air chamber

Any of the air-filled cavities beneath the upper epidermis of the gametophyte thallus in many liverworts of the order Marchantiales. Such air chambers increase the surface area of the internal photosynthetic cells and so facilitate gaseous exchange. At the same time they maintain a humid atmosphere around  these cells, thus reducing transpiration. Connection of the air chambers with the external atmosphere is facilitated by pores in the upper surface of the thallus.