Fertilisation
$\displaystyle \small \bullet$ The fusion of two dissimilar sexual reproductive units (gametes) is called fertilization. This process was discovered by Strasburger (1884).
$\displaystyle \small \bullet$ Germination of pollen grain on stigma and growth of pollen tube.
$\displaystyle \small \bullet$ Pollen grains reach the receptive stigma of the carpel by the act of pollination.
$\displaystyle \small \bullet$ Pollen grains, after getting attached to the stigma, absorb water and swell.
$\displaystyle \small \bullet$ Subsequent to mutual recognition and acceptance, the pollen grain germinates to produce a pollen tube which grows into stigma towards the ovarian cavity.
$\displaystyle \small \bullet$ Entry of pollen tube into ovule
$\displaystyle \small \circ$ After reaching ovary, the pollen tube enters the ovule. Pollen tube may enter the ovule by any one of the following routes:
i. Porogamy: When the pollen tube enters the ovule through micropyle, it is called porogamy. It is the most common type. Eg: Lily.
ii. Chalazogamy: It was first observed by Treub (1981) in Casuarina. The entry of pollen tube into the ovule from chalazal region is known as chalazogamy. Chalazogamy is less common.
Ex: Casuarina, Juglans, Betula, etc.
iii. Mesogamy: The pollen tube enters the ovule through its middle part i.e. through integument (ex: Cucurbita, Populus) or through funicle (eg: Pistacia)
$\displaystyle \small \bullet$ Entry of pollen tube into embryo sac
$\displaystyle \small \circ$ The pollen tube enters the embryo sac only from the micropylar end irrespective of its mode of entry into the ovule.
$\displaystyle \small \circ$ The pollen tube either passes between a synergid and the egg cell or enters into one of the synergids through filiform apparatus.
$\displaystyle \small \circ$ The synergids direct the growth of pollen tube by secreting some chemical substances (chemotropic secretion).
$\displaystyle \small \circ$ The tip of pollen tube enters into one synergid.
$\displaystyle \small \circ$ The penetrated synergid starts degenerating.
$\displaystyle \small \circ$ After penetration, the tip of pollen tube enlarges and ruptures releasing most of its contents including the two male gametes and the vegetative nucleus into the synergid.
Double Fertilization
$\displaystyle \small \bullet$ After entering the one of the synergids, each pollen grain releases two male gametes.
$\displaystyle \small \bullet$ One male gametes fuse with egg nucleus. This is known as syngamy or fertilization and produces a diploid zygote
$\displaystyle \small \bullet$ The other male gamete fuse with two polar nuclei and forms a triploid nucleus or primary endosperm nucleus (PEN). As it involves the fusion of three haploid nuclei, it is known as triple fusion.
$\displaystyle \small \bullet$ Since two sets of fertilization, syngamy and triple fusion occur in the embryo-sac, it is called double fertilization.
$\displaystyle \small \bullet$ After double fertilization
$\displaystyle \small \circ$ The diploid zygote (2n) divides mitotically to form embryo of the seed.
$\displaystyle \small \circ$ The triploid nucleus (3n) or PEN gives rise to a mass of tissue that develops into the endosperm of the seed.
$\displaystyle \small \circ$ Endosperm provides nourishment to the growing embryo.
Post-Fertilization: Structures and Events
After double fertilization, following events occur in a flower
1. Development of endosperm
2. Development of embryo
3. Formation of seed
4. Formation of fruit
Development of endosperm
$\displaystyle \small \bullet$ The primary endosperm cell divides many time to forms triploid endosperm tissue having reserve food materials.
$\displaystyle \small \bullet$ Types of endosperm development
i. Free nuclear: PEN undergoes successive nuclear divisions to give rise to free nuclei. Cytokinesis begins at maturity. Ex: coconut water, maize, wheat, rice, sunflower etc.
ii. Cellular: Nucleus divides and every nuclear division is followed by cytokinesis, making the endosperm cellular from the beginning. Ex: kernel surrounding coconut water (white part).
iii. Helobial: First mitotic division is followed by cytokinesis forming two unequal cells. Subsequent divisions are free nuclear making the endosperm cellular later by cytokinesis.
