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Reproduction in

Flowering Plants

C27: BIOLOGY OF SEED PLANTS BSc III (VI Semester)

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ANGIOSPERMS

KEY

CONCEPTS

The flower is the site of sexual reproduction in angiosperms.

A typical flower consists of four whorls: sepals, petals, stamens, and carpels.

Pollen grains are transported to stigmas by a variety of agents, such as insects, animals and wind.

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Double fertilization results in a plant embryo and endosperm.

The seed is a mature ovule, and the fruit is a mature ovary.

Many vegetative organs (roots, stems, and leaves) have evolved modifications for reproducing asexually.

KEY

CONCEPTS

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THE FLOWERING PLANT LIFE CYCLE

ALTERNATION OF GENERATIONS

In which they spend a portion of their life cycle in a multicellular haploid stage

and

a portion in a multicellular diploid stage.

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The haploid portion, called the gametophyte generation, gives rise to gametes by mitosis.

When two gametes fuse during fertilization, the diploid portion of the life cycle, called the sporophyte generation. The sporophyte generation produces haploid spores by meiosis.

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In flowering plants the diploid sporophyte generation is larger and nutritionally independent.

The haploid gametophyte generation, which is located in the flower, is microscopic and nutritionally dependent on the sporophyte.

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THE FLOWER

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The peduncle, a flower stalk, expands slightly at the tip into a receptacle, which bears the other flower parts. These parts, called

sepals, petals, stamens, and carpels,

are attached to the receptacle in whorls (circles)

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Each part of a flower has a specific function

Sepals, which constitute the outermost and lowest whorl on a floral shoot, cover and protect the flower parts when the flower is a bud. Sepals are leaf like in shape and form and are often green. Some sepals, such as those in lily flowers, are colored and resemble petals.

The collective term for all the sepals of a flower is calyx.

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Petals, The whorl just inside and above the sepals, which are broad, flat, and thin (like sepals and leaves) but tremendously varied in shape and frequently brightly colored, which attracts pollinators. Petals play an important role in ensuring that sexual reproduction will occur. Sometimes petals fuse to form a tube or other floral shape. The collective term for all the petals of a flower is corolla.

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Stamens, just inside and above the petals, which are the male reproductive organs. Each stamen has a thin stalk, called a filament, at the top of which is an anther, a saclike structure in which pollen grains form. For sexual reproduction to occur, pollen grains must be transferred from the anther to the female reproductive structure (the carpel),

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Carpels, the female reproductive organs, are located in the center or top of most flowers.

Carpels bear ovules, which are structures with the potential to develop into seeds. The female part of the flower, often called a pistil, Each pistil has three sections: a stigma, on which the pollen grains land; a style, a neck like structure through which the pollen tube grows; and an ovary, a jug like structure that contains one or more ovules and can develop into a fruit.

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GENERALIZED FLOWER

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MALE REPRODUCTIVE STRUCTURE

The stamen consists of two parts: Anther and Filament

The anther is where meiosis occurs to produce haploid pollen The filament is a stalk that supports the anther

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Female Reproductive Structure

The pistil consists of the stigma, style and ovary

The sticky stigma receives the pollen from the anther The pollen grows a tube down through the style

Meiosis occurs in the ovary to produce haploid ovules

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Development of female and

male gametophytes

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The basic function of a flower is reproduction. A flower is pollinated when pollen from an anther is delivered to a stigma. The pollen germinates on the stigma and forms a pollen tube that delivers sperm to the egg within the ovule.

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The fertilized egg develops into an embryo.

The polar nuclei are also fertilized and develop into endosperm, which is nutritive tissue for the embryo. The seed is the embryo plus the endosperm.

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Development of Endosperm

The primary endosperm nucleus is normally located directly below the egg cell and undergoes division almost immediately after its formation.

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One of the most critical events during a plant's life cycle is survival at germination.

Many types of structural and functional specializations occur in seeds, which increase the probability of survival.

An important specialization is the storage of seed reserves.

Endosperm is a seed storage tissue formed within the embryo sac from a second fertilization within the central cell

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Flowering plants are unique in exhibiting double fertilization.

In these plants two male gametes are delivered into the embryo sac, by the pollen tube, of which one fuses with the egg cell (female gamete) and the other fertilizes the polar nuclei or the secondary nucleus (the fusion product of the two polar nuclei).

The fertilized egg cell i.e. zygote (2n) develops into the embryo and the central cell, with the fertilized secondary nucleus i.e. primary endosperm nucleus (3n), develops into the endosperm.

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The endosperm is predominantly a triploid tissue.

However it is diploid in all the members of the family Onagraceae (evening primrose family) and pentaploid in Fritillaria.

Primrose Fritillaria

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The products of double fertilization i.e. the zygote and the primary endosperm nucleus.

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THE EMBRYO

The embryo sac (female gametophyte) always remains embedded in the ovule. The embryo likewise remains nurtured within the embryo sac.

This results in a remarkable nesting of generations: new sporophyte (embryo) within female gametophyte (embryo sac) within old sporophyte (ovule).

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INTRODUCING THE COTYLEDON(S)

The first appendage produced by the embryo, the cotyledon. The origin of the term (Greek, meaning “cup-shaped hollow”) is obscure.

