Where are fern spores found on a frond?

In most ferns, spores are produced within structures called sporangia, which are typically found in clusters known as sori (singular: sorus) on the undersurface of the fronds (leaves).12

These sori often appear as brown patches on the underside of the leaf, which is referred to as a sporophyll when it bears reproductive structures. While this is the most common arrangement, some ferns produce their sporangia on highly modified leaves or specialized axes rather than on the typical vegetative fronds.21

The arrangement and protection of these sori vary by species:
* Indusiate sori: Many ferns have sori covered by a protective, scalelike structure called an indusium.31
* Naked sori: Some ferns, such as those in the genus Polypodium, have sori that remain uncovered.31
* Marginal protection: In some cases, the sporangia are protected during development by the rolled-over margin of the leaf.31