Development of Embryo
$\displaystyle \small \bullet$ The zygote develops at the micropylar end of the embryo-sac.
$\displaystyle \small \bullet$ Most of the zygote divide only after a certain amount of endosperm is $\displaystyle \small \bullet$ formed. It is an adaptation to make sure that the developing embryo has enough nutrition.
$\displaystyle \small \bullet$ The zygote forms an embryo by mitotic divisions.
$\displaystyle \small \bullet$ The early stages of the embryo development is called embryogeny.
$\displaystyle \small \bullet$ The sequence of events
$\displaystyle \small \circ$ The zygote divides transversely to form two cells.
$\displaystyle \small \circ$ The cell towards the micropyle is the suspensor cell and the one farthest away is the embryo cell.
$\displaystyle \small \circ$ The embryo cell divides mitotically and gives rise to a structure called proembryo.
$\displaystyle \small \circ$ Proembryo undergoes a number of divisions giving rise to the globular, heart-shaped and mature embryo.
$\displaystyle \small \circ$ As the embryo develops, the integument cells become lignified and form a covering called testa or the seed coat.
$\displaystyle \small \circ$ A few cells of embryo nearest to suspensor develop into hypocotyl and radicle. Other cells give rise to epicotyl, plumule and cotyledons.
Dicot embryo
$\displaystyle \small \circ$ Consists of an embryonal axis and two cotyledons.
$\displaystyle \small \circ$ Portion of embryonal axis above the level of cotyledons is called epicotyl. It terminates into the plumule.
$\displaystyle \small \circ$ Portion of embryonal axis below the level of cotyledons is called hypocotyl. $\displaystyle \small \circ$ It terminates into the radicle or rooot tip. The root tip is covered by root cap.
Monocot embryo
$\displaystyle \small \circ$ Consists of an embryonal axis and one cotyledon.
$\displaystyle \small \circ$ The single cotyledon is much reduced and is known as scutellum.
$\displaystyle \small \circ$ Portion of embryonal axis above the scutellum is called epicotyl. It has short apex called plumule, surrounded by protective sheath of hollow foliar structure called coleoptile.
$\displaystyle \small \circ$ Portion of embryonal axis below the scutellum is called hypocotyl. It has radicle and the root cap enclosed in an undifferentiated sheath called coleorrhiza.
Formation of Seed
$\displaystyle \small \bullet$ A seed is a mature fertilized ovule that possess an embryo, stores food material in the cotyledons or endosperm and a protective seed coat.
$\displaystyle \small \bullet$ An ovule undergoes a series of changes after fertilization, as a result of which seed is formed.
$\displaystyle \small \bullet$ Two integuments develops into seed coats. Testa (outer coat) and Tegmen (inner coat)
$\displaystyle \small \bullet$ The egg cell grows into an embryo
$\displaystyle \small \bullet$ The definitive nucleus gives rise to the endosperm.
$\displaystyle \small \bullet$ Types of seeds depending on presence or absence of endosprerm
(i) Endospermic/Albuminous seeds: They contain endosperm with stored food while the cotyledons are small, thin and papery. The endosperms supports the growth of the embryo during seed germination. Ex: rice, maize, coconut, castor bean.
(ii) Non-endospermic/Ex-albuminous seeds: They do not contain endosperm as it gets used up by the developing embryo in the early stages. Cotyledons become thick and fleshy. The food is stored in cotyledons. They provide food the the developing embryo during seed germination. Ex: gram, pea, bean and pulses.
$\displaystyle \small \bullet$ Types of seeds depending on number of cotyledons
(i) Dicotyledons seeds have two cotyledons. Ex: gram, bean, pea, castor etc.
(ii) Monocotyledons seeds have one cotyledon and mostly endospermic. Ex: rice, maize, wheat etc.
