Sexual Reproduction in Plants

This post is written specially keeping it consistent to the  C.B.S.E curriculum for class XII. Nevertheless  students from other boards can benefit from it too :)
Sexual reproduction in flowering plants (angiosperms) are carried out with the help of sexual organelles of the plant, i.e Flowers.Angiosperms: Angiosperms (Gr. Angios: Covered, Spermae: seed) are plants that have their seeds enclosed in a ovule inside the ovary of their flowers.

• Angiosperm flowers have male parts (androecium) and female part (gynaecium).
• Flowers are modified shoots of the plant which are formed after several hormonal and structural changes leading to differentiation and development of the floral premordium, where inflorescence develop. Inflorescence bears the floral buds which further bears the floral bud that develops and differentiates into flower.

The Outer Parts of an Angiosperm Flower

There is a huge diversity among flowers of the angiosperms but all flowers have these structures:

• The peduncle: It is the stalk or stem that attaches the flower to the plant.
• The receptacle: It is the top part of the stalk where all the flower parts attach to on the peduncle.
• The sepal: The small green leaves that protect a new flower bud.
• The petals: These are the colourful parts that attract pollinators such and bees, birds and butterflies.
  
  

If both male and female flowers are borne on the same plant (e.g., corn or peas), the species is called monoecious (meaning "one home"). Species with male and female flowers borne on separate plants (e.g., C. papaya or Cannabis)are termed dioecious, or "two homes."

The ovary, which may contain one or multiple ovules, may be placed above other flower parts (referred to as superior); or it may be placed below the other flower parts (referred to as inferior).

Structure of Stamen, Anther, Pollen Sac/Microsporangium and Pollen Grain in Plants!
(a) The Stamen:
Stamen in a flower consists of two parts, the long narrow stalk like filament and upper broader knob-like bi-lobed anther (Fig. 2 A). The proximal end of the filament is attached to the thalamus or petal of the flower. The number and length of stamens vary in different species.
b) Structure of anther:
A typical angiosperm anther is bilobed with each lobe having two theca, i.e they are bithecous or dithecous anther is made up of two anther lobes, which are connected by a strip of sterile part called connective. The anther is a four-sided (tetragonal) structure consisting of four elongated cavities or pollen sacs (microsporangia) the four microsporangia are located at the corners, two in each lobe. The microsporangia develop further and become pollen sacs in which pollen grains are produced.(c) Structure of microsporangium
In a transverse section, a typical microsporangium appears circular in outline, consisting of two parts, microsporangial wall and sporogenous tissue.
i) Microsporangial Wall: Includes the epidermis, endothecium, middle layers and the tapetum. The outer three wall layers perform the function of protection and help in dehiscence of anther to release the pollen. The innermost wall layer is the tapetum, its cells have dense cytoplasm, become large, multinucleate and are specialized in nourishing the developing pollen grains.

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Functions of Tapetum

• It provides nourishment to growing sporogenous cells, microspore mother cells and young microspores.
• Tapetum provides enzyme callase for dissolution of callose binding the microspores.
• It secretes hormones e.g. IAA that are stored in pollen grains for their early growth.
• Provides compatibility and incompatibility proteins to pollen grains.

ii) Sporogenous tissue
It fills the interior of the microsporangium, all the cells are simmilar and called sporogenous cells. Sporogenous cells devide regularly to from the diploid microspore mother cells. The microspore mother cell devides to form pollen grains.
M icrosporogenesis : As the anther develops, the cells of the sporogenous tissue is capable of giving rise to a microspore tetrad. Each one is a potential pollen or microspore mother cell. The process of formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogenesis. The microspores, as they are formed, are arranged in a cluster of four cells–the microspore tetradmeiotic divisions to form microspore tetrads.

                                                           Types of microspore tetrads
As the anthers mature and dehydrate, the wall of the microspore mother cell degenerates and the microspores dissociate from each other and develop into pollen grains. Inside each microsporangium several thousands of microspores or pollen grains are formed that are released with the dehiscence of anther.

