Why do gymnosperms and grasses produce more pollen than most angiosperms

production of pollen grains depends MOSTLY on the agents of pollination ( Insects, wind,  animals , and birds ).

There is a great deal of wastage of pollen when it is carried by wind ( Anemophylous plant = wind pollinated plants

Explanation:

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.