How large are fern gametophytes?

Fern gametophytes, often called prothalli, are typically very small, delicate structures. In the majority of fern species, these gametophytes are kidney-shaped and generally measure less than 8 mm (0.3 inch) in length.1

More specifically, the average size of a fern gametophyte at the time of fertilization is approximately 2 to 8 mm (0.08 to 0.32 inch) long and up to 8 mm wide. While their exact dimensions can vary, they rarely exceed 13 mm (0.5 inch) in diameter. These structures are typically one cell layer thick, except in the center where they are thicker.23