NEET Biology Notes Reproduction in Plants Sexual Reproduction in Flowering Plants
Sexual Reproduction in Flowering Plants
It involved fusion of male and female gamete formed as a result of gametogenesis,. The two common process of gamete formation in plants are microsporogenesis and megaspdfogenesis found in male reproductive (androecium) and – female reproductive (gynoecium) part of flower respectively. The androecium is composed of stamens, while gynoecium of carpels.
The whole process of sexual reproduction in flowering plants can be divided into the following four steps
- Pollen grain formation
- Embryo sac formation
Pollen Grain Formation
The formation of ‘ microspores or pollens is called microsporogenesis. Pollen grain or microspore formation takes place in fertile portion of anther called pollen sacs.
Anther is a bilobed, pollen grain containing structure. Each anther lobe is having two chambers or pollen sacs called microsporangia, thus each anther consists of four microsporangia.
- A typical angiospermic anther is dithecous (i.e. having two anther lobes and four microsporangia).
- Anther develops from a group of cells (eusporangiate development). A young anther is made up of homogenous mass of meristematic cells surrounded by an epidermis.
- Tapetum consists of cells with large nuclei and dense cytoplasm. The main functions of the tapetum are the production and transport of enzymes, hormones and food materials.
- Callase enzyme is secreted by tapetum, which dissolves callose.
- Ubisch bodies are secreted by tapetum. These are lipid in nature and get covered with sporopollenin, which increases the thickness of exine (i.e. outer layer of pollen wall).
- Sporopollenin present in the exine of pollen grains is resistant to microbial and chemical decomposition.
- Pollen grain exine has prominent apertures called germ
pores, where sporopollenin is absent. ‘
- Pollen kit is an oily layer present on the outside of the mature pollen grains of many insect pollinated species; and consists of lipids and carotenoids.
- In microsporogenesis, the primary sporogenous cells undergo mitosis and form diploid; miprospore mother cells or pollen mother cells or microsporocytes. Each microspore mother cell (diploid) divides by meiosis to form a tetrad of haploid pollen grains.
- In polyspory, more than four pollen grains are produced from one microspore mother cell. Pollen grain is spherical Pollen kit prevents the pollen grains by UV-radiation and 25-50 micrometer in diameter.
Development of Male Gametophyte
- It begins inside the microsporangium. First division of cell is mitotic producing two unequal cells, i.e. a larger vegetative cell and a small generative cell.
- The generative cell produce two male gametes and the vegetative cell forms pollen tube after pollination. Pollen grain at the time of pollination may be 2-celled or 3-celled.
- In over 60% of angiosperms, pollen grains are shed at 2-celled stage (i.e. vegetative cell and generative cell). In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains shed.
- Thus, in these species pollen grains shed at 3-celled stage. Pollen grains of many species (e.g. Parthenium or carrot grass) cause severe allergies and bronchial afflictions in some people often leading to chronic respiratory disorders such as asthma, bronchitis, etc.
- In rice and wheat pollen grains lose viability with in 30 minutes of their release. In some members of Rosaceae, Leguminoseae and Solanaceae, pollen grains maintains viability for months. It is also possible to store pollen grains of a large number of species for years in liquid nitrogen (- 196°C).
Development of Female Gametophyte
- Gynoecium is the female reproductive part of the flower. It may be monocarpellary (single pistil) or polycarpellary (more than one pistil). The pistils may be fused (syncarpous) or may be free (apocarpous).
- Each pistil has three parts, i.e. the stigma, style and ovary. The placenta is located inside the ovary. From the placent arises the megasporangia called ovules.
- Megasporangium (ovule) is the integumented indehiscent, which develops as a small outgrowth from the tissue of placenta. It develops into seed after fertilisation.
- Orthotropous is most primitive and simplest type and anatropous is most common type ovule found in angiosperm.
- The ovule is a rounded structure attached to the placenta by a stalk called funiculus.
- The attachment place of funiculus to the body of ovule is known as hilum.
