Aerenchyma is the spongy tissue occurring mainly in the stems of many aquatic or marsh plants. Freshwater marsh in Florida In geography, a marsh is a type of wetland, featuring grasses, rushes, reeds, typhas, sedges, and other herbaceous plants (possibly with low-growing woody plants) in a context of shallow water. ...

The tissue is formed by the phellogen layer, and in many cases results from the separation of the cell walls, leaving extensive intercellular spaces. Such porous tissue gives great buoyancy to the stem and so helps keep the leaves up in the air and permits gas diffusion within the plant. Their stomata are situated at the upper surface This is not about surgically created bowel openings; see stoma (medicine) In botany, a stoma (also stomate; plural stomata) is a tiny opening or pore, found mostly on the undersurface of a plant leaf, and used for gas exchange. ...

The term aerenchyma is frequently applied to any loose porous tissue found in plants, eg lenticels.