Pollination

Pollination is defined as the process of transfer of pollen grains from the anther of a flower of a plant to the stigma of the same flower or of different flower of same plant or genetically similar or different plants. 

Types of Pollination

It’s of two types

1.     Self-Pollination

2.     Cross Pollination

Self-Pollination

Self-pollination is the transference of pollen grains from the anther of a flower to the stigma of either the same or genetically similar flower. It is basically independent to the external agency. Self-pollination can be observed in legumes such as orchids, sunflowers, peas, peanuts, oats, peaches, potatoes, wheat, and others. It’s of two types:

1. Autogamy

It is a type of self-pollination where the transfer of pollen grains from the anther to the stigma takes place within the same flower. Opening and exposure of anther and stigma are necessary for autogamy. There are two conditions for autogamy to takes place:

• Anther-stigma synchronization; when the pollen is released, stigma should be ready to receive it.


• The position of or distance between anther and stigma. Both should be close enough for pollinations.

In chasmogamous flowers, anther and stigma are exposed. The exposed reproductive parts give a chance of cross-pollination in chasmogamous flowers. While in cleistogamous flowers anther and stigma are not exposed but lie close enough for transfer. Thus, the chances of cross-pollination in cleistogamous flowers are almost none. In addition, they barely require a pollinating agent.

2. Geitonogamy

Geitonogamy is the type of self-pollinations where the transfer of pollen grains from the anther to the stigma takes place between different flowers in the same plant. Though it seems like cross-pollination and takes place with the help of pollinator, both the gametes have the same plant as their origin.

Self-pollination may occurs when the flowers are open (chasmogamy) or closed (cleistogamy). Chasmogamous self-pollination occurs only when their anthers and stigma mature at the same time (homogamy).  The self-pollinated flowers are generally small, inconspicuous, colorless, odorless and nectar less. Self-pollination occurs by following methods:

1.    Homogamy:  Anthers and stigma of the bisexual flowers of some plants mature at the same time. They are brought close to each other by growth, bending or folding to ensure self-pollination. This condition is called homogamy. e.g., Mirabilis (Four O, clock), Catharanthus (= Vinca), Potato, Sunflower, Wheat, Rice, etc.

Also, it is distinguished as direct (accomplished by the movement of floral parts) and indirect (accomplished by outer factors instead of close contact of plant parts).

2. Cleistogamy: Some plants never open to ensure complete self-pollination. This condition is called cleistogamy, e.g., Commelina bengalensis, Oxalis, Viola, etc. The cleistogamous flowers are bisexual small, inconspicuous, colorless and do not secrete nectar.

Some of the advantages and disadvantages of self-pollination are given below:

Cross pollination

Cross pollination is the transfer of pollen grains from anther to stigma of a genetically different flower. It is also known as allogamy or xenogamy . Cross pollination is accomplished with the help of external agencies like:

1.    Anemophily ( through wind)

2.    Hydrophily ( through water )

3.    Zoophily ( through animal )

1.  Anemophily

Anemophily or wind pollination is pollination carried out with the help of the wind. This pollination is called anemophilous pollination. The flowers which carry out such type of pollination are wind pollinating flowers. They are small, lightly coloured and do not produce scent or nectar.

CHARACTERISTICS OF ANEMOPHILOUS FLOWERS

·         Flowers are small and inconspicuous.

·         Non-essential parts are either absent or reduced.

·         The flowers are colorless, odorless and nectar less.

·         In case of unisexual flowers, the male flowers are more abundant. In bisexual flowers, the stamens are generally numerous. Pistils are generally uniovuled.

·         Flowers are produced above the foliage, before the appearance of new foliage or placed in hanging position.

2.  Hydrophily

The pollination is carried out with the help of water and is known as hydrophilous type of cross pollination.

It is generally seen in aquatic plants where pollens are produced in large numbers and with specific weight which makes them float below the surface.

In Vallisneria, the male flower floats on the surface of the water till it comes in contact with the female flowers.

Hydrophily is of two types :

• 
  Hypohydrophily: Plants which are pollinated inside the water e.g., Zostera, Ceratophyllum, Najas, etc.
  


• 
  Epihydrophily: Plants which are pollinated outside the water. e.g., Vallisneria (Ribbon weed).
  

CHARACTERISTICS OF HYDROPHILOUS FLOWERS

·         Flowers are small and inconspicuous.

·         Perianth and other floral parts are unwettable.

·         Nectar and odor are absent.

·         Pollen grains are light and unwettable due to presence of mucilage cover.

·         Stigma is long, sticky but unwettable.

