1. Plural of prothallus; the small, flat, green plant body that develops from a fern spore and produces the sexual organs (archegonia and antheridia).
Usage: botanical term; also called gametophyte in ferns

Examples

• Fern prothalluses are typically heart-shaped and only a few millimeters wide.
• The prothalluses of different fern species vary in size and appearance.
• Under a microscope, you can observe the archegonia and antheridia on the prothalluses.
• Moisture is essential for the prothalluses to survive and reproduce sexually.
• After fertilization occurs on the prothalluses, a new fern sporophyte begins to develop.