It is the transfer of pollen grains from the anthers of one flower to the stigma of a genetically dissimilar flower. It is also called xenogamy. Both xenogamy and geitonogamy are included under allogamy though this term is more commonly used for cross pollination. It requires external agents. They are of two types, abiotic and biotic.

Anemophily (Wind Pollination): In this, air currents pick up pollen from dehiscing anthers. The receptive stigmas pick up the pollen grains from air currents. A very large number of pollen grains are produced, e.g. 500,000 per flower in Cannabis, 25 million by a tassel of maize. Flowers may occur in hanging spikes or catkins. Stigmas are exserted, sticky, and hairy. Pollen grains are dry and unwettable.Pollen is very light. They may have air sacs or wings. Pollen grains of Pinus are found 100 of kilometers away from the plants.

Entomophily: It is pollination brought about through the agency of insects. It is the most common and specialized type of pollination. The important traits of entomophilous flowers are: Flowers are coloured.One sepal is enlarged in Mussaenda, bracts are coloured in Bougainvillea while involurce is conspicuous in Euphorbia. Commonly, petals are coloured. Bluish- purple-violet-yellow flowers attract bees while reddish flowers attract butterflies and wasps. Flowers commonly possess an aroma or scent. Visiting insects are fed by either nectar. Pollen grains have a sticky surface due to pollenkitt. Stigmas are sticky.

Hydrophily: It is the pollination brought about through the agency of water. Water pollination is of two types-epihydrophily and hypohydrophily. Stigma is trifid. Male flowers are drawn in the depression round a female flower to effect pollination. After pollination the female flower is brought down into water.

Ornithophily: It is allogamy performed by birds. Two types of long- beaked small birds take part in pollination- sun birds and humming birds. Others birds performing pollination are Cow, Bulbul, Parrot, Mynah, etc. Ornithophilous flowers are large and strong with abundant nectar or edible part.

Malacophily: It is cross pollination brought about by the agency of snails, e.g. Arisaema and some other aroids.

Chiropterophily: It is allogamous pollination performed by bats. The flowers produce strong aroma abundant nectar and a large number of stamens in Adansonia.

Importance of Cross Pollination is that it is useful in increasing yield and adaptability. It eliminates defective traits and is helpful in production of new varieties. Cross pollination may bring in hybrid vigour. It is, however wasteful and may dilute very good characters of the race.