- Ovules are enclosed in ovary and consist of a central mass of parenchymatous cells known as nucellus. Nucellus is bitegmic in monocots and primitive dicots (those with crassinucellate ovules) and is unitegmic or monotegmic as in higher dicots.
- From the base of the ovule, a third integument develops in the form of aril in many plants namely litchi, Trianthema, Asphodelus, etc.
- A hypodermal cell from the micropylar end of ovule differentiates into a sporogenous cell, which eventually becomes megaspore mother cell (2n).
- The megaspore mother cell divides by meiosis to form four haploid megaspores. One megaspore on chalazal end remains intact, while other three degenerate.
The functional megaspore at the chalazal end grows and many vacuoles appear in the cytoplasm.
Embryo Sac Formation
Out of the three nuclei, located at the micropylar end, one serves as egg and the other two as synergids together constituting the egg apparatus. The mature structure with two polar nuclei, antipodals and egg apparatus is called embryo sac. In majority of flowering plants, one of the magaspores is functional, while the other three degenerate. Only the functional megaspore develops into the female gametophyte (embryo sac). This is called monosporic development.
- The nucleus of functional megaspore divides mitotically to form two nuclei, which move to the opposite poles, forming the 2-nucleate embryo sac.
The two more sequential mitotic nuclear divisions result in the formation of 4-nucleate and later 8-nucleate stages.
- After 8-nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte.
- The typical embryo sac (monosporic, 8-nucleate or Polygonum type) is having 8 nuclei but 7 cells (3 micropylar cells, 3 chalazal cells and two polar nuclei forming a single central cell with diploid secondary nucleus in later stages).
- The synergids are also known as helpers. They help in distribution of nutrients in the embryo sac with the help of filiform apparatus. They also help in attracting pollen tube towards egg.
- Antipodals also help in nutrition of embryo sac. A tissue called obturator also helps in growth of pollen tube towards egg. Ovules are located in locules of ovary.
The transfer of pollen grains from anther of a flower to the stigma of same or other flower is known as pollination. It is mainly of two kinds:
Self-pollination or autogamy or xenogamy is the transfer of pollen grains from the anther of a flower to the stigma of the same flower or from one flower to another on the same plant. Self-pollination produces homozygous characters in the progeny and good characters can be as such retained. Self-pollination is disadvantageous in the sense that no variations are produced and the progeny becomes genetically weak.
Adaptations for Self-Pollination
Homogamy In this condition, the anthers and stigma mature at the same time and the stigma is receptive at the time, when the pollens shed, e.g. Mimbilis and Vinca rosea.
Cleistogamy Sometimes, the flowers do not open and remain closed throughout their life in bud form. The pollination occurs in the bud itself. Most of the plants with cleistogamous flowers, also bear chasmogamous flowers,
i. e. the flowers, which open normally. Thus, cleistogamy is a facultative character. Example of exclusively, cleistogamous flowers are very few and include Sibularia aquatica and Polycarpon tetraphyllum.
Cross-pollination or allogamy is the transfer of pollen from anther of a flower to the stigma of the another flower, borne on separate plants of the same species. Cross-pollination results in combination or mixing up of characters thus, improving the quality or vigour of the species.
Agencies of Cross-Pollination
Various agencies helpful in cross-pollination are broadly categorised into two typfts, i.e. biotic agencies and abiotic agencies.
- Pollination with the help of air or wind is called as anemophily and pollination taking place with the help of water is called as hydrophily.
- When biotic agents are involved in cross-pollination, the process is collectively, called as zoophily.
- Most common type of zoophily is entomophily, i.e. pollination with the help of various insects. Pollination through birds, is called as omithophily, cheiropterophily is the pollination acquired through bats and malacophily is pollination by snails.
- Myrmecophily is the name of beneficial association between ants and flowers, in which flowers get pollinated in return of giving juicy secretions to the ants.
- Geitonogamy is a type of pollination in which pollen grains of one flower is transferred to stigma of another flower belonging to same plant or genetically similar plant.