Advantages of Seeds to Angiosperms
$\displaystyle \small \bullet$ Seeds have better adaptive strategies for dispersal to new habitats.
$\displaystyle \small \bullet$ As the seeds have sufficient food reserves, young seedling can be nourished until they are capable of photosynthesis.
$\displaystyle \small \bullet$ Hard seed coat provides protection to young embryo.
$\displaystyle \small \bullet$ Seeds being the product of sexual reproduction, bring about new genetic combinations, leading to variations in the plants.
$\displaystyle \small \bullet$ Long term viability of most of the seeds.
$\displaystyle \small \bullet$ Can be easily stored for future usage.
Dormancy of seeds
$\displaystyle \small \bullet$ Seed dormancy is a resting state in which the metabolic activity of the seed is greatly reduced.
$\displaystyle \small \bullet$ After dormancy period is over or it is broken and necessary favorable condition are present, the dormant embryo resumes its metabolic activities and seed germination occurs.
Micropyle: Small opening on seed coat, it facilitates entry of $\displaystyle \small H_{2}O$ & $\displaystyle \small O_{2}$ into seeds (for germination).
Hilum: Scar on the seed coat.
Perisperm: Remnants of nucellus that is persistent. Ex: Black pepper.
Formation of fruit
$\displaystyle \small \bullet$ After fertilization, the ovary of flower begins to grow and develops into fruit.
$\displaystyle \small \bullet$ Fruit may be regarded as a mature ovary containing seeds.
$\displaystyle \small \bullet$ A fruit consists of two parts
1. Pericarp developed from the ovary wall
$\displaystyle \small \circ$ Epicarp forms the outer skin
$\displaystyle \small \circ$ Mesocarp forms the edible fleshy region
$\displaystyle \small \circ$ Endocarp forms the innermost hard region that encloses the seed.
2. Seed developed from the ovules
Types of Fruits
Fleshy fruits: All three regions of pericarp are distinct. ex: orange, mango
Dry fruits: All three regions of pericarp are not distinct as the pericarp is dry. Ex: groundnut, mustard
True fruits: Fruits formed solely from the ovary with ovules inside it.
False fruits: Fruits are formed from the ripened ovary along with some other flower parts like the base or receptacle, the perianth etc. Ex: apple, strawberry, cashew.
Parthenocarpic fruits: Fruits developed without fertilization. These fruits are seedless. Eg: banana.
Apomixis and Polyembryony
Special modes of reproduction are
1. Apomixis or agamospermy
2. Polyembryony
Apomixis
$\displaystyle \small \bullet$ Seeds are formed without fusion of gametes (fertilization).
$\displaystyle \small \bullet$ It can be considered as a form of asexual reproduction.
$\displaystyle \small \bullet$ In this, a diploid cell of the ovule, wither from nucellus or megaspore develops into an embryo.
$\displaystyle \small \bullet$ Rest of the ovule develops into a diploid seed and the ovary into a fruit.
$\displaystyle \small \bullet$ The seed produced is genetically identical to mother plant.
$\displaystyle \small \bullet$ Organisms the reproduce by apomixis is called an apomict.
$\displaystyle \small \bullet$ Ex: Asteraceae, grasses
$\displaystyle \small \bullet$ Parthenogenesis: Type of apomixis in which seeds develop without fertilization from unfertilized female gamete. OR development of seeds from an egg without fertilization. Ex: grapes, pear, banana, pineapple.
Polyembryony
$\displaystyle \small \bullet$ Refers to many embryos in the same seed.
$\displaystyle \small \bullet$ It may occur due to
$\displaystyle \small \circ$ Presence of more than one egg cell in the embryo sac or more than one embryo-sac in the ovule and all the eggs gets fertilized.
$\displaystyle \small \circ$ Number of embryos develop simultaneously from different parts of an ovule like synergids and antipodal cells.
$\displaystyle \small \circ$ Number of embryos develop from the tissues of nucellus and integuments.
$\displaystyle \small \circ$ Ex: onion, groundnut, mango, lemon orange.
