prothallium
/proʊˈθæliəm/
noun
- The small, flat, heart-shaped structure that grows from a fern spore and produces the sex cells for reproduction.
- Under the microscope, the prothallium looked like a tiny green heart.
- The fern's prothallium must stay moist for the sperm to swim to the egg.
- In biology class, we observed a prothallium growing on damp soil.