- Sexual reproduction is the process of development of new organisms through the formation and fusion of gametes.
- The flower is the main structure concerned with reproduction, The reproductive organs or the sporophylls are produced within the flowers. The sporophylls are of two types microsporophylls ( stamen) and megasporophylls ( carpel)
- Stamen is distinguished as filament, anther and connective. Carpel is distinguished as ovary bearing ovule, style and stigma
- The whole process of sexual reproduction in flowering plants can be divided into three steps:- i) Pre-fertilization ii) Double fertilization iii) Post-fertilization
PRE-PERTILISATION : STRUCTURE AND EVENTS
The pre-fertilisation events can be studied under following points:-
i) Pollen grain formation ii) Embryo sac formation iii) Pollination iv) Pollen pistil interaction
POLLEN GRAIN FORMATION
Male reproductive unit ( Stamen)
- A stamen is the male reproductive unit of angiosperms. It consists of an anther and a filament. The anther is bilobed and the lobe encloses four pollen sacs or microsporangia. Each pollen sac contains number of pollen grains. The four pollen sacs in a dithecous anther appear to lie in the four corners of anther
- The wall of anther consists of four layers of cells
- An anther dehisces by slits to liberate pollen grains
- The anther initiates its development in the form of a homogenous mass of a meristematic cells surrounded by epidermis. It becomes four lobed and four longitudinal rows of aechesporial cells are differentiated. Each of these cells divides to form a primary parietal cell and a primary sporogenous cell. The parietal cell divides several times to form the anther wall and the sporogenous cell divides a few times to form the microscopes or pollen mother cells (PMC). The innermost layer of cell wall in contact with the PMC’s form the tapetum which plays a significant role in pollen development. The layer below the epidermis later becomes the endothecium.
Wall layers of anther
- Epidermis – one cell thick and protective in function
- Endothecium – Second wall layer usually single layered. Cells have a cellulose thickening with a little pectin and lignin. It help in anther dehiscence
- Middle layers – The number of middle layer ranges from 1-6. The middle layer degenerate at the maturity of the anther
- Tapetum – This is the innermost layer of anther wall which surrounds the sporogenous tissue. Tapetal cells are nutritive. They are multinucleated and polyploidy. In these cells the ubisch bodies which is deposited in the exine of microscope wall. The tapetum is of two types:- (i) Secretary / glandular – The tapetal cells remain in situ all through the development of microscope and finally they degenerate. (ii) Amoeboid / periplasmodial – The radial wall of tapetum cell break up releasing the protoplast into the pollen chamber. All such protoplast now fuse to form the periplasmodium.
- The formation and differentiation of microspore is called microsporogenesis. The PMCs divide meiotically each forming generally tetrahedral tetrads, Cytokinesis may be successive or simultaneous.
- Successive type is advanced type. Tetrad are of five types, tetrahedral, isobilateral, decussate, T shaped, linear tetrahedral is most common
- In successive type, the cell wall is formed after meiosis –I as well as meiosis –II thus an isobilateral pollen tetras is formed. It is a characteristic feature of monocot
- In simultaneous type, each nuclear division in microspore mother cell is most followed by cell wall formation
- The microspores separate from the tetrahedral configuration and get surrounded by a two layered wall, outer exine and inner intine. The pollen grains are the first cells of the male gametophyte.
- The tapetum get used up, the anther becomes dry structure and pollen are liberated by dehiscence of the anther.
- Mostly, all the four nuclei in a tetrad remain functional to form four microspores. However, in cyperaceae only one functions and therefore only one microspore instead of four is formed by one meiosis. In some cases, all the four pollens remain attached forming compound pollen grains e.g. Juncus jatropha. In family asclepiadaceae and orchidacae, all the microspores in a sporangium adhere together in a single mass called pollinium.
- Pollen grains may be oval, ellipsoidal, triangular, lobed or even crescent shaped. It is generally round with size of 25 – 30μm
- Pollen grain is haploid, unicellular body with single nucleus. Therefore is an outer wall and 2-3 celled interior.
- Wall or sporoderm is made of two covering, outer thick exine of sporopollenin and inner thin intine of pecto-cellulose
- The outer layer exine is thick and sculptured or smooth. It is cuticularised and cutin is of special type called sporopollenin which is resistant to chemical and biological decomposition so pollen wall is preserved for long periods. It also possess proteins for enzymatic and compatibility reactions.
- Exine is differentiated into inner endexine and outer ektexine. Ektexine is further divided into inner continuous foot layer, middle discontinuous baculate layer and outermost discontinuous tectum.
- Tectum is helpful for identifying pollen grain and referring them to their family, genus or species.
