File:Angiosperm Leaf Midrib Vascular Bundle in The Hydrophyte Potamogeton (36716448871).jpg

cross section: Potamogeton leaf magnification: 400x

Both the upper (adaxial) and lower (abaxial) epidermis are uniseriate, lightly cutinized and contain chloroplasts. The air exposed and adaxial epidermis contains numerous stomata that open into large substomatal air spaces. The submerged and abaxial epidermis lacks stomata.

Deep to the upper epidermis are several layers of tightly packed palisade mesophyll, the chief photosynthetic tissue of the leaf. Most of the leaf interior is occupied by a deeper zone of spongy mesophyll, with large regularly arranged air spaces that extend throughout the leaf and into the stem. Spongy mesophyll gives the floating leaf buoyancy while providing abundant space for the storage of photosynthesis gases. Air chamber walls, termed diaphragms, consist of one or two layers of photosynthetic parenchyma with small perforations that allow easy movement of gases while blocking the movement of water.

Vascular bundles are centrally located with xylem toward the adaxial surface and phloem to the abaxial surface. All vascular tissues are reduced with xylem lacking in all but the largest bundles. In smaller bundles xylem is replaced by air chambers or xylem lacuna. Phloem is better developed.