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.