The orientation of foliicolous lichen campylidia with respect to water runoff and its significance for propagule dispersal.

PREMISE OF THE STUDY: Some common leaf-dwelling lichen fungi produce asexual spores (conidia) within curved, dorsiventral structures called campylidia. Their shape and tendency to face in the same direction have generated speculation about how dispersal is accomplished. Here we tested the hypothesis that campylidia orient their spore-producing surface against runoff currents and examined the effects of hydration to better understand the spore dispersal mechanism.

METHODS: Palm leaves bearing lichens (Calopadia) were surveyed with a dissecting microscope for campylidia with fibrous debris entangled around the base. Where possible, the direction of runoff flow was inferred from the position of the entangled debris; the angle between this direction and that toward which the spore-producing side faced was calculated for 67 campylidia. Other fresh-collected campylidia were photographed in the air-dry state and again after hydration.

KEY RESULTS: Orientation of campylidia was strongly correlated with direction of runoff flow, such that the spore-producing side faced against oncoming runoff. Hydration of campylidia quickly resulted in swelling of the conidial mass beneath a thin flap of tissue covering the conidiogenous surface. The flap then bulged outward, exposing the conidial mass from above within its pocket-like compartment.

CONCLUSIONS: Our results support previous contentions that water impact against campylidia is important in spore dispersal. However, the morphology of hydrated campylidia and their strong tendency to face upstream suggest that water currents impact laterally upon the thin tissue covering the hydrated conidial mass, thereby extruding spores apically. We contrast these findings with previous suggestions that campylidia act as splash cups.