Reproduction in Plant (Part - 5) - Notes, Botany, Class 12 PDF Download

Reproduction in Plant

DEVELOPMENT OF ENDOSPERM

•  First of all endosperm develops from the primary endosperm nucleus after the fertilization which stored food materials. It is utilized by the embryo during the early development then after at the time of seed germination.

Food is present in the form of starch in endosperm . The endo sperm is of three types on the basis of development :-

1 . NUCLEAR ENDOSPERM :

•  This type of endosperm mostly found in Dicotyledon [Polypetalae] . Nuclear endosperm is also present in Capsella . Such type of endosperm develops by free nuclear divisions of nucleus of primary endosperm nucleus, Thus a multinucleated endosperm is formed. Later on cytokinesis takes place, so that multicellular endosperm is formed.
•  This type of endosperm is the most common in Angiosperms.
•  The milky fluid is found in green Coconut is example of nuclear endosperm , which is called liquid syncytium.
•  In Melas toma , cytokinesis never take place so that it is always remains nuclear endosperm .

2 . CELLULAR ENDOSPERM :

•  This type of endosperm is found in Gamopetalae group. During the development, each division of primary endosperm nucleus is followed by cytokinesis. So that endosperm is remains cellular from the biginning.

3 . HELOBIAL ENDOSPERM :

•  During the development of this type of endosperm, first division of primary endosperm nucleus is followed  by unequal cytokinesis so that two unequal sized cells are formed (Cell towards the micropyle is large). Now free nuclear divisions takes place in each cells, results it becomes multinucleated. Eventually cytokinesis takes place later on so that it is changed into a cellular endosperm. This type of endosperm is found in Monocots . It is intermediate type of endoperm.

SPECIAL POINTS :

•  After the observation of first two divisions of primary endosperm nucleus, endosperm can be identified whether it would be nuclear , or cellular endosperm .
•  Endosperm is absent in some of Angiosperms e.g. In Orchid aceae , Podostemaceae and Trapaceae .
•  Exceptionally, some of the plants have diploid endosperm instead of triploid such as in Oenothera .
•  Maize and Tom a to have mosaic endosperm in which patches of different colours are present.
•  Theen dosperm in Betalnut is rough surfaced. It is known as "ruminate endosperm " .
•  The drinking portion is nuclear endosperm and edible portion is cellular endosperm in Coconut .

 DEVELOPMENT OF EMBRYO IN DICOT

Development of embryo in Capsella is first time discovered by " Hanstein" .

• In Angiosperm, Zygote undergoes in resting phase. When the endosperm is formed, development of zygote starts. In the beginning it absorb food from the endosperm and increase in size then after a layer secreted by itself. Now it is called Oospore .
•  The first division of Oospore is transverse , results two cells are formed. The one cell lies towards micropyle is called basal cell or suspensor cell . The other cell is formed towards the Chalaza is called apical cell or terminal cell or embryonal cell .
•  The basal cell and embryonal cell divide simultaneously

•  The basal cell divides transversly and apical cell divides vertically resulting, two suspensor cells and two embryonal cells formed. This stage is made up of four cells which are arranged in ' T' shaped structure. Embryonal cells divide vertically to form four Embryonal cells. This is the quadrant stage of embryo.
•  The two suspensor cells divided by transverse divisions forming a 6-10 celled long filament like structure is termed suspensor. The main function of suspensor is to pushe s the developing embryo into food laden endosperm to provide nutrition.
•  The micropyler cell of the suspensor swells up. This cell of suspensor is known as haustorial-cell .
•  The cell of suspensor lies near the embryonal cells is called hypophysis. T his cell combined with radicle to form the apex of root [Root cap].
•  These four cells quadrent embryo further divide transversely to produce eight. The eight ce lled stage of embryo is called octant stage . The eight cells of octant are arranged in two tiers.
•  The four cells of the octant embryo lies near the hypophysis is known as hypobasal cells and four cells present towards the chalaza termed epibasal cells .
•  The hypobasal cells gives rise to radicle and hypocotyl and epibasal cells giverise to two cotyledons and plumule of the embryo.
•  All the cells of octant divided by periclinal division so that a 16 celled globular embryo is formed [Proembryo].
•  Due to fast division of embryonal cells of globular embryo, a heart shaped embryo is formed. All the cells of this embryo are meristematic.
•  Due to the fast growth in two lobes of heart shaped embryo, they develop into two cotyledons . Both the growing cotyledons turn in downwards due to the curved position of body of ovule of Capsella .
•  The tissues are present above the joining place of both the cotyledons are responsible to form plumule and behind it epicotyl is formed.
•  The tissues present opposite to the plumule give rise to radicle .
•  This curved position of the embryo is called Torpedo or Chordate stage .
•  An axis is present between  plumule and radicle is called embryonal axis . It is also called Tigellum [ main embryonal axis]. 
•  Both the cotyledons are present at lateral position of embryonal axis and plumule is formed in terminal position in Dicotyledon embryo.
•  This type of development of embryo i s known as Cruci fer type or Onagrad type . It is the mos t c om mon ty pe of development in Di c o ts .
• Crucifer type of development is also found in Capsella, so that it is considered as typical Angiosperm for the s tud y of em br yonic deve lo pm ent of A ngio sper ms .
• During the development of embryo, embryo is formed from the some part of zygote so that is called meroblastic development.
• Ovule m odifi ed in to s e ed in which t es ta i s f orm ed by o uter i nteg um en t and teg m en i s fo rme d by i nn er integument.
• Only mi c ropy le of ovu le remains unc hange d and also present in s e ed .
• Entire o va ry modified into fruit . This fruit is formed by fertilized ovary so that it is called true fruit .
• In some of the Angiosperms fruit is formed from the ovary without fertilization known as parthenocarpic fruit.
• In some fruit parthenocarpy is useless (If edible part is endosperm or seed)

DEVELOPMENT OF EMBRYO IN MONOCOTYLEDON
The Lil ium ty pe of emb ryonic development is found in monocotyledons. The first division is transverse division in Oospore. Results two cells are formed the upper cell chalazal i s called emb ryona l cell and lower micropylar cell is termed as basal cell. The basal cell does not divide further and later on it increases in size and form single celled ve si cul ar suspensor .
Only embryonal cell divides transversly in which terminal cell is called c o ty ledon cell and lower (Middle) cell is known as embryonal axis cell.

A transverse division takes place in embryonal axis cell to gives rise two cells.
The one cell out of two, gives rise to plumule initial and another gives ris e to radicle initial .

• The plumule initial divides to form a plumule of the embryo.
• Radicle initial divides to form radicle. In this both the initials are responsible to form embryo in Lateral position.
• An apic al c otyled on is formed by the continuous divisions of c otyled on c ell .

• In Angiosperms, dev elop ment of embryo is meroblast ic a nd endoscopic [towards the chalaza].
• In the grass family the cotyledon is called scutellum that is situated towards one side (lateral) of the embryonal axis. At its lower end, the embryonal axis has the radical and root cap enclosed in an undifferentiated sheath called c oleo rrhiza .

The portion of the embryonal axis above the level of attachment of scutellum is the epicotyl. Epicotyl has a shoot apex and a  few leaf primordia enclosed in a hollow foliar structure, the coleoptile.