How does a fern spore grow into a gametophyte?

When a fern spore lands in a suitable, moist environment, it undergoes germination to form a gametophyte, also known as a prothallium.12

The process occurs as follows:

• Emergence: The spore wall cracks, allowing a nongreen rhizoid to emerge and anchor the plant to the surface, alongside a green mother cell that initiates the development of the rest of the gametophyte.3
• Initial Growth: In most homosporous ferns, the gametophyte begins as a single filament of cells. If lighting is weak, it may remain filamentous; however, under optimal light, it develops into a two-dimensional sheet of cells.3
• Maturation: The structure eventually becomes a layered, three-dimensional, typically kidney-shaped or heart-shaped plant, usually one cell layer thick except in the center. Growth is driven by an apical meristem located in an apical notch, flanked by flat, platelike protrusions called prothallial wings.132

Once mature, this tiny structure—typically less than 8–13 mm in diameter—produces the sex organs necessary for fertilization.12