Pollen grain:
Pollen grains are male reproductive propagule or young male gametophyte which is formed in the anther and is meant for reaching the female reproductive organ through a pollinating agent. Pollen grains are generally spherical measuring about 25-50 micrometers in diameter. The pollen grains are coverd by a two-layered wall called sporoderm. The two layers of sporoderm are inner intine and outer exine.
1. Intine: It is the inner wall of the pollen grain and is a thin and continuous layer made up of cellulose and pectin. Some enzymatic proteins also occour in the intine.
2. Exine: The exine is the hard outer layer made up of sporopollenin which is one of the most resistant organic material known. It can withstand high temperatures and strong acids and alkali. No enzyme that degrades sporopollenin is so far known. Pollen grains where sporopollenin is absent can be easily identified by the presence of prominent apertures called germ pores. The exine surface may be smooth, pitted, reticulate, spiny, warty etc, the exine surface sculpting are specific for each type of pollen grain. Pollen grains are also well preserved as fossils because of the presence of sporopollenin, and thus are helpful in studying the evolutionary history of the plant.
The cytoplasm of a mature pollen grain is surrounded by a plasma membrane and contains two cells, the vegetative cell and generative cell. The vegetative cell is bigger, has abundant food reserve and a large irregularly shaped nucleus. The generative cell is small and floats in the cytoplasm of the vegetative cell. It is spindle shaped with dense cytoplasm and a nucleus. In over 60 per cent of angiosperms, pollen grains of a microsporeare shed at this 2-celled stage. In the remaining species, the pollen grain generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage).
Pollen grains of many species cause severe allergies and bronchial afflictions in some people often leading to chronic respiratory disorders – asthma, bronchitis, etc. However they are rich in neutrients and thus often consumed as food suppliment.

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The Pistil, Megasporangium (ovule) and Embryo sac
The female reproductive parts of the flower are knwon as carpels, and are collectively called as gynoecium. Gynoecium may consist of a single pistil (monocarpellary) or may have more than one pistil (multicarpellary). When there are more than one carpel the pistils may be fused together (syncarpous) or may be free (apocarpous).
Each pistil has three parts, the stigma, style and ovary. The stigma serves as a landing platform for pollen grains. The style is the elongated slender part beneath the stigma. The basal bulged part of the pistil is the ovary. Inside the ovary is the ovarian cavity (locule). The placenta is located inside the ovarian cavity.

Structure of a Megasporangium (Ovule)
The ovule is a small structure attached to the placenta by means of a stalk called funicle. The body of the ovule fuses with funicle in the region called hilum. Thus, hilum represents the junction between ovule and funicle. Each ovule has one or two protective envelopes called integuments. Integuments encircle the nucellus except at the tip where a small opening called the micropyle is organised. Opposite the micropylar end, is the chalaza, representing the basal part of the ovule. The main body of the ovule is composed of parenchymatous mass called nucellus. Cells of the nucellus has abundant reserve of food. Located in the nucellus is the embryo sac or female gametophyte. An ovule generally has a single embryo sac formed from a megaspore.

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Megasporogenesis
It is the process of formation of haploid megaspore from the diploid megaspore mother cell (MMC). Usually a single MMC differentiates in the micropylar region. It is a large cell containing dense cytoplasm and a prominent nucleus. The MMC undergoes meiotic division. which results in the production of four haploid megaspores, arranged generally in the form of a linear tetrad.

Female gametophyte or Embryo sac: Only one of the megaspores is functional while the other three degenerate. The functional megaspore develops into the female gametophyte (embryo sac).

• The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 2-nucleate embryo sac. Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac.
• Two groups of four nucleii are formed at the two ends, one nucleus from each group pass towards the centre and they are called polar nuclei.
• There is a characteristic distribution of the cells within the embryo sac. Three cells are grouped together at the micropylar end and constitute the egg apparatus. The egg apparatus, in turn, consists of two synergids and one egg cell.
• The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which play an important role in guiding the pollen tubes into the synergid. Three cells are at the chalazal end and are called the antipodals. The antipodal cells are vegetative cells and take part in absorbing nourishment from the nucellus.
• Laying down of cell wall results in the formation of seven cells and a large bi-nucleate central cell and three cells at the poles. The embryo sac is therefore called seven celled and eight nucleate.
  

Pollination
Pollination is the process of transferring pollen from the stamens to the stigmatic surface in angiosperms or the micropyle region of the ovule in gymnosperms. Depending on the source of pollen, pollination can be divided into three types.

1. Autogamy- In this type, pollination is achieved within the same flower. Transfer of pollen grains from the anther to the stigma of the same flower requires synchrony in pollen releaseand stigma receptivity and also, the anthers and the stigma should lie close to each other so that self-pollination can occur.
2. Geitonogamy- Transfer of pollen grains from the anther to the stigma of another flower of the same plant.
3. Xenogamy- Transfer of pollen grains from anther to the stigma of a different plant. This is the only type of pollination during which pollination brings genetically different types of pollen grains to the stigma.

Pollination mechanisms
Pollen transfer can be facilitated by the aid of abiotic (wind, water), abiotic (insects, birds, mammals).In some cases, pollen is transferred simply by gravity and the proximity of the anthers to the stigma.