$\displaystyle \small \bullet$ The fusion of two dissimilar sexual reproductive units (gametes) is called fertilization. This process was discovered by Strasburger (1884).
$\displaystyle \small \bullet$ Germination of pollen grain on stigma and growth of pollen tube.
$\displaystyle \small \bullet$ Pollen grains reach the receptive stigma of the carpel by the act of pollination.
$\displaystyle \small \bullet$ Pollen grains, after getting attached to the stigma, absorb water and swell.
$\displaystyle \small \bullet$ Subsequent to mutual recognition and acceptance, the pollen grain germinates to produce a pollen tube which grows into stigma towards the ovarian cavity.
$\displaystyle \small \bullet$ Entry of pollen tube into ovule
$\displaystyle \small \circ$ After reaching ovary, the pollen tube enters the ovule. Pollen tube may enter the ovule by any one of the following routes:
i. Porogamy: When the pollen tube enters the ovule through micropyle, it is called porogamy. It is the most common type. Eg: Lily.
ii. Chalazogamy: It was first observed by Treub (1981) in Casuarina. The entry of pollen tube into the ovule from chalazal region is known as chalazogamy. Chalazogamy is less common.
Ex: Casuarina, Juglans, Betula, etc.
iii. Mesogamy: The pollen tube enters the ovule through its middle part i.e. through integument (ex: Cucurbita, Populus) or through funicle (eg: Pistacia)
$\displaystyle \small \bullet$ Entry of pollen tube into embryo sac
$\displaystyle \small \circ$ The pollen tube enters the embryo sac only from the micropylar end irrespective of its mode of entry into the ovule.
$\displaystyle \small \circ$ The pollen tube either passes between a synergid and the egg cell or enters into one of the synergids through filiform apparatus.
$\displaystyle \small \circ$ The synergids direct the growth of pollen tube by secreting some chemical substances (chemotropic secretion).
$\displaystyle \small \circ$ The tip of pollen tube enters into one synergid.
$\displaystyle \small \circ$ The penetrated synergid starts degenerating.
$\displaystyle \small \circ$ After penetration, the tip of pollen tube enlarges and ruptures releasing most of its contents including the two male gametes and the vegetative nucleus into the synergid.
Double Fertilization
$\displaystyle \small \bullet$ After entering the one of the synergids, each pollen grain releases two male gametes.
$\displaystyle \small \bullet$ One male gametes fuse with egg nucleus. This is known as syngamy or fertilization and produces a diploid zygote
$\displaystyle \small \bullet$ The other male gamete fuse with two polar nuclei and forms a triploid nucleus or primary endosperm nucleus (PEN). As it involves the fusion of three haploid nuclei, it is known as triple fusion.
$\displaystyle \small \bullet$ Since two sets of fertilization, syngamy and triple fusion occur in the embryo-sac, it is called double fertilization.
$\displaystyle \small \bullet$ After double fertilization
$\displaystyle \small \circ$ The diploid zygote (2n) divides mitotically to form embryo of the seed.
$\displaystyle \small \circ$ The triploid nucleus (3n) or PEN gives rise to a mass of tissue that develops into the endosperm of the seed.
$\displaystyle \small \circ$ Endosperm provides nourishment to the growing embryo.
Post-Fertilization: Structures and Events
After double fertilization, following events occur in a flower
1. Development of endosperm
2. Development of embryo
3. Formation of seed
4. Formation of fruit
Development of endosperm
$\displaystyle \small \bullet$ The primary endosperm cell divides many time to forms triploid endosperm tissue having reserve food materials.
$\displaystyle \small \bullet$ Types of endosperm development
i. Free nuclear: PEN undergoes successive nuclear divisions to give rise to free nuclei. Cytokinesis begins at maturity. Ex: coconut water, maize, wheat, rice, sunflower etc.
ii. Cellular: Nucleus divides and every nuclear division is followed by cytokinesis, making the endosperm cellular from the beginning. Ex: kernel surrounding coconut water (white part).
iii. Helobial: First mitotic division is followed by cytokinesis forming two unequal cells. Subsequent divisions are free nuclear making the endosperm cellular later by cytokinesis.
