Ferns pass genes to their children in tiny invisible spores, rather than seeds. The spores grow in cases called sporangia. Groups of sporangia, called sori or fruitdots, may be clustered on the underside of a fertile frond.

Fertile fronds are usually taller, to reach into the breeze and scatter the spores when they are released. That's their main job.

Sterile fronds make the food, and sometimes grow facing up to the sky.

Dimorphism means having two forms; that the sterile and fertile fronds look different. All ferns are dimorphic to at least a small degree, but in some species the sterile and fertile fronds look remarkably unalike.

Fertile fronds can be very useful in identifying the species, but sometimes the plant doesn't produce any.

So when you're out looking at ferns this summer, see if you can find their spores (or at least their sori.)

MONOMORPHIC

Fertile fronds are all about genetic propagation. But actually, most individual fern plants did not reporduce genetically. There are various vegetative ways that ferns propagate, often just by the horizontal growth of underground rhizomes. A few have bits that break off and start new plants, clones of the parent. Bracken [Pteridium aquilinum] is so good at vegetative reproduction that it seldom produces a fertile frond at all.

fertile Bracken

Lady ferns [Athyrium filix-femina] are common lacy plants that reproduce easily by spores. The curved sori form a characteristic herringbone pattern.

Marginal Wood fern (Dryopteris marginalis) is common on steep slopes and rocky ground. The round sori are located very close to the edge of the leaflets, hence the name.

 

fertile Lady fern	fertile Marginal Wood fern (credit: Lucy Mallary Bugbee)

DIMORPHIC

Thelypteris ferns - New York, Massachusetts, and Marsh [Thelypteris palustris] - have narrowed pinnae on their fertile fronds.   Christmas ferns [Polystichum acrostichoides] have narrowed pinnae at the ends of the fertile fronds, with sori blanketing the underside.   Cinnamon fern [Osmunda cinnamomea] is named for the cinnamon-colored sporangia on its fertile fronds, which have no green leaves at all. They form a beautiful display around Memorial Day each year.

fertile Christmas

https://gobotany.newenglandwild.org/genus/thelypteris/
fertile & sterile Marsh (credit: Cheryl Lowe)	sterile & fertile Cinnamon

Sensitive and Ostrich ferns have their own strategy. They do not even produce fertile fronds until mid-summer, and their spores are produced late in the season. They do not scatter the spores in the fall though, but hold them until the following spring, and release them when the snow melts. They get a head start on the following season. Since they release spores after winter, when competing undergrowth has been flattened by snow, they do not need to reach above it. The fertile fronds are shorter than the sterile fronds that will grow later in the spring. Sensitive fern [Onoclea sensibilis] is probably the easiest species to identify in the winter, with stiff fertile fronds sticking up through the snow.

sterile Sensitive fern frond

fertile Sensitive in snow	Sensitive fern, old and new fertile fronds (credit: Lucy Mallary Bugbee)

A few uncommon ferns - Grape ferns, Moonworts, and Adder's Tongue [Ophioglossum pusillum] - do not have separate sterile and fertile fronds. They grow a spore-bearing structure from the same stem as the sterile part of the frond. Adder's Tongue fern (credit: Lucy Mallary Bugbee)