Sexual Reproduction in Flowering Plants Shot Notes // Grove Production

 

   CHAPTER 2 –    SEXUAL REPRODUCTION IN FLOWERING PLANTS

 

All flowering plants show sexual reproduction.

At least five flowers of ornamental value that are commonly cultivated at homes and in garden.

PRE-FERTILISATION : STRUCTURES AND EVENTS :-

Androecium consists of a whorl of stamens representing the male reproductive organ and the gynocium represents the female reproductive organ.

 Stamen – the long and slender stalk called the Filament.

Bilobed structure called the anther.

The proximal end of the filament is attached to the thalamus or the petal of the flower.

The number and length of stamens are variable in flowers of different species.

A typical angiosperm anther is bilobed with each lobe having two theca, i.e, they are dithecous.

The anther is a four- sided (tetragonal) structure consisting of four microsporangia located at the corners, two in each lobe.

The microsporangia develop further and become pollen sacs.

It nourishes the developing pollen grains. Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus.

The anther is young, a group of compactly arranged homogenous cells called the Sporogenous Tissue occupies the centre of each microsporangium.  

The process of formation of microspores from a pollen mother cell (PMC) through meiosis is called Microsporogenesis.

The microspores, as they are formed, are arranged in a cluster of four cells- the Microspore Tetrad.

Pollen Grains: The pollen grains represent the male gametophytes.

Pollen grains are generally spherical measuring about 25-50 micrometer in diameter.

Exine is made up of sporopollenin which is one of the most resistant organic material known. It can withstand high temperatures and strong acids and alkali.

No enzymes that degrades sporopollenin is so for known.

Pollen grain exine has prominent apertures called germ pores.

The inner wall of the pollen grain is called the intine. It is a thin and continuous layer made up of cellulose and pectin.

The pollen grain is mature it contains two cells , the Vegetative cell and Generative.

Placenta are the Megasporangia, commonly called Ovules.

The Placenta is located inside the ovarian cavity.

The ovule is a small structure attached to the placenta by means of a stalk called funicle .

The body of the ovule fuses with funicle in the region called hilium.

Each ovule has one or two protective envelopes called Integuments.

Megasporogenesis :  The process of formation of megaspores from the megaspore mother cell is called megasporogenesis.

Female Gametophyte :  In a majority of flowering plants, one of the megaspores is functional while the other three degenerate.

The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 2- nucleate embryo sac.

Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo Sac.

Six of the eight nuclei are surrounded by cell walls and organized into cells; the remaining two nuclei , called polar nuclei are situated below the egg apparatus in the large Central Cell.

Three cells are grouped together at the micropylar end and constitute the egg apparatus.

The egg apparatus , in turn, consists of two synergids and one egg cell.

Three cells are at the chalazal end and are called the Antipodals.

A typical angiosperm embryo sac, at maturity , through 8-nucleate is 7-celled.

Pollinatioin is the mechanism to achieve this objective. Transfer of pollen grains (shed from the anther ) to the stigma of a pistil is termed pollination.

Kinds of Pollination :- Depending on the source of pollen, pollination can be divided into three types .

(i) Autogamy :-  Pollination is achieved within the same flower . Transfer  of pollen grains from the anther to the stigma of the same flower.

(ii) Geitonogamy :-  Tranfer of pollen grains from the anther to the stigma of another flower of the same plant.

(iii) Xenogamy :-   Tansfer of pollen grains from anther to the stigma of a different plant. This is the only type of pollination which during pollination brings genetically different types of pollen grains to the stigma.

Outbreeding Devices :- Majority of flowering plants produce hermaphrodite flowers and pollen grains are likely to come in contact with the stigma of the same flower.

Pollen –Pistil Interaction :- Pollination does not guarantee the transfer of the right of pollen (compatible pollen of the same species as the stigma ).

Artificial hybridization :- It is one of the major approaches of crop improvement programme.

Emasculated flowers have to be covered with a bag of suitable size, generally made up of butter paper, to prevent contamination of its stigma with unwanted pollen. This process is called Bagging.

One of the male gamete moves towards the egg cell and fuses with its nucleus thus completing the Syngamy.

The formation of a diploid cell, the Zygote.

The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid Primary Endosperm Nucleus (PEN).

The fusion of three haploid nuclei it is termed triple fusion.

Fusion , syngamy and triple fusion take place in an embryo sac the phenomenon is termed double Fertilisation.

The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm while the zygote develops into an embryo.

POST-FERTILISATION :- Double fertilization, events of endosperm and embryo development, maturation of ovules(s) into seed(s) and ovary into fruit, are collectively termed Post-fertilisation events.

Embryo :-  Embryo develops at the micropylar end of the embryo sac where the zygote is situated. Most zygotes divide only after certain amount of endosperm is formed .

The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped and mature embryo.

A typical dicotyledonous embryo , consists embryonal axis and two cotyledons .

The portion of embryonal axis above the level of cotyledons is the epicotyl, which terminates with the plumule or stem tip.

The root tip is covered with a root cap.

The grass family the cotyledon is called scutellum that is situated towards one side (lateral) of the embryonal axis.

The embryonal axis has the radical and root cap enclosed in an undifferentiated sheath called Coleorrhiza.

The seed is the final product of sexual reproduction . It is often described as a fertilized ovule. Seeds are formed inside fruits.

Persistent nucellus is the Perisperm.

The embryo may enter a state of inactivity called Dormancy.

The wall of the ovary develops into the wall of fruits called Pericarp.

The thalamus also contributes to fruit formation . Such fruits are called False Fruits.

Most fruits however develop only from the ovary and are called True fruits.

The species, fruits are the results of fertilization, there are a few species in which fruits develop without fertilization. Such fruits are called Parthenocarpic Fruits. Banana is one such example.  

                

 

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