What Is a Costa? The Midrib of a Moss Leaf
A costa is the midrib of a moss leaf. The University of Illinois moss guide defines the costa as the midrib of a leaf, while the Minnesota DNR glossary calls it the nerve or vein of a moss leaf, sometimes single plus sometimes double.
The short answer
A costa is the central rib down a moss leaf. The University of Illinois notes a leaf without one is ecostate, plus a costa that runs past the leaf tip is excurrent. The Minnesota DNR notes a double costa can be nearly impossible to see with a hand lens. Use a lens on single leaves.
What is a costa on a moss leaf?
The Minnesota DNR glossary notes the costa is the nerve, or vein, of a moss leaf that forms a midrib. UConn Bryology notes moss leaves often have a midrib, while liverwort leaves lack one, so the costa is a quick clue to separate them.
How do you see a costa?
Pull one leaf from a moss stem plus view it under a hand lens. The Minnesota DNR notes some double costae are so faint they count as ecostate.
| Term | Meaning | Source |
|---|---|---|
| Costa | Midrib of a leaf | Illinois |
| Ecostate | Leaf without a costa | Illinois |
| Excurrent | Costa runs past the leaf tip | Illinois |
| Double costa | Two short nerves | Minnesota DNR |
| Lamina | Leaf blade beside the costa | Minnesota DNR |
Source: University of Illinois moss guide plus Minnesota DNR.
Why does the costa matter for ID?
The Minnesota DNR glossary notes the costa can be single or double, plus the University of Illinois notes it can stop short or run past the tip. Such details help sort mosses. Iowa State University Extension notes moss favors shade, low fertility plus poor drainage, so damp shady spots offer plenty to study.
How to: step by step
- Find a moss patch in shade.
- Pick one stem.
- Pull off a single leaf.
- Set it on a dark card.
- View it with a hand lens.
- Note if the costa is single or double.
Troubleshooting
No midrib: an ecostate leaf. The University of Illinois notes this term.
Faint double lines: a dim double costa. The Minnesota DNR notes these are hard to see.
Rib past the tip: an excurrent costa. The University of Illinois notes this form.
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Sources
- A visit to the miniature forest (mosses), University of Connecticut Bryology
- How to manage moss in the lawn and garden, Iowa State University Extension
- Moss guide glossary, University of Illinois
- Mosses and liverworts of Minnesota illustrated glossary, Minnesota DNR
FAQ
Do all mosses have a costa?
No. The University of Illinois moss guide notes a leaf without a costa is called ecostate.
Is a costa the same as a vein?
The Minnesota DNR glossary describes the costa as the nerve, or vein, of a moss leaf.
Can you see a costa without a lens?
A lens helps. The Minnesota DNR notes some double costae are nearly impossible to see even with a hand lens.
What does excurrent mean?
The University of Illinois moss guide notes an excurrent costa extends beyond the end of the leaf.
Can a costa be double?
Yes. The Minnesota DNR glossary notes the costa is sometimes double plus sometimes single.
