilisation. E.g. Some species of Asteraceae and grasses.
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In many species (e.g. Citrus & Mango) some nucellar cells divide, protrude into the embryo sac to form embryos. Thus each ovule contains many embryos. Occurrence of more than one embryo in a seed is called polyembryony.
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Importance of apomixis in hybrid seed industry: If the hybrids are made into apomicts, there is no segregation of characters in the hybrid progeny. So farmers can keep on using hybrid seeds to raise new crop.
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