POST- FERTILIZATION : STRUCTURE AND EVENTS
Events of endosperm and embryo development, maturation of ovule into seed and ovary into fruit, are collectively termed as post-fertilization events.
· Development of endosperm takes place before the embryo development.
· Primary endosperm cell divides repeatedly to form a triploid endosperm.
· Cells are filled with reserve food material and are used for the nutrition of the developing embryo.
· PEN undergoes successive nuclear division to give rise to free nuclei. This is called free-nuclear endosperm.
· Subsequently cell wall formation takes place and become cellular endosperm.
· The coconut water is free nuclear endosperm and the white kernel is the cellular endosperm.
· Endosperm may be consumed completely during embryo developed or it may be consumed during germination of seed.
· Zygote formed and placed at the micropylar end of the embryo sac.
· Zygote starts its development only after some amount of endosperm formed.
· Embryo development takes place in following stages:
o Globular stage
o Heart shaped
o Matured embryo.
· A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons.
· Embryonal axis above the cotyledon is the epicotyls.
· Terminal part of the epicotyls is the plumule (gives rise to the shoot).
· Embryonal axis below the cotyledon is the hypocotyl.
· The terminal part of the hypocotyl is called the radicle (root tip).
· The root tip is covered by the root cap.
· Possesses only one cotyledon
· In grass family the cotyledon is called scutellum.
· Scutellum situated towards one side of the embryonal axis.
· Radicle and the root cap enclosed by a sheath called coleorhiza.
· The portion of the embryonal axis above level of attachment of scutellum is called epicotyls.
· Epicotyl has the shoot apex or plumule enclosed by hollow foliar structure called coleoptile.
· Seed is the final product of the sexual reproduction.
· Seed consists of seed coat, cotyledon and an embryo axis.
· Cotyledon stores the reserve food material for development and germination.
· Matured seed without endosperm called non-albuminous. (Ground nut)
· A part of the endosperm retained in matured seed is Albuminous.
· Remnants of nucellus in the matured seed are called perisperm. E.g. black pepper, beet.
· The wall of the ovary develops into the wall of fruit called pericarp.
· Fruit developed from the ovary is called true fruit.
· In apple, strawberry, cashew, the thalamus contributes in the fruit formation is called false fruit.
· Fruit developed without fertilization is called Parthenocarpic fruits.
APOMIXIS AND POLYEMBRYONY.
· Apomixis is very common in Asteraceae and grasses.
· Seeds are produced without fertilization.
· Apomixis is a type of asexual reproduction which mimics the sexual reproduction.
· Diploid egg cell is formed without meiosis and develops into seed without fertilization.
· In Citrusand Mango the nucellar cells starts dividing, protrude into the embryo sac and develop into embryo.
· Ovule having more than one embryo is termed as polyembryony.
· Hybrid plants are developed by apomixis to maintain the genetic identity.
Microsporogenesis is the process of formation of microspores from pollen mother cell. Megasporogenesis is the formation of megaspores from megaspore mother cell. Functional megaspore develops into female gametophyte.
Pollination is a process of transfer of pollen grains from male anther to the stigma of flower.All animals or insects that tranfer pollen from one plant to another are called pollinators.
Artificial hybridization is the process where desired pollen grains are introduced to the stigma through pollination. This helps to avoid unwanted pollen rejection and saves time. Double fertilization is the process where syngamy and triple fusion takes place simultaneously.