Gametophyte

When the spore wall cracks under appropriate moist conditions, the fern gametophyte is formed. Emerging from the spore at the time of germination are a nongreen rhizoid (rootlike organ), which attaches the plant to the growing surface, and a green single cell—the mother cell that gives rise to the rest of the gametophyte. At first, in most homosporous ferns, growth is in the form of a single filament, and it may continue in this fashion if lighting conditions are weak. If lighting is optimal, however, the gametophyte becomes a two-dimensional sheet of cells and later a layered three-dimensional structure. The apical cell, which initiates growth, is soon replaced by a growth zone, or meristem, which, as a result of the directions of cell division and enlargement, comes to lie in an apical notch in the gametophyte, surrounded on either side by the prothallial wings—flat platelike protrusions, one cell thick. The average size of the 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.