Post Fertilization In Flowering plants
Post Fertilization is a process of events after fertilization in which, the Ovule develops into a seed and ovary transformed into a fruit.
The major events include are-
a) Development of endosperm from triploid primary endosperm nucleus in embryosac
b) Development of embryo from diploid zygote
C) Development of seeds from Ovule
d) Development of fruit from ovary
i) Development of endosperm
Endosperm is the food laden tissue formed by the development of angiospermous seeds which provides essential nutrients to the growing embryo and also the young seedlings at the time of seeds germination.
In gymnosperms, the female gametophyte is known as endosperm. In angiosperms, the endosperm develops from triploid (3n) primary endosperm nucleus which is formed as a result of vegetative fertilization, triple fusion or fusion of male gamete with secondary nucleus of central cell.
Types of Endosperm
Endosperm is of three types -
i) Nuclear endosperm
ii) Cellular endosperm
iii) Helobial endosperm
i) Nuclear endosperm
In this type, the primary endosperm nucleus divides by repeated mitotic division without the formation of walls which results in the formation of large number of free nuclei in the central wall of the embryosac. A big vacuole develops in the embryosac pushing all the nuclei to the peripheral cytoplasm.The free nuclei come to lie between the embryosac cell wall and vacuolar membrane of central cell. The second phase begins with the cleavage of multinucleate peripheral cytoplasm which results the formation of centripetal cell wall. Finally, the endosperm is converted into a cellular tissue. Nuclear endosperm is most common type of endosperm.E.g Acalypha indica
Cellular endosperm -
In this type, the first Nuclear division of primary endosperm nucleus occurs by the formation of either longitudinal or transverse cell wall in central cell.So, Nuclear division and wall formation results in the formation of cellular endosperm. E.g Petunia, Datura, Balsam
iii) Helobial endosperm
In this the primary endosperm nucleus moves to the chalazal end of the embryosac. The first Nuclear division of the primary endosperm nucleus is followed by formation of a transverse wall resulting in the formation of a small chalazal cell and a large micropylar end. Generally, the chalazal end doesn't divide further and function as haustorium. Nucleus of large micropylar end divides by repeated free division and further development takes place in the same way as in nuclear endosperm.
E.g in monocotyledons.
b) Development of Embryo
In Dicotyledons:
i) In these , the zygote enlarges in size, secrete its own cellulose cell wall and divides by the transverse division which form the two unequal cells. The large basal cell i.e suspensor cell which lies towards the micropylar end of the embryosac and the smaller terminal cell i.e embryonal cell which lies towards the chalazal end of the embryosac.
ii) The basal suspensor cell divides by transverse division and the terminal embryonal cell by longitudinal division to form a tetrad.
iii) Due to the suspensor cell division, lower most cell enlarges in size towards micropylar end to form a vesicular basal cell. The other cell divides to form a row of 7-8 cells of a long suspensor cells.
iv) Each of two terminal cells of the tetrad stage of proembryo divide longitudinally forming four celled quadrant stage.
v) Each four celled quadrant stage divide to form eight celled octant stage.
vi) Each eight celled octant now becomes sixteen celled by having periclinal division to form an outer prodermal celland an inner cell.
vii) By anticlinal division, epidermis has been formed. The eight inner cells divide in different planes to form the ground meristem and procambium of hypocotyledon, hypocotyl and plumule.
viii) The suspensor cell situated at the terminal end of suspensor is called hypophysis.
ix) The hypophysis divides by the transverse and longitudinal division to form three tiers of cells.The cells of inner tiers form the cortical initial of radicle, middle and outer tiers form the initial root periderm and root cap.
x) At this stage, the developing embryo is globular and shows radial symmetry. This is called proembryo. The proembryo is transformed into embryo with the development of radicle, plumule and cotyledons..
xi) Two cotyledons arise from two ridges of tissues appeared at the distant region of the proembryo which becomes a heart shaped embryo
xii) Now cotyledons grow rapidly whereas a few cells to form a dome shaped plumule.
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2. In monocotyledons
xiii) Early phases of embryogenesis are very similar in both Dicotyledons and monocotyledons upto the formation of globular proembryo . In monocotyledons, about half of terminal cells derived functions as shoot apex whereas remainder of the cells show a rapid rate of cell division in the formation of single terminal cotyledon.
iii) Development of Seed
After fertilization, the Ovule transformed into a seed. The following changes usually occurs in the Ovule during seed development.
a) The zygote develops into an embryo.
b) The triploid primary endosperm nucleus gives rise to a nutritive tissue called endosperm. The endosperm may persist or completely digested during embryogenesis.
The seeds having copious amount of endosperm, tissue are called albuminous and the seeds in which endosperm is used up are called exalbuminous . They usually store reserve food materials.
c) The nucellus is generally usedup during the development of embryo and form a thin layer, called perisperm in some cases.
d) The outer integument is hard and leathery testa which ensures survival of seeds
e) The inner integument persist form the tegmen
f) The micropyle remains in the form of a fine pore on the surface of seeds. Funicle is transformed into stalk of the seeds. The hilum marks the point of attachment to the stalk.
Development of fruits
The ovary is transformed into fruit after developing the seeds. After receiving the necessary stimuli , the ovules present in the ovary which transformed into seeds and the wall of the ovary transformed into pericarp or fruit wall.
Types of fruits -
The fruits are divided into two categories -
1. True fruits - The fruit that derived from ovary of a flower, not associated with the noncarpellary part, is called true fruits.
E.g Mango, Banana, grapes,pea, cucumber etc
2. False fruits - The fruit that derived from ovary along with other accessory floral parts , are called False fruits or pseudocarp.