phyllids

Phyllids are leaf-like organs found in many bryophytes, particularly leafy liverworts, where they form the photosynthetic blades that surround a stem axis. In leafy liverworts, the thallus is differentiated into a collection of phyllids arranged along one or more stems, and these leaf-like structures are the principal sites of photosynthesis. The morphology of phyllids varies widely among taxa; they are often elongated or oblong and may bear a midrib or costa, while the surface cells are typically thin-walled and specialized for efficient light capture. In many liverworts, phyllids lack stomata, a feature that helps distinguish bryophyte groups, though exceptions exist.

Phyllids can be arranged in two-ranked or multi-ranked patterns around the stem and may be simple or lobed, with some lineages showing highly specialized leaf forms. Their development arises from apical cells of the growing axis and can reflect broad evolutionary trends in bryophyte leaf evolution. Functionally, phyllids perform photosynthesis and contribute to water absorption and retention on the plant body, aiding bryophyte moisture management in terrestrial habitats.

In bryology, phyllids are a key diagnostic feature that helps distinguish leafy liverworts from thallose liverworts and from other bryophyte groups. The term is sometimes used more broadly for leaf-like structures on bryophytes, but its most established use is in the context of leafy liverwort morphology. Related outgrowths along stems, called paraphyllidia, may resemble phyllids but are distinct in their development and function.