The spore that becomes a pollen grain in seed plants is called a microspore.

Here's why:

* Microspores are produced by meiosis within the anther of the flower, the male reproductive part.

* Each microspore undergoes mitosis to form a pollen grain. This pollen grain contains the male gametophyte, which includes the sperm cells.

* Pollen grains are the male gametes of seed plants. They are transported by wind, water, or insects to the female part of the flower (the pistil), where they fertilize the egg cell.

In contrast:

* Megaspores are produced in the ovule, the female reproductive part of the flower.

* They develop into the female gametophyte, which contains the egg cell.

So, while both microspores and megaspores are spores, only microspores develop into pollen grains.