The most reduced size gametophyte is found in angiosperms. Angiosperms are a group of plants that produce flowers and fruits. They are the most diverse and dominant group of plants on Earth, with over 300,000 species.

Angiosperms have a unique reproductive cycle that involves the alternation of generations between a sporophyte and a gametophyte. The sporophyte is the dominant phase in the life cycle of angiosperms and is the plant that we typically see above the ground. It produces flowers, which contain the reproductive structures of the plant.

The male gametophyte, also known as the pollen grain, is produced within the flower. It is formed through a process called microsporogenesis, where microspore mother cells undergo meiosis to produce four haploid microspores. Each microspore then undergoes mitosis to produce a pollen grain.

The pollen grain is the male gametophyte, and it is incredibly small in size. It consists of just a few cells, including two haploid sperm cells, a tube cell, and a generative cell. The entire male gametophyte is contained within the pollen grain, which is typically around 20-100 micrometers in diameter.

The female gametophyte, also known as the embryo sac, is found within the ovule of the flower. It is formed through a process called megasporogenesis, where megaspore mother cells undergo meiosis to produce four haploid megaspores. Only one of these megaspores survives and undergoes mitosis three times to produce an eight-nucleate embryo sac.

The female gametophyte is also incredibly small in size, but it is larger than the male gametophyte. It is typically around 100-500 micrometers in length and is composed of eight cells and seven cells.

Overall, the gametophytes of angiosperms are highly reduced in size compared to the gametophytes of other plant groups, such as bryophytes, pteridophytes, and gymnosperms. This reduction in size is thought to be an adaptation that allows for more efficient reproduction and increased success in diverse environments.