Angiosperms depend on external agents for pollination because ________ .

In angiosperms, both male and female gametes are non-motile. The male gametes are carried within the pollen grains, which are also non-motile. Since the pollen grains cannot move on their own to reach the stigma of the pistil, they require external agents such as wind, water, or animals for their transfer. This process of transfer is called pollination. Thus, the dependence on external agents is due to the non-motile nature of pollen grains.

Answer: pollen grains are non-motile.