1. Anemophily (Gk. Anemos-wind; philein: to love) wind pollination (anemophily) occurs most often in plants with separate male and female flowers, often on separate plants. It occurs in both gymnosperms and angiosperms, but is the primary mechanism for pollination in gymnosperms.
   1. Gynmosperms evolved prior to the rise in insects and therefore most modern day gymnosperms rely on wind pollination to move male gametes to the female. In gymnosperms, the male strobili shed the wind borne pollen.
   2. In conifers, the pollen is produced in abundance and winged to help long range pollen flight.
   3. Many of the angiosperms like oaks, chestnuts and hazelnuts, are wind-pollinated.
      
      

      2. Hydrophily Water pollination (hydrophily) occurs in aquatic plants and requires the pollen to move in the water to the female flower. It occurs in only in only about 4% of plants. It is believed, particularly for some bryophytes and pteridophytes, that their distribution is limited because of the need for water for the transport of male gametes and fertilisation. Some examples of water pollinated plants are Vallisneria and Hydrilla which grow in fresh water and several marine sea-grasses such as Zostera.
Both wind and water pollinated flowers are not very colourful and do not produce nectar.
   3. Zoophily It is a mode of pollination in which the biotic agents bring about pollination in flowering plants. Zoophily has several subtypes eg. Entomophily (by insects) malacophily (by snails ) chiropterophily (by Bats), ornithophilly (by birds eg. Humming bird), myrmecophily (by ants), anthrophily (by Humans).

Flower traits associated with different pollination agents

Advantages and Disadvantages of Cross Pollination
Advantages
1. A number of plants are self-sterile, that is, the pollen grains cannot complete growth on the stigma of the same flower due to mutual inhibition or incompatibility, e.g., many crucifers, solanaceous plants. Several plants are pre-potent, that is, pollen grains of another flower germinate more readily and rapidly over the stigma than the pollen grains of the same flower, e.g., Grape, Apple. Such plants of economic interest give higher yield only if their biotic pollinators like bees are available along-with plants of different varieties or descent
2. Cross pollination introduces genetic re-combinations and hence variations in the progeny.
3. Cross pollination increases the adaptability of the offspring towards changes in the environment.
4. It makes the organisms better fitted in the struggle for existence.
5. The plants produced through cross pollination are more resistant to diseases.
6. The seeds produced are usually larger and the offspring have characters better than the parents due to the phenomenon of hybrid vigour.
7. New and more useful varieties can be produced through cross pollination.
8. The defective characters of the race are eliminated and replaced by better characters.
9. Yield never falls below an average minimum.
Disadvantages
1. It is highly wasteful because plants have to produce a larger number of pollen grains and other accessory structures in order to suit the various pollinating agencies.
2. A factor of chance is always involved in cross .pollination.
3. It is less economical.
4. Some undesirable characters may creep in the race.
5. The very good characters of the race are likely to be spoiled.
Outbreeding Devices
As continued self-pollination result in inbreeding depression, flowering plants have developed many devices to discourage self- pollination and to encourage cross-pollination.

• In some species, pollen release and stigma receptivity are not synchronised. Either the pollen is released before the stigma becomes receptive or stigma becomes receptive much before the release of pollen.
  
• In some other species, the anther and stigma are placed at different positions to prevent autogamy.
  
• Self-incompatibility-is a genetic mechanism and prevents self-pollen (from the same flower or other flowers of the same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.
  
• Another device to prevent self-pollination is the production of unisexual flowers.
  

Pollen-pistil Interaction: Pollination does not guarantee the transfer of the right type of pollen (compatible pollen of the same species as the stigma). The pistil has the ability to recognise the pollen, whether it is of the right type (compatible) or of the wrong type (incompatible). If it is of the right type, the pistil accepts the pollen and promotes post-pollination events i.e fertilisation. However if the pollen is of the wrong type, the pistil rejects the pollen by preventing pollen germination and formation of the pollen tube. Pollen-pistil interaction and acceptance of the compatible pollen type or rejection of the incompatible type results from a continuous dialogue between pollen grain and the pistil. This dialogue is mediated by chemical components of the pollen interacting with those of the pistil.
Artificial hybridisation is one of the major approaches of crop improvement programme. Here only the desired pollen grains are used for pollination and the stigma is protected from contamination (unwanted pollen). This is achieved by emasculation and bagging techniques. Anthers from the flower bud before the anther dehisces using a pair of forceps is necessary. This step is referred to as emasculation.

• Emasculated flowers have to be covered with a bag of suitable size, generally made up of butter paper, to prevent contamination of its stigma with unwanted pollen. This process is called bagging.
  
• When the stigma of bagged flower attains receptivity, mature pollen grains collected from anthers of the desired male parent are dusted on the stigma, and the flowers are rebagged, and the fruits allowed to develop.
  
  

Ref: NCERT Biology for Class 12
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