The post fertilization events includes:-
1. Endosperm development
This is a product if triple fusion and develops from cell of embryo sac. It is generally a triploid tissue. The cells of this tissue are filled with reserve food materials and are used for the nutrition of the developing embryo. It is absent in families such as Orchidaceae, podostemaceae and Trapaceae.
Plants | Nature of Endosperm |
Cereals (e.g. Rice Wheat) | Starchy |
Cereals (e.g Maize) | Proteinaceous (Aleurone layer) |
Castor, Coconut | Oily |
Ivory palm | Cellulosic |
Date palm | Hemicellulosic |
Mode of Development of Endosperm
(i) Free nuclear endosperm development:- It is most common type of endosperm development, PEN undergoes successive nuclear divisions to give rise to free nuclei. Finally, wall formation begins and makes the endosperm a multicellular.
Examples:- Cotton, maize
(ii) Cellular mode of development:- Primary endosperm nucleus divides many times and each division is followed by wall formation.
Example:- Pentunia
The coconut water from tender coconut is free nuclear endosperm (made up of thousands of nuclei) and the surrounding white kernel is the cellular endosperm.
Fate of Endosperm:- Endosperm is meant for nourishing the embryo. Endosperm can be completely consumed during development of embryo or it can persists in mature seeds.
Embryo Development:-
The development of embryo from a zygote is called as embryogeny.
(i) Embryogeny in Dicot Plants
(a) Zygote (oospore) divides into two unequal cells, larger suspensor cell towards micropyle and a smaller embryonal cell (= terminal cell) towards antipodal region.
(b) The suspensor cell undergoes transverse divisions forming 6-10 celled long suspensor.
(c) The first cell of the suspensor (towards micropyle) is large and called haustorium or vesicular cell.
(d) The last cell of the suspensor towards embryo cell is hypophysis. It forms radical tip.
(e) Embryonal cell divides twice vertically and once transversely to produce two tiered eight-celled embryo.
(f) The epibasal tier forms two cotyledons and a plumule while hypobasal tier produces only hypocotyl and most of radical.
(g) For this the octant embryo undergoes periclinal divisions producing protoderm, procambium and ground meristem. It is initially globular but with the growth of cotyledons it becomes heart-shaped and then assumes the typical shape.
(II) Embryogeny in Monocotyledons:-
(a) The zygote or oospore divides transversely producing a suspensor cell towards micropylar end and embryo cell towards chalazal end.
(b) The embryo cell divides transversly again into a top and a middle cell
(c) The terminal cell divides vertically and transversely into a globular embryo.
Structure of a typical monocot embryo:-
(a) A single cotyledon called an scutellum that is situated towards one side (lateral) of embryonal axis.
(b) At its lower end, the embryonal axis has radical and root cap enclose in an undifferentiated sheath called coleorhizae.
(c) The portion of embryonal axis above the level of attachment of scutellum is the epicotyls. It has a shoot apex and few leaf primordial in a hollow foliar structure called coleoptile.
(d) Remains of second cotyledon occur in some grasses. It is called as epiblast
Seed:- A fertilized ovule is called a seed.
Seeds may be:-
(i) Endospermic/Albuminous seeds:- e.g. Wheat, maize, barley, coconut and castol.
(ii) Non-endospermic/Exalbuminous seeds:- e.g. Pea, bean, groundnut
(iii) Perispermic seeds:- Seeds in which remains of nucellus is seen. The residual, persistent nucellus is called perisperm, e.g. blackpepper, beet.
Structure of seed:–
Atypical seed consists of:-
(i) Seed coat
(ii) Endosperm
(iii) Embryo
Dormancy and Seed Germination:-
(i) Dormancy:– It is a state of inactivity of embryo when the seed is not able to germinate.
(ii) Germination:– The ability of a seed to produce a seeding in presence of favourable environmental condition
Advantage of Seed to Angiosperms
(i) Seeds have better adaptive strategies for dispersal to new habitat.
(ii) It has sufficient food reserves for nourishment pf ypung seedings
(iii) Protection is is provided to young embryo by the hard seed coat.
(iv) Generate new genetic recombination as it is a product of sexual reproduction.
Seeds- Basis of our agriculture
Seeds form the basis of our agriculture as they show; Dehydration and Dormancy.
These two features help in strong of seeds which can be used as food throughout the year and also to raise crops in the next season
Seed Viability:–
The period for which the seeds retain their power of germination is called seed viability.
(a) It is variable example few months in Oxalis.
(b) The oldest is that of a lupine. Lupinus arcticus excavated from Arctic Tundra.
The seed germinated and flowered after an estimated record of 10,000 years of dormancy.
(c) During an archaeological excavation at King Herod’s Palace near the Dead sea, a 2000 years old viable seed of date palm, phoenix dactylifera was found.
The number of seeds in fruit is generally equal to or less than the number of ovules in a ovary. It never exceeds the number of ovules’
(i) Orchid’s fruit contains thousands of tiny seeds
(ii) Parasitic species like Orobanche and Striga also contain many seeds.
Fruit
(i) A ripened ovary is called a fruit.
(ii) The wall of the ovary forms pericarp
(iii) Fruit may be:
(a) True fruit
(b) False fruit
(c) Parthenocarpic fruit
The first stimulus for fruit development comes from pollination while second stimulate is received from developing seeds and the third stimulus is provided by the availability of nutrients.
Post-fertilization events include several key stages:
Cleavage: Rapid cell division of the fertilized egg (zygote) to form a multicellular structure called the blastocyst.
Implantation: The blastocyst attaches to and embeds itself into the uterine wall.
Gastrulation: The process where the blastocyst reorganizes into a three-layered structure known as the gastrula, which forms the primary germ layers (ectoderm, mesoderm, and endoderm).
Organogenesis: The development of organs and organ systems from the germ layers.
Cleavage is the series of rapid mitotic cell divisions that a zygote undergoes after fertilization. It results in the formation of a multicellular embryo called a blastocyst. Cleavage increases the number of cells without increasing the overall size of the embryo, preparing it for implantation in the uterine wall.
Implantation occurs when the blastocyst reaches the uterus and embeds itself into the uterine lining (endometrium). This process involves the following steps:
The blastocyst hatches from its surrounding zona pellucida.
It adheres to the endometrial epithelium.
The trophoblast cells of the blastocyst invade the endometrium, securing the embryo and establishing the initial connection to the maternal blood supply.
Gastrulation is a critical process during embryonic development where the single-layered blastula reorganizes into a trilaminar (three-layered) structure known as the gastrula. This process forms the three primary germ layers:
Ectoderm: Will develop into the nervous system, skin, and hair.
Mesoderm: Will develop into the skeletal system, muscles, and circulatory system.
Endoderm: Will develop into the internal organs such as the lungs, liver, and digestive tract.
Organogenesis is the stage of embryonic development where the three germ layers formed during gastrulation differentiate into specific organs and tissues. Each germ layer gives rise to different parts of the organism:
The ectoderm forms structures such as the brain, spinal cord, and skin.
The mesoderm forms muscles, bones, and the circulatory system.
The endoderm forms the gastrointestinal tract, lungs, and other internal organs.