What Is an Indusium? The Flap Covering Fern Spore Dots
An indusium is a thin membrane that covers each sorus, or spore dot, on the underside of many fern fronds. University of Georgia Extension notes the presence plus structure of the indusium are used to identify ferns.
The short answer
An indusium is a flap over a cluster of spore cases. The USDA Forest Service notes sori are groups of sporangia, plus young sori are commonly covered by flaps called indusia. UGA notes some ferns have none, while others have a false indusium formed by an in rolled leaf margin.
What does an indusium do?
It shields the sorus as it develops. The USDA Forest Service notes young sori are commonly covered by indusia, plus maturity affects how useful they are for identification, since the flap can shrivel or fall away.
What shapes can an indusium have?
The USDA Forest Service lists many shapes.
| Shape | Example fern | Source |
|---|---|---|
| Umbrella shaped | Sword fern | USDA Forest Service |
| Hood like | Fragile fern | USDA Forest Service |
| Kidney shaped | Northern wood fern | USDA Forest Service |
| Linear | Maidenhair spleenwort | USDA Forest Service |
| False indusium | Maidenhair | USDA Forest Service |
| None | Polypody | USDA Forest Service |
Source: USDA Forest Service Fern structure.
How does it help identify ferns?
UGA Extension notes the presence plus structure of the indusium are used for identification. NC State Extension notes marginal wood fern carries its sori at the margins of the pinnules. Iowa State University Extension notes mature sori on frond undersides turn dark brown or black.
How to: step by step
- Pick a fertile frond.
- Flip it over.
- Find the sori.
- Use a hand lens.
- Note the flap shape.
- Compare with the table above.
Troubleshooting
No flap seen: a fern without one. The USDA Forest Service notes polypody sori have no indusia.
Flap shriveled: mature sori. The USDA Forest Service notes maturity affects ID.
Edge rolled over dots: a false indusium. UGA notes it forms under in rolled margins.
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Sources
- Ferns in the landscape (B 987), University of Georgia Extension
- Growing ferns from spores, Iowa State University Extension, Yard and Garden
- Fern structure, USDA Forest Service
- Dryopteris marginalis (marginal wood fern), NC State Extension Gardener Plant Toolbox
FAQ
Do all ferns have an indusium?
No. UGA Extension notes some ferns have none, plus the USDA Forest Service notes polypody sori have no indusia.
Is the indusium the same as a spore case?
No. The USDA Forest Service notes sori are groups of sporangia, which hold the spores, plus the indusium is the flap that covers them.
When do fern spores ripen?
Iowa State University Extension notes sori on the frond underside turn dark brown or black when mature.
Can you grow ferns from spores?
Yes. Iowa State University Extension has a how to on growing ferns from spores collected from mature sori.
What is a false indusium?
UGA Extension notes some ferns have a false indusium under in rolled leaf margins. The USDA Forest Service gives maidenhair as an example.
