Aerenchyma

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Aerenchyma is the spongy tissue occurring mainly in the stems of many aquatic or marsh plants.

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

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