Perhaps the name was given because cotyledons of many dicot seedlings gradually wither (decay) and turn up around the edges as their stored food is utilized, which gives them a somewhat cuplike appearance.

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The cotyledon of the mature embryo has the unexpected distinction of providing the primary taxonomic character that delimits and names the two large angiosperm groups;

Monocotyledoneae (monocots) have one and Dicotyledoneae (dicots) have two.

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Among monocots two common features probably contribute:

1. The sheath like single cotyledon completely encircles the shoot apex, leaving no space for a second cotyledon.

1. The monocot cotyledon typically remains in the seed as a haustorium, enlarging as it absorbs endosperm upon germination for translocation to the seedling, and leaving little room in the seed for a second cotyledon.

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THE ZYGOTE

The sperm nucleus that merges with the egg nucleus (technically called Karyogamy

“marriage of nuclei”) contributes a second set of chromosomes, which restores the 2N sporophytic genetic constitution and transforms the egg cell into the zygote (Greek, meaning “yoked together”), the first cell of the embryo.

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The zygote of most species does not divide immediately after fertilization; instead, it waits until the primary endosperm cell (PEC) has produced some endosperm (see Table on next slide). A reasonable speculation is that a certain amount of PEC activity is needed to provide some nutritional, hormonal, or other stimulation.

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PROEMBRYO INITIATION

In the majority of endosperms the zygote divides transversely, resulting in a small apical cell (conventionally designated ca) towards the interior of the embryo sac and a large basal cell (conventionally designated cb) towards the micropyle.

NOTE: Rarely, the division of zygote may be vertical (Loranthaceae) or obligate (Triticum spp.)

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The variation in the developmental pattern of embryo during early embryogeny are common to monocotyledons and dicotyledons.

Differences appear when the initiations of plumule and cotyledon(s) are laid down.

From the 2-celled stage until the initiation of organs, the embryo is commonly called proembryo.

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A dividing zygote always produces a basal cell that does not become part of the embryo regardless of whether it will produce a functional suspensor. It seems reasonable to speculate that a basal cell is necessary to orient or otherwise prepare the apical cell to initiate the proembryo.

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Development of Embryo in Monocotyledons

The zygote elongates and then divides transversely to form basal and terminal/apical cells. The basal cell (towards micropylar end) produces a large swollen, vesicular suspensor cell. It may function as haustorium. The terminal cell divides by another transverse wall to form two cells.

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The top cell after a series of divisions forms plumule and a single cotyledon. Cotyledon called scutellum, grows rapidly and pushes the terminal plumule to one side. The plumule comes to lie in a depression.

The middle cell, after many divisions forms hypocotyl and radicle. It also adds a few cells to the suspensor. In some cereals both plumule and radicle get covered by sheaths developed from scutellum called coleoptile and coleorhiza respectively.

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Stages in Development of

Monocot Embryo

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The embryos of monocotyledons have only one cotyledon. In grass family (Gramineae), this cotyledon is called scutellum. It is situated towards lateral side of embryonal axis. This axis at its lower end has radicle and root cap enclosed in a sheath called coleorhiza.

The part of axis above the level of attachment of scutellum is called epicotyl. It has as shoot apex and few leaf primordia enclosed in a hollow foliar structure called coleoptile. Epiblast represents rudiments(undeveloped/immature) of second cotyledon.

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Development of Embryo in dicotyledons

According to Soueges, the mode of origin of the four-celled pro-embryo and the contribution made by each of these cells makes the base for the classification of the embryonal type. However, Schnarf (1929), Johansen (1945) and Maheshwari (1950) have recognized five main types of embryos in dicotyledons based on the plane of division of the apical/terminal cell in the 2-celled proembryo and contribution of cb & ca in the formation of embryo proper.

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1. The terminal cell of the two-celled pro- embryo divides by longitudinal wall.

(i) Crucifer type:

Basal cell plays little or no role in the development of the embryo.

(ii) Asterad type:

Basal and terminal cells play an important role in the development of the embryo.

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2. The terminal cell of the two-celled proembryo divides by a transverse wall, Basal cell plays a little or no role in the development of the embryo.

(i) Solanad type: Basal cell usually forms a suspensor of two or more cells.

(ii) Caryophyllod type: Basal cell does divide further.

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(iii) Chenopodiad type: Both basal and terminal cells take part in the development of the embryo.

Here citing the example of Capsella bursa- pastoris (Shepherd’s purse), the detailed study of Crucifer type of the development of the embryo has been given.

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Stages in Development of Dicot Embryo

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A typical dicotyledonous embryo (See the figure in above slide) consists of an embryonal axis and two cotyledons. The part of embryonal axis above the level of cotyledons is called epicotyl. It terminates with the stem tip, called plumule (future shoot). The part below the level of cotyledons is called hypocotyl which terminates in the root tip called radicle (future root). The root tip is covered with a root cap (calyptra).

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In Capsella bursa-pastoris, the elongating cotyledons curve due to the curving of the ovule itself. With the growth of embryo, the ovule enlarges. Its integuments ultimately become hard to form protective coverings.

Now the embryo undergoes rest and the ovule gets transformed into seed. In some plants the embryo remains in the globular or spherical form even at the time of seed shedding without showing any distinction of plumule, radicle and cotyledons, e.g., Orobanche, Orchids, Utricularia.

References

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