Why are sori useful for identifying fern species?

Sori are essential for identifying fern species because their structure, arrangement, and protective features are highly characteristic of different genera. Traditionally, these reproductive organs have served as the most important characters for fern classification.12

Key features used for identification include:

• Arrangement: Sori may be organized in specific patterns, such as linear arrangements along veins or along the leaf edge, or they may be distributed across the entire undersurface of the blade.1
• Presence of an Indusium: Identification often depends on whether the sorus is "naked" (lacking a covering) or protected by an indusium—a specialized, membranous, or scalelike outgrowth of the leaf.12
• Indusium Shape: When present, the shape of the indusium—such as kidney-shaped, shieldlike, or narrow—provides a reliable diagnostic trait for distinguishing between various groups, such as Asplenium and Dryopteris.1

By examining these specific morphological traits on the undersurface of the fronds, botanists can effectively categorize and identify different species of ferns.12