What Is a Pinnule? The Smallest Divisions of Fern Fronds
A pinnule is a segment of a pinna on a fern frond that is divided more than once. Iowa State University Extension notes the leaf blade is made of pinnae, while NC State Extension notes the marginal wood fern has a bipinnate blade with pinnules on each pinna.
The short answer
A fern frond has a stalk called the stipe, a midrib called the rachis plus leaflets called pinnae. Iowa State notes that on much divided fronds the pinnae branch again. Those smaller segments are pinnules. NC State notes marginal wood fern carries its sori at the pinnule margins.
Where are pinnules on a fern?
Iowa State notes the stipe runs from the rhizome to the base of the blade, plus the rachis is the midrib where pinnae attach. On much divided fronds a secondary rachis forms, plus if divided again, a tertiary rachis. Pinnules sit on those branches.
How is a pinnule different from a pinna?
A pinna is the first division of the blade. A pinnule is a division of a pinna. NC State describes marginal wood fern as bipinnate, so each pinna carries pinnules.
| Part | Meaning | Source |
|---|---|---|
| Frond | The whole leaf | Iowa State |
| Stipe | Stalk from rhizome to blade | Iowa State |
| Rachis | Midrib holding pinnae | Iowa State |
| Pinna | First division of the blade | Iowa State |
| Pinnule | Division of a pinna | NC State |
| Sori | Spore clusters under fronds | Iowa State |
Source: Iowa State University Extension plus NC State Extension.
Why do pinnules matter for fern ID?
Iowa State notes sori form patterns, ribbons or dots on the underside of fertile fronds. NC State notes marginal wood fern has sori at the margins of the pinnule undersides, with heavily veined pinnules plus blunt tips. UC Berkeley notes sporangia sit beneath fronds.
How to: step by step
- Find the stipe at the base of the frond.
- Follow the rachis up the middle.
- Note the pinnae along each side.
- Check whether each pinna splits again.
- Call those smaller segments pinnules.
- Turn the frond over to look for sori.
Troubleshooting
No pinnules seen: frond divided once. Iowa State notes fronds can be undivided, divided or much divided.
Levels hard to tell: a lacy frond. Iowa State notes secondary plus tertiary rachises form.
Dots under pinnules: a fertile frond. Iowa State notes sori sit on the underside.
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Sources
- Growing ferns in Iowa, Iowa State University Extension, Yard and Garden
- Fern reproduction, UC Botanical Garden at Berkeley
- Extension Gardener Handbook, chapter 3: botany, NC State Extension
- Dryopteris marginalis (marginal wood fern), NC State Extension Gardener Plant Toolbox
FAQ
Do all ferns have pinnules?
No. Iowa State University Extension notes fronds may be undivided, divided or much divided. Only fronds divided more than once carry pinnules.
Can pinnules carry spores?
Yes. NC State Extension notes marginal wood fern carries its sori at the margins of the pinnule undersides.
What is a bipinnate frond?
A bipinnate blade is divided twice, so each pinna carries pinnules. NC State describes marginal wood fern this way.
What is the rachis on a fern?
Iowa State University Extension notes the rachis is the midrib of the frond where the pinnae attach.
What is a fiddlehead?
Iowa State University Extension notes the curled frond at emergence is called a crozier or fiddlehead.
