1. A swollen air-filled region of the thallus seen in many algae of the order Fucales, notably Fucus vesiculosus (bladder wrack). The air bladders serve to increase buoyancy and may also play a part in respiration.
2. Any of the air-filled protuberances that develop by separation of the exine layers in the pollen grains of certain gymnosperms, e.g. Pinus. They give the pollen a characteristic winged appearance and assist the wind dispersal of the pollen. See also bladder