R Camerarius was the first to describe sexual reproduction in plants. The flower is the main reproductive part of the angiospermic plants. In which, pollen grain is male gametophyte and embryo sac is female gametophyte.
The process of sexual reproduction is divided into three steps:
- Double fertilization
Pre-Fertilization events in sexual reproduction in flower
Important events of Pre-fertilization are:-
- Formation of the pollen grain
- formation of embryosac
- Pollen postil interaction
Pollen Grain formation
Pollen grains are the male gametophyte. Microgametogeneis is the process of the formation of male gamete. Pollen grains are developed from the microspores cells, that are present in the anther.
Anther is bilobed and each lobe having two microsporangia.
The wall of microsporangia is consists of 4 layers :
- Middle layer
- Endothecium is made up of radially elongated cells and helps in the dehiscence of anthers.
- Tapetum is the innermost layer of the wall of microsporangia. Tapetum cells have more than one prominent nuclei. It provides nutrition to the pollen grains, during pollen formation. It produces pollen kit material, sporopollenin and secretes the callase enzyme.
- On the basis of behavior, tapetum is of two type – Amoeboid and Secretory
- The amoeboid tapetum is also called as invasive tapetum or periplasmodial tapetum. Amoeboid tapetum cells breakdown its radial and inner wall completely to release their protoplasmic contents in the cavity of the sporangium. During pollen development, this content surrounds the developing pollen grains and provides them proper nourishment. e.g. – Butomus
- Cells of secretory tapetum provide Ubisch bodies which help in the ornamentation of exine as they have a chemical sporopollenin which is deposited on them.
The process of microspore formation is called Microsporogenesis. In this, the diploid microspore Mother Cell (MMC) produce haploid microspores.
In this process, the young anthers have a homogenous mass of hypodermal cell bounded by the epidermis. Inside the epidermis, some cells have a prominent nucleus and abundant protoplasm, known as Archesporial cells.
These archesporial cells undergo periclinal division and form outer parietal cells and inner sporogenous cells.
The parietal cells from the microsporangial wall and the sporogenous cells undergo mitotic division and function as Microspore Mother cells (MMCs). The MMCs divide by meiosis and form four haploid microspores. These haploid microspores remain inside the microspore mother cells and form microspore tetrads.
- Sometimes microspores in a sporangium are united together to form a single structure called pollinium. e.g. Calotropis.
When these microspores released from the tetrad are called as pollen grains.
They undergo meiotic division and give rise to a larger vegetative cell ( helps in the formation of pollen tube) and a smaller generative cell.
During meiosis of the microspore mother cell, two patterns of cell wall formation seen:-
- Successive type – Seen in Monocots.
- Simultaneous type – Seen in Dicots.
Pollen grain structure
The wall covering of the pollen grain is called sporoderm. It is made of two layers, outer exine, and the inner intine. The exine is chiefly made up of sporopollenin. Sporopollenin is one of the most resistant biological known material. It is resistant to high temperature, acids, alkali and cannot be degraded by any enzyme. Exine also has prominent apertures called germ pores where sporopollenin is absent. This germ pore is the point from where the pollen tubes emerge which carry male gametes in them.
2. Embryo Sac Formation
The body of ovule consists of a mass of parenchymatous cell called Nucellus. It may be
- well-developed – Crassincellate
- Poorly developed – Tenuinucellate.
The outer most multicellular covering of ovule is called integument and it leaves a narrow pore at one end to the ovule called micropyle.
Types of ovules
On the basis of the relative position of funiculus, chalaza, and micropyle the ovules can be classified into following six type:
- Atropus or Orthotropous
- Simplest and primitive type
- Micropyle, chalaza, and funiculus all lie in one vertical plane.
- Anatropous or inverted
- Most common type of ovules
- In families – Butomaceae (Butamus)
- The body of ovule is turned through 90o.
The megaspore mother cells (2n) (MMCs) divide meiotically to produce a tetrad of four linear megaspores. Three megaspores degenerate and only one megaspore remain functional. The functional megaspore divides three times by mitotic division at the time megagametogenesis and forms eight haploid nuclei and seven cells are formed.
Three types of embryo sac on the basis of the number of megaspore nuclei participating in embryo sac formation:
- Monosporic embryo sac
- Polygonum type – formed by the chalazal megaspore of the tetrad and is eight nucleate.
- Oenothera type – formed by the micropylar megaspore of the tetrad and it is four nucleate.
- Bisporic embryo sac
- Allium type – It develops from chalazal dyad.
- Endymion type – It develops from micropylar dyad.
- Tetrasporic embryo sac
The synergids develop haustoria and thus provide nutrition to the developing embryo sac. Example – Quinchamalium.
3.Pollination-Sexual reproduction in flowering plants
Pollination is mainly of three types:
- seen in open flower or chasmogamous flowers.
- The anther and stigmas mature at the same time.
- It occurs in cleistogamous (Closed) flowers.
- Viola, Oxalis, and Commelina produce both type s of flower, ( Chasmogamous and Cleistogamous).
- Bud pollination
- It involves the transfer of pollen grains from the anther of a flower to the stigma of another flower of the same plant. So, genetically it is self-pollination but functionally it is cross-pollination.
- Xenogamy (Allogamy)
- It takes place between two genetically different plants thus, offsprings formed are of new types.
- Recombinationation is the result of cross-pollination, which increases heterosis (Increase in vigor).
Adaptation in Xenogamy:
- Dichogamy – Maturation of male and female gamete occurs at different times.
