Reproduction in plant

Shoot apical meristem develop into reproductive meristem which later develops into inflorescence axis over which floral primordia develop.The primordia gives rise to buds and into flower in which the androecium and gynoecium develop.

Stamen:

The anther is a bilobed structure often a longitudinal grove runs b/w them.

It’s a four-sided structure consisting of four microsporangia which form pollen sac and gives rise to pollen grains. Thus anther is tetraspornagiate.

The structure of Microsporangia:

It consists of homogenous meristematic cells called the primary sporogenous cells which form microspore mother cell.

Anther wall:

Structure of Microsporangia

                   

1.               Epidermis: It’s the outermost layer. Provides protection. The epidermis of Arceuthobium develops fibrous thickening and is called exothecium.

2.                Endothecium:  It has alpha cellulosic fibers arising from an inner tangential wall. It helps in dehiscence of anther due to hygroscopic nature.

3.                Middle layer

4.               Tapetum: It’s the most innermost layer and nourishes the developing pollen grain. It has dense cytoplasm and polyploid.

Pollen grain:

It represents the male gametophyte.

Pollen grain has two layers:

·     Exine: outer layer and made up of sporopollenin. It is absent on germ pore.

·     It helps in fossilization.

·     Gives rise to the array of designs which are of taxonomic importance.

Withstand adverse condition.

Intine: the thin continuous layer made up of cellulose and pectin.

The cell wall of pollen is spermoderm.

The pistil:

The ovary has the ovarian chamber that has placenta inside from which arises the megasporangia.

MMC is a large cell consisting of dense cytoplasm and a prominent nucleus.

MMC divides into linear tetrad out of which one(chalaza) remains functional and the other three(micropyle) degenerates.

Embryo sac:

v    P.Maheshwari  classified the embryo sac on  the  basis of no. of  megaspore nuclei

Ø    Monosporic embryo sac:  one nucleus. Eg. Polygonum, Oenothera

Ø    Diasporic embryo sac: two nuclei. eg. Allium, Endymion.

Ø    Tetrasporic embryo sac: four nuclei.

Development of embryo sac

The functional megaspore (chalazal) divides mitotically giving rise to 2 nucleate stage. It is followed by two more division to form 4 and 8 nucleate stage. One nucleus from each pole comes to the middle and form polar nuclei. These mitotic divisions are strictly free nuclear followed by cell wall formation.

This is called Polygonum type of embryo sac studied by Strasburger in Polygonum.

a)  The three nuclei get organized at the chalazal end called antipodal

b) While other three nuclei get organized at micropyle called synergid and egg cell.

c)  The polar nuclei are present in the central cell.

 This constitutes a 7 celled and 8 nucleate embryo sac.

Embryo sac

Organization of embryo sac:

a)  Synergid: These cells possess micropyle nucleus and chalazal vacuole. Synergid lacks cell wall at the chalazal end and one synergid degenerate just with pollination. They have filiform apparatus which guides the pollen tube.  They secrete the chemotropic substance that helps in the growth of pollen tube inside the embryo sac.

b) Egg cell: They show cytoplasmic polarity and have cell wall thicker at the micropyle end. They have micropyle vacuole and chalazal nucleus. Plasmodesmata connection is present b/w synergid and egg cell.

c)  Antipodal: Vegetative cell

d) Central cell: Largest cell.

The structure of ovule:

1.Funicle: The stalk of the ovule by which it is attached to the placenta.

2.Hilum: The region where the ovule attaches to the placenta.

3.Integuments: Protective envelope around the nucellus.

4.Nucellus: It’s a parenchymatous mass of tissue which forms the body of the ovule.

Depending on the development of the nucellus, ovule is of two types:

a. Crassinucleate ovule: well-developed

b.Teninucleate ovule: poorly developed.

5.Micropyle: The tip where the integument is absent.

6.Chalaza: The basal part of the ovule

Ovule

Types of ovule:

1. Orthotropous: The micropyle, chalaza, and funicle lie in the straight line. eg. Piper, Polygonum, Cyprus.

2. Anatropous:  The ovule turns 180.

3.Hemianatropus: The ovule turns 90. eg. Ranunculus

4.Campylotropous: The ovule is more or less  90.eg.Leguminosae, Cruciferae

5.Amphitropous: The embryo sac is horse shaped. eg.Lemna, Poppy, and Alisma

6.Circinotropous: the ovule turns 360. eg. Plumbaginaceae, Opuntia

Types of ovule

                   

.

Pollination: Types of pollination-

        i.            Autogamy: transfer of pollen grain b/w anther to the stigma of the same flower.

Adaptation:

·     Homogamy: maturation of anther and stigma at the same time.

·     Bisexuality:  the presence of both the whorls in the same flower.

·     Cleistogamy: The flower does not open at all. The anther and stigma lie very close to each other and ensure pollination in absence of pollinating agent and produce assured seed set.

·     Bud pollination: When pollination takes place in bud stage before opening. Eg. Pea, Rice.

    ii.            Geitonogamy: The transfer of pollen grain from the anther to the stigma  of one flower to another.

 iii.            Xenogamy: Transfer of pollen grain from an anther to stigma in different flower.

Agents of pollination:

·     Abiotic  agent:

1.   The Wind(Anemophily):

                                                      i.            The pollen grains are nonsticky and light.

                                                  ii.            The stamen are well exposed to disperse pollen grains.

                                               iii.            The stigma is large and feathery to capture the pollen grains.

                                               iv.            The presence of single ovary.

                                                   v.            Flowers packed in inflorescence.

It’s common in grass.

2.   Water (Hydrophily):

Pollination by water is quite rare in flowering plant and is limited to 30 genera mostly monocots.

                                                      i.            On the surface: (epi hydrophily)

Eg.Vallisneria

The female flower has long pedicel which reaches the surface and the pollens are released by the male flower and carried passively by water current which is received by the stigma.

                                                  ii.            Beneath  the surface: (hypo hydrophily)

Eg. Zostera

The female flower is submerged under water where the pollen –ribbon-shaped is released under water.

In the majority, the flowers emerge above the water level surface and are pollinated by bees or the wind. Eg. Water Hyacinth or Water Lilly.

Biotic agent:

Insect (entomophily):

Birds	Orinthophily	Bombax, Callistemon
Bats	Chiropterophily	Anthocephalus
Snakes	Ophiophily	Santalum, Machelia
Snails	Malacophily	Arisaema

Outbreeding devices:

a.    Unisexuality:

·     Monoecious: male and female are in the same plant. It prevents Autogamy and but not geitonogamy.

·     Delicious:

b.   Self-incompatibility: inhibits the growth of pollen tube genetically.

c.    Dichogamy:

Pollen-pistil interaction:

The stigma is able to recognize the right type of pollen by constant chemical dialogue b/w pollen grain and stigma. Its acceptance is followed by germination of pollen on stigma to produce pollen tube in style. The content of pollen grain moves through the stigma. The generative cell divides mitotically to produce two male gametes.

Entry of pollen tube into ovule takes place through:

     Micropyle (Porogamy): Most common type.

·     Chalaza(Chalazogamy): eg. Casuarina

·     Integuments (Monogamy) eg.Cucurbita.

The pollen-pistil interaction is a dynamic process.