Leaf symmetry

Leaf symmetry describes how a leaf is oriented and shaped in cross-section. A dorsiventral leaf is flat, held horizontally, and shows a clearly different upper and lower surface, as in most dicotyledons (Tridax). An isobilateral leaf is held vertically upright, so both its surfaces face sideways and receive similar light, as in many monocotyledons (grasses). A centric leaf is more or less cylindrical and oriented upward or downward, as in pine and onion.

Heterophylly is the occurrence of two visibly different kinds of leaf on the same plant. It is common in aquatic plants, where the floating or aerial leaves are typically broad, fully expanded and undivided or only lobed, while the submerged leaves are narrow, ribbon-like, linear or finely dissected - an adaptation to the two very different environments the plant experiences above and below the water surface (water crowfoot Ranunculus aquatilis, water plantain Alisma plantago, arrowhead Sagittaria, Limnophila heterophylla). Terrestrial plants can also show heterophylly, but tied to developmental stage rather than environment: young leaves are lobed or dissected wh …