- Protandry – When anther mature first.
- Protogyne – When gynoecium mature first.
- Herkogamy – A mechanical barrier exists between the compatible pollen and stigma so that self-pollination becomes impossible.
- Self-incompatibility – When pollen from the same flower comes on the stigma, it is incapable of bringing about fertilization, due to the presence of similar self-sterile genes.
Abiotic agencies for cross-pollination
- Anemophily ( By wind)
- Non-essential whorls such as calyx, corolla, and bracts, bracteoles are not showy.
- Pollen grains are dry, lightweight, very small sized, non-sticky and unwettable.
- Stigma may be large and feathery (grasses).
- Hydrophily (By water)
- Hypohydrophily – when pollination occurs in submerged plants. eg. – Zostera.
- Epihydrophily – When pollination occurs in floating hydrophytes. eg. – Vallisneria. In Vallisneria spiralis, flowers are borne under water. Male flower gets detached from the parent plant after maturation. Female flowers at the time of pollination are brought to the surface by their long stalks. As it arrives on the surface it forms a cup-like depression to grab male fower and complete the pollination.
Some other abiotic agencies for cross-pollination
- Turn pope mechanism – Salvia
- Translator mechanism – Calotropis
- Trapdoor mechanism – Ficus
- Pseudocopulation mechanism – Ophrys
Biotic agencies for cross-pollination
- Entomophily (by insects)
- Nectariferous glands of flower produce nectar, which attracts the pollinators for feeding.
- Flowers are bright colored, fragrant and omit scent.
- Ornithophily (by birds)
- Bigonia pollinated by Humming birds.
- Chiropterophily (by bats)
- Malacopily (by snails and slug)
- Ophiophily (by snake)
The growth and path of pollen tube through the style is also determined by specific chemicals.
Pollen tube after reaching the ovary enter into ovule via following ways:
- Porogamy – through micropyle
- Mesogamy – through integument. eg.- Cucurbita.
- Chalazogamy – through chalazal tissues. eg.- Casuarina.
In the ovule, the pollen tube is attracted by secretions of synergids. Usually, the pollen tube enters the embryo sac by passing into one of the two synergids which starts degenerating. The pollen tube burst up by absorbing hydrolytic substances secreted by degenerating synergids.
4.Double Fertilization-Sexual reproduction in flowering plants
Double fertilization consist of two events that are as follows:
- Syngamy or Amphimixis – Fusion of egg nucleus with one male gamete and form a diploid cell, the zygote.
- Triple Fusion – Along with syngamy, the other male gamete moves towards the two polar nuclei located in the center cell and fuses with them to produce a triploid Primary Endosperm Mother (PEM) cell.
- Egg – haploid (n)
- Secondary nucleus – Diploid (2n), contain two polar nuclei.
- Primary Endosperm mother cell – Triploid (3n), Product of triple fusion
- Zygote – Diploid (2n) – Product of syngamy
- Antipodals and synergids degenerate after fertilization and nucellus change into perisperm.
5. Post-fertilisation: Structure and events
Development of embryo is called embryogeny.
The mature monocot embryo contains a laterally situated plumule and a single, terminal cotyledon. Monocotyledons have only one cotyledon. In Gramineae, this cotyledon is called scutellum. The part of axis above the level of attachment of scutellum is called epicotyl.
Epiblast present in monocot embryo represents secondary cotyledons.
Effect of pollen on endosperm is called Xenia.
In Orchidaceae, Podostemaceae and Trapaceae families, endosperm is absent.
In Oenothera type of embryo sac, there is haploid secondary nucleus (n). Thus, the ploidy level of the endosperm is diploid (2n) in Oenothera type embryo sac.
In cereals, the cells of the outermost layers of the endosperm become morphologically and physiologically specialized and form a layer of cells called aleurone layer.
On the basis of its development, the endosperm is classified into three main groups:
- Cellular endosperm
- Particularly in dicotyledons.
- Nuclear endosperm
- In coconut, Primary Endosperm Nucleus undergoes a number of free nuclear divisions. Hence the embryo sac gets filled with a clear fluid, this fluid is known as coconut water.
- Helobial endosperm
- Particularly in monocotyledons.
- Chalazal endosperm haustorium with fingers like appendages occurs in Lomatia.
- Only lateral endosperm haustoria arise in monochoria. Mature endosperm with any degree of irregularity and unevenness in its surface is called ruminate endosperm.
- In mosaic endosperm, the tissue of endosperm is not homogenous, e.g. Zea mays.
Depending on the presence or absence of endosperm, seeds may be:
- Endospermic or Albuminous seed
- Endosperm is present in the seed.
- Examples – Wheat, maize, sunflower, and caster, etc.
- Non-endospermic or Exalbuminous seed
- Endosperm is absent in the seed.
- Examples – Pea, bean, groundnut, etc.
Fruits that developed from other floral parts such as thalamus along with the development of ovary wall are known as False fruit, eg. – Apple, etc.
Cruncle is a fleshy, whitish structure present o micropylar end of the seed. It arises due to the proliferation of cells at the tip of outer integument on the side of funiculus or all around the micropyle. It helps in seed dispersal.
Adventive polyembryo is formed by diploid nucellus or integument cells. eg. – Opuntia, Citrus.
In induced polyembryony, each viable cell of a plant can be converted into an embryo by providing suitable nutritional requirements and environmental conditions in culture vials. Embryos formed in such a way are called adventive embryos.
Reproduction in Organisms
More- Sexual reproduction in flowering plants