Development of Embryo
$\displaystyle \small \bullet$ The zygote develops at the micropylar end of the embryo-sac.
$\displaystyle \small \bullet$ Most of the zygote divide only after a certain amount of endosperm is $\displaystyle \small \bullet$ formed. It is an adaptation to make sure that the developing embryo has enough nutrition.
$\displaystyle \small \bullet$ The zygote forms an embryo by mitotic divisions.
$\displaystyle \small \bullet$ The early stages of the embryo development is called embryogeny.
$\displaystyle \small \bullet$ The sequence of events
$\displaystyle \small \circ$ The zygote divides transversely to form two cells.
$\displaystyle \small \circ$ The cell towards the micropyle is the suspensor cell and the one farthest away is the embryo cell.
$\displaystyle \small \circ$ The embryo cell divides mitotically and gives rise to a structure called proembryo.
$\displaystyle \small \circ$ Proembryo undergoes a number of divisions giving rise to the globular, heart-shaped and mature embryo.
$\displaystyle \small \circ$ As the embryo develops, the integument cells become lignified and form a covering called testa or the seed coat.
$\displaystyle \small \circ$ A few cells of embryo nearest to suspensor develop into hypocotyl and radicle. Other cells give rise to epicotyl, plumule and cotyledons.
Dicot embryo
$\displaystyle \small \circ$ Consists of an embryonal axis and two cotyledons.
$\displaystyle \small \circ$ Portion of embryonal axis above the level of cotyledons is called epicotyl. It terminates into the plumule.
$\displaystyle \small \circ$ Portion of embryonal axis below the level of cotyledons is called hypocotyl. $\displaystyle \small \circ$ It terminates into the radicle or rooot tip. The root tip is covered by root cap.
Monocot embryo
$\displaystyle \small \circ$ Consists of an embryonal axis and one cotyledon.
$\displaystyle \small \circ$ The single cotyledon is much reduced and is known as scutellum.
$\displaystyle \small \circ$ Portion of embryonal axis above the scutellum is called epicotyl. It has short apex called plumule, surrounded by protective sheath of hollow foliar structure called coleoptile.
$\displaystyle \small \circ$ Portion of embryonal axis below the scutellum is called hypocotyl. It has radicle and the root cap enclosed in an undifferentiated sheath called coleorrhiza.
Formation of Seed
$\displaystyle \small \bullet$ A seed is a mature fertilized ovule that possess an embryo, stores food material in the cotyledons or endosperm and a protective seed coat.
$\displaystyle \small \bullet$ An ovule undergoes a series of changes after fertilization, as a result of which seed is formed.
$\displaystyle \small \bullet$ Two integuments develops into seed coats. Testa (outer coat) and Tegmen (inner coat)
$\displaystyle \small \bullet$ The egg cell grows into an embryo
$\displaystyle \small \bullet$ The definitive nucleus gives rise to the endosperm.
$\displaystyle \small \bullet$ Types of seeds depending on presence or absence of endosprerm
(i) Endospermic/Albuminous seeds: They contain endosperm with stored food while the cotyledons are small, thin and papery. The endosperms supports the growth of the embryo during seed germination. Ex: rice, maize, coconut, castor bean.
(ii) Non-endospermic/Ex-albuminous seeds: They do not contain endosperm as it gets used up by the developing embryo in the early stages. Cotyledons become thick and fleshy. The food is stored in cotyledons. They provide food the the developing embryo during seed germination. Ex: gram, pea, bean and pulses.
$\displaystyle \small \bullet$ Types of seeds depending on number of cotyledons
(i) Dicotyledons seeds have two cotyledons. Ex: gram, bean, pea, castor etc.
(ii) Monocotyledons seeds have one cotyledon and mostly endospermic. Ex: rice, maize, wheat etc.
