How do sori form on fern fronds?

Sori form on fern fronds through the development of specialized regions called receptacles, which serve as the site of origin for clusters of sporangia. These clusters, known as sori, arise from meristematic areas—regions of active cell growth—on the undersurface of the frond or, in some species, on highly modified leaves.12

The formation process involves the continuous activity of these meristematic zones, which can produce sporangia over several weeks. As new sporangia mature, older ones are pushed aside, resulting in clusters that may contain sporangia of varying ages.2

To protect these developing reproductive structures from environmental hazards like drying, ferns employ several strategies:
* Indusia: Many ferns produce a true indusium, a specialized, often papery, scalelike structure that covers the sorus.21
* False Indusia: Some species utilize a "false indusium," where the margin of the leaf rolls over to shield the developing sporangia.13
* Naked Sori: In certain genera, such as Polypodium, the sori remain uncovered.21

The arrangement and structure of these sori are key characteristics used in the classification of ferns.2