fertile fronds of purple cliffbrake (Pellaea atropurpurea)
So if ferns aren’t flowering plants, how do they reproduce? The answer is fairly technical, and detailed descriptions are only a search away. Here’s a good one. A short answer is: they produce spores rather than seeds. (Although, as with seed plants, there are vegetative means of reproduction as well.)
Spores generally, but not always, form on the underside of a frond. Not all fronds are fertile (that is, spore-bearing). If you look on the back of a fern frond and see lots of little dots, you’re looking at a fertile frond.
The spores are contained within structures called sporangia. The sporangia, in turn, are often (but not always) clustered together in structures called sori (singluar sorus), which are the small dots that you can see with the naked eye. Sometimes the sorus will be covered by a flap of tissue called an indusium (plural indusia).
Noting the characteristics of the sori – how they’re shaped, how they’re placed on the pinnae – can be an important step in identifying the species of fern.
And now, the meat of this post, which I’ve written in part because this stuff is fascinating, but mostly because I’ve been having fun taking “studio” pictures.
the reverse side of purple-stem cliffbrake; in the lead-in photo, the white margins on the pinnae are a clue that the fronds are fertile (an unusual characteristic)
linear sori along the midveins of the pinnae on ebony spleenwort (Asplenium platyneuron); some indusia are also visible near the rachis
orbicular sori along the margins of the pinnae of common polypody (Polypodium virginianum)
marginal woodfern (Dryopteris marginalis)
closeup of marginal woodfern
the unusual fertile frond of rattlesnake fern (Botrypus virginianus)
linear sori scattered along the frond of walking fern (Asplenium rhizophyllum)
blunt-lobed cliff fern (Woodsia obtusa)
masses of sori on christmas fern (Polystichum acrostichoides)