Adaptations for Cross-Pollination
Most of the flowers in nature are cross-pollinated and are thus variously adapted for it. For avoiding self-pollination and for exclusively carrying out cross-pollination, the adaptations are as follows:
Self-Sterility or Self-Incompatibility
In this adaptation, either the pollens of a flower are unable to grow on the stigma of same flower or if they do so, they grow very slowly, e.g. Malva and Abutilon.
When maturing times of stigma and anthers is such that either stigma becomes receptive before anthers mature (protogyny) or the anthers are ready for dehiscence before stigma becomes receptive (protandry),
e.g. in Aristolochia and Scrophularia protogyny occurs and in rose, sunflower, Impatiens protandry is observed.
In some members of Oxalidaceae, Rubiaceae, Polygonaceae, etc., the flowers are dimorphic, i.e. of two different forms. One form has long stamens and styles are very small. The anthers are well above stigma thus minimising the chances of self-pollination. The other form has long styles and the stamens are having small filaments. Thus, anthers lie below the receptive part of the stigma, e.g. primrose. This condition is termed as dimorphic heterostyly.
The cross-pollination takes place between flowers with same length of styles. Physiology of flower also plays a role and it has been seen that cross-pollinating flowers axe physiologically resembling to each other.
- Unisexuality (Dicliny)
The presence of only one kind of reproductive whorl in a flower is known as unisexuality. A plant may be monoecious, i.e. carrying two different flowers as male and female flowers on the same plant. In this case, both cross or self-pollination can occur.
However in dioecious plants, i.e. plants in which male and female flowers are borne on different plants, cross-pollination is the only way of pollination.
In some flowers, a mechanical barrier exists between compatible pollen and stigma so that self-pollination is not possible. Sometimes a hood-like covering covers the stigma as in Iris and Calotropis, the pollens are grouped in pollinia and stick to surface till they are carried away by insects.
- Pollen-Pistil Interaction
The pistil has the ability to recognise the suitable pollen type. If it is of the right type, the pistil accepts the pollen and promotes post-pollination events that leads to fertilisation. The acceptance or rejection1 of pollen by the pistil is mediated by chemical components of the pollen interacting with those of the pistil.
Following compatible-pollination, the pollen grain germinates on the stigma to produce a pollen tube, through one of the germ pores. The content of polllen grain move into the pollen tube. Pollen tube, grows through the tissues of the stigma and style and reaches the ovary. All these events from pollen deposition on the stigma until pollen tubes enter the ovule are referred to as pollen-pistil interaction. In artificial hybridisation, the desired pollen grains are used for pollination and the stigma is protected from contamination (from unwanted pollen). This is done by emasculation and bagging.
Fusion of male and female gametes is known as fertilisation. In angiosperms, the pollen tube has two male gametes. The phenomenon of fertilisation was first reported by Strasburger (1884) in Monotrapa. The pollen tube moves by its tip towards micropylar end of ovule, where egg is situated. Male gametes are discharged in one of the synergids, which later degenerates. Fertiliruion occurs after this and viable seeds are formed after double fertilisation.
It was discovered by Nawaschin in 1898 in Lilium and Fritillaria.
One male , gamete fuses with egg (syngamy) forming a diploid zygote (2n). The second male gamete fuses with two polar nuclei (or secondary nucleus) forming a triploid primary endosperm nucleus (3n). The process of fusion of two male gametes in a single embryo sac is called as double fertilisation and the formation of triploid nucleus by fusion of one male gamete with secondary nucleus is called triple fusion. Total five nuclei takes part in double fertilisation.
The pollen tube enters the ovule in majority of cases through micropyle by a process termed porogamy or it may enter through chalaza (chalazogamy) as in Casuarina and Juglans. In ‘rare cases, the pollen tube may pierce through integuments (mesogamy) as in Pistacia and Cucurbita. Chalazogamy was first reported in Casuarina.
The filliform apparatus of synergids secrete some chemical substances which direct the pollen tube towards micropyle of ovule.