Pilobolus The Hat Thrower
The sporangia grow overnight on animal dung. They are mature by early morning. The light strikes the swollen vesicle and the curvature acts like a convex lens and the light is refracted and falls on a photosensitive zone (red) at the apex of the sporangiophore just below the vesicle (yellow). Growth of the sporangiophore takes place just below the light sensitive zone on the side that receives the light signal and the sporangiophore bends towards the source of incident light (rising sun). Growth will continue until the light signal is equalized around the light sensit ve zone and the sporangiophore is facing the light source (approximately 45 degrees dependent on latitude and time!). At the same time, turgor pressure builds up in the vesicle. There is a line of weakness between the apex of the vesicle and the cap (sporangium). When the osmotic pressure of the vesicle exceeds the resistence of this line of weakness, the cap is shot off violently for a distance of up to six feet. It is shot off in a squirt of protoplasm and sticks to grass blades at some distance from the animal dung and persists on the blade until eaten by a browsing herbivore. The sporangium breaks down during the digestive process and spores are released into the gut and eventually defecated to renew the cycle.