What Is a Sorus? Spore Dots on Fern Fronds
A sorus is a cluster of spore cases on a fern frond. The plural is sori. They often show as dots on the underside.
The short answer
Iowa State says fern spores form in a case called a sporangium. These cluster in groups called sori on the underside of fertile fronds. Sori turn from green to rusty brown to nearly black as they mature. The spores are then released plus carried by wind or water.
What is a sorus on a fern?
Ferns make no seeds, Iowa State says. They make spores in sporangia that cluster into sori. The sori are visible to the naked eye on the underside of fronds. UC Botanical Garden says sporangia may show as green, brown or black dots or stripes.
| Option | What it does | Source |
|---|---|---|
| Sporangium | Spore case | Growing ferns in Iowa |
| Sorus | Cluster of cases | Growing ferns in Iowa |
| Dots or stripes | Seen under fronds | Fern reproduction |
How does a sorus release spores?
Iowa State says mature sori turn dark brown or black then release spores. At maturity the spores are forcibly released. USDA Forest Service says a spore must land on a moist sheltered spot to grow. It forms a small heart shaped gametophyte.
Dark sori: ready to release. Wind or water: carry spores. Moist spot: spore grows.
How does sorus shape help identify ferns?
Iowa State says the location, size, pattern, color plus shape of sori are among the most reliable features to tell look alike ferns apart. Some form patterns or ribbons. Others form random dots or lines. UC Botanical Garden says these patterns inform fern naming.
Pattern: names species. Ribbons or dots: vary by fern. Taxonomy: uses sori.
Troubleshooting
No dots on a frond? It may be a sterile frond per Iowa State.
Green dots? Young sori that darken with age.
Two frond types? Dimorphic ferns per Iowa State.
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Sources
- Growing ferns in Iowa, Iowa State University Extension, Yard and Garden
- Growing ferns from spores, Iowa State University Extension, Yard and Garden
- Fern reproduction, USDA Forest Service
- Fern reproduction, UC Botanical Garden at Berkeley
FAQ
What color are mature sori?
Iowa State says sori turn from green to rusty brown to almost black as they mature.
Do all ferns have a sorus on each frond?
No. Iowa State says fertile fronds bear spores while infertile ones do not. Dimorphic ferns have two frond types.
Do sori help identify ferns?
Yes. Iowa State says sori location, size, pattern, color plus shape help tell similar ferns apart.
Can you grow ferns from spores?
Yes. Iowa State says to set a fertile frond with dark sori on white paper so spores drop. Sow them on moist medium.
How long do fern spores take to sprout?
Iowa State says spores may take two to eight weeks to germinate.
