Where are fern sporangia usually located?

In ferns, sporangia (spore cases) are typically located on the undersurfaces of the fronds, or vegetative leaves. In many species, these sporangia are organized into clusters known as sori.12

While the undersurface of the leaf is the most common location, the placement can vary depending on the fern species:

• Modified leaves: Some ferns produce their sporangia on specialized, highly modified leaves or specific portions of the plant.1
• Fertile spikes: In more primitive ferns, such as Ophioglossum and Botrychium, spores are borne on a specialized axis called a fertile spike rather than on typical fronds.1

The sori themselves may be naked, covered by the leaf margin, or protected by a specialized outgrowth called an indusium.21