3.  Zoophily

This type of pollination takes place by different types of animals. It is of different kinds:

(a) Entomophily: When pollination is brought about by the agency of insects, it is known as entomophily or insect pollination. About 80% pollination occurs by insects like moths, beetles, butterflies, wasp, etc. All the flowers pollinated by insects are brightly coloured, have a sweet smell and produce nectar. Entomophilous flowers produce a small amount of pollen which has a spinous and sticky exine due to presence of pollen kitt. The stigmas of such flowers are long rough and sticky. Salvia is excellent example of insect pollination is which pollination occurs by lever or turn pipe mechanism. Other examples of insect plants are Yucca (by Tageticula moth), Orchid Ophrys speculum (by Colpa aurea a hairy wasp), Ficus (by Blastophega), etc. Yucca is pollinated by Pronuba (= Tegaticula) yuccasella which passes its larval stage inside the ripening ovary. The flower of orchid ophrys resemble in shape colour and odour to female wasp of colpa aurea (mimicry). The male wasps pollinate the flowers mistaking them as female (pseudocopulation).

(b) Ornithophily : When flowers are pollinated by birds, the phenomenon is known as ornithophily. The most common bird pollinators are Sun bird, Humming bird, Crow, Bulbul, Parrot, Mynah, etc. The birds visit a large variety of flowers such as Bombax (red silk cotton), Erythrina (Coral tree), Callistemon (Bottle brush), Bignonia, Agave, etc. Flowers are brightly coloured and produce plenty of nectar and large quantities of pollen. Humming bird pollinates while hovering over the flowers and sucking nectar. The bird can derive about half of its body weight of nectar in a single day. The nectar is chiefly made of sugars and provides a sweet drink to the bird.

(c) Chiropterophily : It is a mode of pollination performed by bats. The flowers they visit are large, dull-coloured and have a strong scent. Chiropterophilous flowers produce abundant pollen grains. These flowers secrete more nectar than ornithophilous flowers and open at night emit a good fragrance. e.g., Kigelia pinnata (Sausage tree), Adansonia (Baobab tree), Bauhinia megalandra, Anthocephalus (Kadam tree), etc.

(d) Malacophily : Pollination by slugs and snails is called malacophily. Land plants like Chrysanthemum and water plant like lemna shows malacophily. Arisaema (aroid; snake plant) is often visited by snails.

(e) Myrmecophily : Pollination by ants. e.g., Anemone nemarosa (fruit).

Devices to ensure cross pollination / Contrivances

Nature favors cross pollination. All unisexual flowers and a large number of bisexual flowers are naturally cross pollinated.

The main contrivances ensuring cross pollination are as follows:

(i) Diclincy or Unisexuality : In unisexual flowers stamens and carpels are found in different flowers. Unisexuality can be of two types :

• 
  Monoecious plant : When male and female flowers are borne on the same plant. e.g., Maize, Cucurbits, Castor.
  


• 
  Dioecious plant : When male and female flowers are borne on different plants. e.g., Carica papaya, Cannabis.
  

(ii) Dichogamy : In bisexual flowers, when two sexes mature at different intervals and thus avoid self pollination is known as dichogamy. When stamens mature earlier than the stigma, it is known as protandry and the flowers are called protandrous e.g., Coriander, Jasmine, Sunflower, Lady’s finger, etc. When stigma matures earlier than the stamens, it is known as protogyny and the flowers are called protogynous. e.g., Rose, Tobacco, Crucifers, etc.

(iii) Heterostyly : The plants of some species in which flowers are dimorphic. Thus facilitate cross pollination. Some of them possess a long style but short stamens and are known as pin-eyed while others have short style and long stamens. These are known as thrum-eyed. e.g., Oxalis.

(iv) Herkogamy : In some bisexual flowers where the stigma and anthers mature at the same time, self pollination is avoided by some sort of barrier. The flowers show following contrivances :

• The male and female sex organs lie at some distance from each other.


• In some flowers corolla has peculiar forms which act as barrier in self pollination. e.g., Aristolochia.


• In some other flowers, the pollens are held together to form pollinia which can only be carried away by insects. e.g., Orchids and Calotropis.

(v) Self sterility or Incompatibility : When pollen grain of an anther do not germinate on the stigma of the same flower, then such flower is called self sterile or incompatible and this condition of flower is called self sterility, intraspecific incompatibility or self incompatibility. In these flowers cross pollination is the only means for fertilization and production of seeds.

Some of the advantages and disadvantages of cross pollination are given below :

Fertilization

The fusion of two dissimilar sexual reproductive units (gametes) is called fertilization. This process was discovered by Strasburger (1884).

(1) Germination of pollen grain on stigma and growth of pollen tube : Pollen grains reach the receptive stigma of the carpel by the act of pollination. Pollen grains, after getting attached to the stigma, absorb water and swell. Subsequent to mutual recognition and acceptance of pollen grains, the pollen grain germinates (in vivo) to produce a pollen tube which grows into stigma towards the ovarian cavity.

