What Is a Prothallus? The Tiny Heart Stage of a Fern
A prothallus is the small green heart shaped plant that grows from a fern spore. Iowa State Extension calls it the gametophyte stage, the part of the fern life cycle that makes eggs plus sperm.
The short answer
A prothallus is a tiny flat green heart that grows from a germinated fern spore. UC Berkeley garden says it is typically 2 to 5 mm wide plus short lived. It makes eggs plus sperm on its underside. After fertilization a new fern plant grows from it, which Iowa State says takes 2 to 6 months.
How does a spore become a prothallus?
A fern spore, Iowa State says, does not become a fern at once. It grows into a tiny heart shaped prothallus first. Germination takes 2 to 8 weeks. To grow, a spore must land on a moist sheltered spot, USDA Forest Service says. The mature gametophyte of many ferns is a flat green heart about the size of a fingernail.
| Option | What it does | Source |
|---|---|---|
| Spore | Lands on moist ground | Fern reproduction |
| 2 to 8 weeks | Germination | Growing ferns from spores |
| Green heart | Prothallus forms | Growing ferns from spores |
How does a prothallus make a new fern?
UC Berkeley garden says rhizoids grow on the underside. The same underside bears archegonia, which are female, plus antheridia, which are male. Iowa State says sperm swim to the egg through a thin film of water. UC Berkeley says the zygote grows into an embryo. The embryo becomes the fern plant, the sporophyte in USDA Forest Service terms.
Archegonia: hold eggs. Antheridia: release sperm. Film of water: sperm swim.
What does a prothallus look like in a pot?
Iowa State says prothalli can form a green mat on the sowing medium. Thin the mat so plants have room. First small fronds appear in 2 to 6 months. UC Berkeley notes each prothallus is short lived.
Green mat: many prothalli. Thin out: give room. 2 to 6 months: first fronds.
Troubleshooting
Mat but no fronds? Iowa State says sperm need a thin film of water to reach eggs.
Crowded mat? Thin it per Iowa State.
Cannot see one? USDA Forest Service says they are about fingernail size.
Kind Seed Co shares these plant notes for growers of all kinds. For more on plant stages see cannabis leaf anatomy and life cycle of a marijuana plant. Read companion plants when growing marijuana for garden pairings. See growing cannabis near trees for site notes.
Sources
- Fern reproduction, USDA Forest Service
- Fern reproduction, UC Botanical Garden at Berkeley
- Growing ferns from spores, Iowa State University Extension, Yard and Garden
FAQ
How big is a prothallus?
UC Berkeley garden says it is typically 2 to 5 mm wide. USDA Forest Service compares it to a fingernail.
Why does a prothallus need water?
Iowa State says sperm must swim through a thin film of water to reach the egg.
Can you see a prothallus outdoors?
It is small. USDA Forest Service says it is a flat green heart about the size of a fingernail on moist sheltered ground.
How long until a fern grows from it?
Iowa State says the first fern plants appear in 2 to 6 months.
What grows from a prothallus?
UC Berkeley garden says the zygote grows into an embryo that becomes the fern plant.
