Cleistogamy

It occurs in bisexual flowers which are always closed. Such closed flowers can perform only self pollination. Cleistogamy is generally accompanied by geocarpy when the fruits are formed underground, e.g., Groundnut, The phenomenon of having both open and closed flowers is called chasmocleistogamy.
Advantage of self pollination
1. Self-pollination can preserve parental characters indefinitely. Therefore, a useful variety, once evolved in a homozygous form, can be preserved.
2. It helps in maintaining pure lines for experimental hybridization.
3. It is more economical. The plants do not consume more energy in the production of large number of pollen grains.
4. It ensure seed production. The flowers do not take chances. Some flowers (e.g., Oxalis) utilize self-pollination for seed production when cross-pollination fails.
Disadvantage of self pollination
1. Useful characters can not be introduced by this method.
2. The undesirable or defect characters do not get eliminated from a plant.
3. The immunity of race towards infection decreases and ultimately the plant race become susceptible to many diseases.
4. Continuous self-pollination can lead to the death of the species
5. Self-pollination does not cause any verities. Hence, plants become less adapted to the changes in environment.
Cross pollination
Cross pollination is the transfer of pollen grains from the anther of one flower to the stigma of a genetically different flower. It is also known as allogamy or xenogamy (Gk. Xenos-strange, gamos-marriage). Cross pollination is accomplished with the help of an external agencies like wind (anemophily), water (hydrophily) and animals (zoophily). It is artificial carried out by plant breeders for maintaining races of cultivated plants and producing new varieties. On the basis of external agencies, cross pollination is performed by the following modes.
1. Anemophily
2. Hydrophily
3. Zoophily
Anemophily – In this type, pollination is brought about by wind. Wind picks up the pollen grains from the dehisced. Anthers of a flowers and drops them on the stigmas of other flowers. Example of wind pollination plants are Maize, Poplar, Birch, Oak, Urtica. These pants show the following characteristic.
i. Anemophilous plants grow in large groups. This ensures the pollination of flowers of all the plants.
ii. In many herbaceous plants the flowers are produced above the foliage (e.g., Plantago). This ensures the exposure of flowers to the wind.
iii. In deciduous anemophilous trees, flowers are produced before leaves (peach, apple)
iv. A high proportion of pollen grains is wasted in wind pollination. Therefore, plants produced large quantities of the pollen. A tassel of maize gives rise to 20-25 million pollen grains.
v. Pollen grains are small, light or dusty and can be blown to distances of upto 1300 km. Pollen grains of some plants have air sacs for making them light (e.g., Pine)
vi. Pollen grains are dry and unwettable.
vii. Stigmas are sticky, hairy, feathery (e.g., grasses) or branched in order to increases surface are for catching pollen grains.
viii. In wind pollination flowers, the non-essential floral oranges are either absent or reduced. This makes the flowers small, inconspicuous and devoid of colour, nectar or smell.

Pollination

The transference of the pollen grains from the anther to the stigma is called pollination. It is 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 generally not dependent on any external agency for pollination. It is of two types:
a) Autogamy – It is the transfer of pollen grains from anther of a bisexual flower to its stigma. Both stamens and carpel mature at the same time. It occurs in several cereals (wheat, rice) some peas and flowers which do not open.
b) Geitonogamy – It is kind of self pollination where flowers could be bisexual or unisexual but are borne on the same plant. It may required an external agency like insects or wind.
Self pollination may occurs when the flowers are open (chasmogamy) or closed (elesistogamy). Chasmogamous self pollination occurs only when their anthers and stigma mature at the same time (homogamy).
The self pollinated flower are generally small, inconspicuous, colourless, odourless and nectaorless. Self pollination occurs by following methods:
1. Homogamy – In this case the anther and stigmas of a bisexual flower mature simultaneously. The pollen grains reach the mature stigma either by contact, wind, gravity, rain drop or even insects. Self pollination brought about by contact is called direct autogamy and remaining agencies perform indirect autogamy.
I. Direct autogamy – It is accomplished by the movement of floral parts on account of growth, bending or folding, in convolvulus lxoza, catharanthus and Gardenia the anthers are borne at the mouth of the corolla tube. With the growth of style the stigma comes in contact with ripe anthers and pollination occurs. Pollination takes place in Cotton when the stigma in being pushed out of the staminal tube due to the growth of the style.
In Mirabilis ( four o’clock plant) the filaments of ripe anthers bend and bring in contact with stigma. A similar curling of style takes place in Potato. The stigma of Sunflower has brushing hair which helps in pushing the pollen grains from the synantherous tube. The stigma curls back and receives the pollen grains present on the brushing hair. The closing of flowers at night performer self pollination in Argemone Mexicana.
II. Indirect autogamy – It occurs without coming contact of anthers and stigmas. The pollen grains reach the stigma by rain (e.g., Caltha) or gravity (e.g., Lilac). In Lilac the anther lie exactly above the stigmas. The pollen grains of the dehisced anther fall over the low lying stigmas under the influence of gravity.
In sanicula male and female flowers grow side by side. The long style of the female flowers bends over the male flowers to get pollinated.