G.B. Amici (1824) discovered the pollen tube in Portulaca oleracea. Generally, only one pollen tube is produced by a pollen grain (monosiphonous). But some plants like members of Cucurbitaceae produce many pollen tubes (polysiphonous). The pollen tube contains a vegetative nucleus or tube nucleus and two male gametes. Later, the vegetative cell degenerates. The pollen tube now reaches the ovule after passing through the style.

(2) Entry of pollen tube into ovule : After reaching ovary, the pollen tube enters the ovule. Pollen tube may enter the ovule by any one of the following routes :

(i) Porogamy : When the pollen tube enters the ovule through micropyle, it is called porogamy. It is the most common type. e.g., Lily.

(ii) Chalazogamy : The entry of pollen tube into the ovule from chalazal region is known as chalazogamy. Chalazogamy is less common. e.g., Casuarina, Juglans, Betula, etc. It was first observed by Treub (1981) in Casuarina.

(iii) Mesogamy : The pollen tube enters the ovule through its middle part i.e., through integument (e.g., Cucurbita, Populus) or through funicle (e.g., Pistacia).

(3) Entry of pollen tube into embryo sac: The pollen tube enters the embryo sac only from the micropylar end irrespective of its mode of entry into the ovule. The pollen tube either passes between a synergid and the egg cell or enters into one of the synergids through filiform apparatus. The synergids direct the growth of pollen tube by secreting some chemical substances (chemotropic secretion). The tip of pollen tube enters into one synergid. The penetrated synergid starts degenerating. After penetration, the tip of pollen tube enlarge and ruptures releasing most of its contents including the two male gametes and the vegetative nucleus into the synergid.

(4) Double fertilization : The nuclei of both the male gametes are released in the embryo sac. One male gamete fuses with the egg to form the diploid zygote. The process is called syngamy or generative fertilization. This syngamy was discovered by Strasburger (1884).

The diploid zygote finally develops into embryo. The other male gamete fuses with the two polar nuclei (or secondary nucleus) to form the triploid primary endosperm nucleus. The process is called triple fusion or vegetative fertilization. These two acts of fertilizations constitute the process of double fertilization. The process was discovered by S.G. Nawaschin (1898) and Guignard in Lilium and Frittillaria. Double fertilization occurs in angiosperms only.

The zygote divides transversely to form a two-celled proembryo. The cell towards the micropyle is known as the basal cell and the other is known as terminal cell. 

Development of dicot embryo

The zygote divides transversely to form a two-celled proembryo. The cell towards the micropyle is known as the basal cell and the other is known as terminal cell.

The basal cell undergoes several transverse divisions to form a long suspensor. The terminal cell divides longitudinally twice to form four cells. This four-celled stage of terminal cell is called quadrant stage. The four cells of the quadrant stage now divide transversely to form an octant stage of eight cells arranged in two tiers of four cells. The lower tier gives rise to the stem tip and cotyledons, while the upper tier is meant for the formation of hypocotyl.

This is followed by periclinal divisioin the octant cells to give rise to eight outer cells, and eight inner cells. The eight outer cells form the dermatogen, which divide anticlinally and develop into the epidermis. The inner cells form the periblem and plerome. The cortex develops from the periblem and stele from the plerome. The basal cell divides several times to form a long suspensor of six to ten cells. The lowermost cell of the suspensor nearest to the developing embryo is known as hypophysis. The hypophysis, by repeated divisions, gives rise to root cap, epidermis and cortex of the root.

Further enlargement of hypocotyl and cotyledon result in a curvature of the cotyledons. The embryo at this stage appear as a horse-shoe shaped structure. In the mature embryo, the stem tip is terminal and the two cotyledons occupy the lateral position.

There is no essential difference between the monocotyledons and the dicotyledons regarding the early cell divisions of the proembryo, but the mature embryos are quite different in two groups. Here the embryogeny of Sagittaria sagittifolia has been given as one of the examples.

The zygote divides transversely forming the terminal cell and the basal cell. The basal cell, which is the larger and lies towards the micropylar end, does not divide again but becomes transformed directly into a large vesicular cell. The terminal cell divides transversely forming the two cells. of these, the lower cell divides vertically forming a pair of juxtaposed cells, and the middle cell divides transversely into two cells.

In the next stage, the two cells once again divide vertically forming quadrants. The cell next to the quadrants also divides vertically and the cell next to the upper vesicular divides several times transversely. The quadrants now divide transversely forming the octants, the eight cells being arranged in two tiers of four cells each. With the result of periclinal division, the dermatogen is formed.

Later the periblem and plerome are also differentiated. All these regions, formed from the octants develop into a single terminal cotyledon afterwards. The lowermost cell L of the three-celled suspensor divides vertically to form the plumule or stem tip. The cells R form radicle. The upper 3-6 cells contribute to the formation of suspensor.