What are sori on fern leaves?

A sorus (plural: sori) is a cluster of spore-producing structures known as sporangia, typically found on the undersurface of fern leaves, or fronds. These structures are essential for the plant's reproduction, as they serve as the site where spores are generated and released.123

Key characteristics of sori include:

• Protection: Many sori are shielded during development by a specialized scale or flap of tissue called an indusium. In some species, the margin of the leaf itself rolls over to provide this protection, while others, such as Polypodium, have naked sori.23
• Structure and Evolution: Sori vary in their arrangement and maturity. Advanced sori often contain a "mixed" cluster of sporangia of different ages, which allows the plant to extend its period of spore production.3
• Identification: Because their shapes and arrangements are highly diverse, sori have historically been a primary feature used in the classification of different fern species.3

When observed on house ferns, sori are sometimes mistakenly identified as pests or fungal diseases, though they are entirely normal reproductive organs.3