Why are fern gametophytes also called prothalli?

Fern gametophytes are called prothalli (singular: prothallium or prothallus) because they typically grow as a small, flat, green, thalloid structure. In botany, a thallus refers to a plant body that is not differentiated into true roots, stems, and leaves.12

The term reflects the physical form of the gametophyte, which develops from a spore into a simple, often heart-shaped or kidney-shaped sheet of cells. This structure is typically attached to the soil by rhizoids—rootlike organs that anchor the plant—rather than true roots. Because this independent, sexual phase of the fern life cycle lacks the complex vascular organization of the mature sporophyte, it is descriptively classified as a prothallus.1342