Sexual Reproduction In Flowering Plants

Xenogamy is pollination between two flowers of two different plants.

Xenogamy is true cross-pollination, i.e., allogamy which involves the transfer of pollen grains from the anther of one plant to the stigma of another, genetically dissimilar plant.

Some plants such as Viola (common pansy), Oxalis and Commelina produce two types of flowers: chasmogamous flowers which are similar to flowers
of other species with exposed anthers and stigma, and cleistogamous flowers which do not open at all. In such flowers the anthers and stigma lie close to each other. When anthers dehisce in the flower buds, pollen grains
come in contact with the stigma to affect pollination. Thus, cleistogamous flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma. Cleistogamous flowers produce assured seed-set even in the absence of pollinators.

In angiosperms, endosperm nucleus is the product of triple fusion and is 3n (triploid).

Fully developed embryo sac of angiosperm is 8 nucleated and 7 celled structure i.e., egg apparatus at micropylar end is having 3 cells (2 synergids + l egg cell), 3 antipodal cells at chalazal end and a central cell having two polar nuclei at centre.