- Exine is absent over certain areas called germ pores when circular or it is called germ furrow when elongated
- In insected pollinated pollen grain, exine is covered with yellowish, viscous and sticky substance called pollenkitt. Pollenkitt act as an insect attractant and protects the pollen from UV rays
- Intine is thin and elastic. It is made up of cellulose and pectin. It emerges out as the pollen tube from the germ pores during germination
- Internally pollen grains have cytoplasm which is rich in starch and unsaturated oils. Uninucleated protopeast becomes 2-3 celled at the later stages of development.
- In calotropis and orchids, the pollen of each anther lobe formed a characteristics mass called pollinium
- Pollen grains can be monoclopate ( having one germ pores),bicolpate(two germ pores) and triclopate (3 germ pores).
- The branch of study of pollens is called palynology
Development of male gametophyte
- Size of nucleus in pollen grain increases and it divides mitotically to produce a bigger vegetative cell or tube cell and smaller generative cell
- Pollination can occur in two celled ( tube + generative ) or three-celled ( tube + two male gametes)
- However, in plants such as cereals, the male gametes while the pollen is still within the anther. In those cases, where pollen is shed at two celled stage, the generative cell divides after pollen has landed on stigma.
- The cytoplasm contents of generative cell do not possess much of stored food material. Vegetative cell contains fat, starch and protein granules
- Pollen food supplements : Pollen grain contains abundant carbohydrates and unsaturated fat. They are used in form of tablets and syrups for enhancing vital body functions. Pollen consumption increases performance and used by athletes and given to race horses.
- Pollen creams : Pollen grain protect themselves from UV rays. Thus they are used in creams, emulsions for providing smoothness and protection to skin.
The period for which pollen grains remain viable or functional is called pollen viability. It depends upon temperature, humidity. Pollen grains remain viable in 30 minutes. Pollen grain can be cryopreserved in liquid nitrogen ( temp – 196OC) and used as pollen banks.
Pollen grain produce severe allergy. It causes have fever and common respiratory disorders are asthma, bronchitis. Carrot grass ( Parthenium hysterophorus) that came in India along with imported wheat is major source of pollen allergy besides harming internal body organs.
FEMALE REPRODUCTIVE UNIT ( Pistil )
- The pistil or gynoceium of a flower is the female reproductive unit.
- A carpel or pistil has a stigma or receptive region for pollen grains, a stalk or style and basal swollen region or ovary. Ovary contains one to several ovules.
- Ovule is integumented megasporangium which on fertilization ripens into a seed. It is oval and whitish.
- The ovule is attached to placenta by means of a stalk called funiculus or funicle. The point of attachment of funicle to the ovule is known as hilum. A raphe ( ridge) is formed by the fusion of funiculus with the body of ovule.
- The actual megasporangium equivalent is a parenchymatous tissue called nucellus. It may be thin ( tenuinucellate, e.g. compositae ) or massive ( crassinucellate e.g casuarinaceae)
- On the basis of number of integuments, ovules are of following types:- (i) Unitegmic – With one integuments, higher dicots e.g. composital, gymnosperms (ii) Bitegmic – Ovules with two integuments ( monocots and primitive dicots like cruciferae and malvaceae) (iii) Tritegmic – With three integuments ( Asphodelus) (iv) Ategmic – Without integument ( Santalum, Loranthus, Ziriosoma and olax)
- Place of origin of integuments is called chalaza, A pore is present in the integuments at one end. It is known as micropyle. The inner region of integument may provide nourishment to developing embryo sac and it is called endothelium. Outer side of each integument as well as nucellus possesses cuticle.
- In castor bean ( Ricinus ) proliferation of the integumentary cells at micropylar region is called caruncle. It performs two functions:- 1.) It acts as water absorbing pas and helps in seed germination 2.) It is made up of sugary substance and thus seed dispersal occurs by ants.
Forms of ovule
- Orthotropous ( Erect ): – The body of the ovule lies straight and upright over the funicle. Hilum, chalaza and micropyle occurs on the same line. E.g. Polygonum
- Anatropous ( Inverted ):- The body of ovule is inverted and gets fused with funicle forms ridge called raphe. Hilum and micropyle are nearby with chalaza on opposite sides. It is the most common type of ovule. E.g. Ranunculus
- Hemianatroppous:- The body of ovule is placed at right angle (90o) to the funicle e.g. Malpighiaceae.
- Campylotropous:- The body is curved but embryo sac is straight. Hilum, chalaza and micropyle come nearby e.g. Caspells, Capparis, Chenopodiaceac
- Amphitropous:- Both body of ovule and embryo sac are curved e.g. crucifers
- Circinotropous:- The ovule turns at morethan 360O angle so funicle becomes coiled around the ovule. Example opuntia.
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