The structure that encloses the male gametophyte and transports it to another plant in seed plants is called a pollen grain.

Here's why:

* Male gametophyte: This is the male reproductive structure in seed plants. It develops within the pollen grain and contains the sperm cells.

* Pollen grain: This is a tough, protective outer layer that encloses the male gametophyte. It's designed to be dispersed by wind, water, or insects.

* Transportation: When a pollen grain lands on the stigma of a flower (in flowering plants), it germinates and grows a pollen tube. This tube carries the sperm cells down to the ovary, where fertilization can occur.

So, the pollen grain acts as both a protective capsule for the male gametophyte and a vehicle for its transport.