Notes on "Sexual Reproduction In Flowering Plants" Part-2

 Notes on "Sexual Reproduction In Flowering Plants" Part-2

by:-Nagraj Sir

 Megasporogenesis- The process of formation of megaspore from megaspore mother cell by meiotic division is known as megasporogenesis. This process takes place in ovule

Ovule differentiates a single megaspore mother cell (MMC) in the micropylar region of nucellus. MMC undergoes meiotic division that results into the production of four megaspores.

• In most of the flowering plants three megaspores degenerate. 1megaspore develops into female gametophyte (embryo sac).

• The nucleus of functional megaspore divides mitotically to form two nuclei which move to opposite poles to form 2-nucleate embryo sac. Two more sequential mitotic division results into 8-nucleate embryo sac.

• Six of the eight nuclei surrounded by cell wall and remaining two nuclei (polar nuclei) are situated below the egg apparatus.

• Three cells are grouped at micropylar end to constitute egg apparatus and three cells at chalazal end forms antipodal cells. At maturity ,embryosac is 8-nucleate and 7 celled.

Pollination: When pollen grains get transferred ( from the anther) to the stigma of the pistil is called

 Pollination. There are different kinds of Pollination:

a) Autogamy– transfer of pollen grain from anther to stigma of same flower.

i. Cleistogamous – flower which do not open. cleistogamous flowers are autogamous as there is no chance of cross-pollen landing on the stigma. Cleistogamous flowers produce assured seed-set even in the absence of pollinators. e.g Viola (common pansy), Oxalis, and Commelina.

ii. Chasmogamous– exposed anther and stigma.

b) Geitonogamy – transfer of pollen grains from anther to stigma of different flower of same plant. Geitonogamy is functionally cross-pollination involving a pollinating agent, genetically it is similar to autogamy since the pollen grains come from the same plant

c) Xenogamy– transfer of pollen grain from anther to stigma of different plant’s flower of same species.

Agents of pollination includes abiotic (water, wind) and biotic (insects, butterfly, honey bee etc. large number of pollen grains are produced by plants using abiotic mode of pollination as most of pollen grains are wasted during transfer.

Outbreeding Devices– the various mechanisms take discourage self-pollination and encourage cross pollination as continued self-pollination leads to inbreeding depression. It includes

• Pollen release and stigma receptivity not synchronized.

• Anther and stigma are placed at different position.

• Inhibiting pollen germination in pistil.

• Production of unisexual flowers.

Pollen pistil interaction – the pistil has ability to recognize the compatible pollen to initiate post pollination events that leads to fertilisation. Pollen grain produce pollen tube through germ pores to facilitate transfer of male gametes to embryo sac.

Artificial Hybridization

• Crossing diff varieties of species- hybrid individual- with desirable characters of the parent plants
• desired pollen grains for pollination- stigma protected from contamination
• Emasculation : removal of anther
• Bagging : flower covered- bag made up of butter-prevent contamination of stigma from unwanted pollen

Double Fertilisation- after entering the one of the synergids, each pollen grain releases two male gametes. One male gametes fuse with egg (Syngamy) and other male gametes fuse with two polar nuclei (triple fusion) to produce triploid primary endosperm nucleus (PEN). Since two types of fusion takes place in an embryo sac the phenomenon is called double fertilisation. The PEN develops into the endosperm and zygote develops into embryo.

Post fertilisation events include endosperm and embryo development, maturation of ovules into seeds and ovary into fruits.

Endosperm– the primary endosperm cell divides many time to forms triploid endosperm tissue having reserve food materials.

Two types of endosperm development :
(i) Free nuclear type (common method)
(ii) Cellular type

(a) Non-albuminous- endosperm completely utilized- before maturation of seeds. e.g pea, groundnut

(b) Albuminous- a portion of endosperm remain in mature seeds. e.g wheat, maize, castor

Embryo- Embryo develops at the micropylar end of the embryo sac where the zygote is located.

Embryogeny – early stages of embryo development.The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped and mature embryo.

Embryo consists of:
– embryonal axis
– cotyledons
– plumule
– radicle

Monocotyledonous Seed
– Scutellem = Cotyledon
– Coleorrhiza: undifferentiated sheath covering radical & root cap
– Coleoptile: sheath covering plumule

Seed
– Fertilized and mature ovule develops into seed.

Seed consists of:
– cotyledon(s)
– embryonal axis
– Seed coat- double layered- formed by integuments

• Testa (outer coat)
• Tegmen (inner coat)

– Micropyle:- small opening on seed coat, it facilitates entry of H2O & O2 into seeds (for germination)
– Hilum:- scar on seed coat
– Seed – Albuminous / Non-Albuminous
– Perisperm : remnants of nucellus that is persistent. Ex: Black pepper
– Dormancy: state of inactivity

• The wall of ovary develops into wall of fruit called pericarp. In true fruits only ovary contributes in fruit formation by in false fruit thalamus also contributes in fruit formation.

Apomixis
– Form of asexual reproduction- mimics sexual reproduction- seed formed without fertilisation
– Formation of apomictic seeds :
• diploid cell (formed without meiosis)- develop into embryo without fertilization
• cells of nucellus (2n) surrounding embryo sac- protrude into embryo sac- develop into embryos. Ex. Citrus and Mango.

Polyembryony
– Occurrence of more than one embryo in a seed
– Often associated with apomixes. Ex: Citrus, groundnut