A long overdue post on these beautiful, useful, and often-overlooked (though not by professional fern fanciers) structures found on the lower (abaxial) surface of many fern fronds: the sori. A sorus (singular) is a cluster of sporangia, which are very small spore-bearing sacs. So: each of the colored "dots" in some of the above photos is actually made up of many smaller "dots" which actually contain the plant's spores. The individual sporangia are actually visible in the first image, which is a Costa Rican Campyloneurum species.
Sori are often round (as in the first, third, and last images, of Campyloneurum, Niphidium, and Tectaria, respectively), but may also be linear (second and fourth, Adiantum and Pteris), and some groups of ferns have more complex cup-shaped or valved sori. Another important structure when considering sori is the indusium, a piece of tissue that may sit on top of or surrounds the sorus. To complicate matters further, some ferns, like the Adiantum pictured in image #2 at top right, have what is known as a false indusium; their sori are surrounded by an indusium-like flap, but it is not derived from the same source as a true indusium.
The presence and shape of the sori, and the presence or absence of indusia, are some of the most important characters to consider when trying to identify a fern, as they are very often specific to families or genera, or even to a certain species. For example, all members of the genus Adiantum have a false indusium that is simply the edge of the leaf rolled over the sori, and this single diagnostic feature is extremely helpful in positively identifying a fern as a member of this genus.
Well-meaning but uninformed folk sometimes mistake sori for fungal or insect infections on leaves, and scrape them off thinking that they're doing the plant a favor. Big mistake! Doing so could potentially destroy a fern's entire reproductive output if the scraping were thorough enough. And besides, then you couldn't enjoy looking at them anymore.