Advantages of Seeds to Angiosperms
$\displaystyle \small \bullet$ Seeds have better adaptive strategies for dispersal to new habitats.
$\displaystyle \small \bullet$ As the seeds have sufficient food reserves, young seedling can be nourished until they are capable of photosynthesis.
$\displaystyle \small \bullet$ Hard seed coat provides protection to young embryo.
$\displaystyle \small \bullet$ Seeds being the product of sexual reproduction, bring about new genetic combinations, leading to variations in the plants.
$\displaystyle \small \bullet$ Long term viability of most of the seeds.
$\displaystyle \small \bullet$ Can be easily stored for future usage.
Dormancy of seeds
$\displaystyle \small \bullet$ Seed dormancy is a resting state in which the metabolic activity of the seed is greatly reduced.
$\displaystyle \small \bullet$ After dormancy period is over or it is broken and necessary favorable condition are present, the dormant embryo resumes its metabolic activities and seed germination occurs.
Micropyle: Small opening on seed coat, it facilitates entry of $\displaystyle \small H_{2}O$ & $\displaystyle \small O_{2}$ into seeds (for germination).
Hilum: Scar on the seed coat.
Perisperm: Remnants of nucellus that is persistent. Ex: Black pepper.
Formation of fruit
$\displaystyle \small \bullet$ After fertilization, the ovary of flower begins to grow and develops into fruit.
$\displaystyle \small \bullet$ Fruit may be regarded as a mature ovary containing seeds.
$\displaystyle \small \bullet$ A fruit consists of two parts
1. Pericarp developed from the ovary wall
$\displaystyle \small \circ$ Epicarp forms the outer skin
$\displaystyle \small \circ$ Mesocarp forms the edible fleshy region
$\displaystyle \small \circ$ Endocarp forms the innermost hard region that encloses the seed.
2. Seed developed from the ovules
Types of Fruits
Fleshy fruits: All three regions of pericarp are distinct. ex: orange, mango
Dry fruits: All three regions of pericarp are not distinct as the pericarp is dry. Ex: groundnut, mustard
True fruits: Fruits formed solely from the ovary with ovules inside it.
False fruits: Fruits are formed from the ripened ovary along with some other flower parts like the base or receptacle, the perianth etc. Ex: apple, strawberry, cashew.
Parthenocarpic fruits: Fruits developed without fertilization. These fruits are seedless. Eg: banana.
Apomixis and Polyembryony
Special modes of reproduction are
1. Apomixis or agamospermy
2. Polyembryony
Apomixis
$\displaystyle \small \bullet$ Seeds are formed without fusion of gametes (fertilization).
$\displaystyle \small \bullet$ It can be considered as a form of asexual reproduction.
$\displaystyle \small \bullet$ In this, a diploid cell of the ovule, wither from nucellus or megaspore develops into an embryo.
$\displaystyle \small \bullet$ Rest of the ovule develops into a diploid seed and the ovary into a fruit.
$\displaystyle \small \bullet$ The seed produced is genetically identical to mother plant.
$\displaystyle \small \bullet$ Organisms the reproduce by apomixis is called an apomict.
$\displaystyle \small \bullet$ Ex: Asteraceae, grasses
$\displaystyle \small \bullet$ Parthenogenesis: Type of apomixis in which seeds develop without fertilization from unfertilized female gamete. OR development of seeds from an egg without fertilization. Ex: grapes, pear, banana, pineapple.
Polyembryony
$\displaystyle \small \bullet$ Refers to many embryos in the same seed.
$\displaystyle \small \bullet$ It may occur due to
$\displaystyle \small \circ$ Presence of more than one egg cell in the embryo sac or more than one embryo-sac in the ovule and all the eggs gets fertilized.
$\displaystyle \small \circ$ Number of embryos develop simultaneously from different parts of an ovule like synergids and antipodal cells.
$\displaystyle \small \circ$ Number of embryos develop from the tissues of nucellus and integuments.
$\displaystyle \small \circ$ Ex: onion, groundnut, mango, lemon orange.
0 Comments