give 4 examples flowers undergoing  xenogamy ?

Pollen is carried from a flower on one plant to a flower on a different plant is called xenogamy that is it is a transfer of pollen grains from anther to the stigma of a different plant This is the only type of pollination which during pollination brings genetically different types of pollen grains to the stigma. 

Xenogamy occurs in some of the monoecious plants and in all dioecious plants. Date palm, Papaya, Maize, Nepanthes, Cycas,  Cannabis etc..are some of the examples.