Many of the designs exhibited by plants living in water were obvious to early botanists. For example, Agnus Arber published a book in 1920 on aquatic plants, documenting many of the strategies that we still talk about today.
All accounts discuss three basic types of leaves:
Submersed leaves receive low levels of sunlight (PFD) because light energy diminishes rapidly while passing through a water column. Light penetration is especially poor in turbid water with dense surface populations of algae. Such underwater leaves are often so highly dissected that the segments may appear superficially to be macroscopic green algae (e.g., Chara and Nitella). This is a strategy to maximize surface-to-volume (S/V), permitting rapid diffusion of carbon dioxide into the chloroplasts of the cells by having proportionately greater surface area. Certain aquatic species have very high ratios of surface to volume (S/V) by having one- or two-cell layer construction. These leaves have a very thin cuticle (wax), but the wax is porous enough to permit easy diffusion of gases through the surface. On these leaves, stomates are generally absent, and would be useless for submerged plants, where water, not air, continually surrounds the photosynthetic organ. Such leaves have very poor development of xylem tissue (water transport), appropriate inasmuch as shoots are bathed in water. Intercellular air spaces are not well developed, thereby enabling this plant to remain submersed by having greater specific gravity. The highly dissected underwater shoot can be tugged at and pulled by water currents without damaging the segments (i.e., little mechanical resistance to current). In swiftly running streams, these shoots and leaves wave and dance wildly.
broader, without major lobing, and remain flat on the water, taking advantage of full sun. Stomates are present for gas exchange, especially on the upper (adaxial) leaf surface. The upper leaf surface tends to have a very prominent cuticle, thereby permitting water to roll off, and not interfering with photosynthesis or promoting growth of epiphytic algae. Epidermis may be rich in chloroplasts, and a bifacial mesophyll (palisade and spongy layers) is formed. Floating leaves often have well-developed air chambers (lacunae), which provide buoyancy, and they may also have hard cells, sclereids, within the mesophyll that provide some toughness for the leaf and prevent the layers from becoming collapsed.
Emersed (aerial) leaves are essentially like typical leaves of herbaceous angiosperms that inhabit full-sun environments. Such leaves are emergent from the water and, consequently, have a waxy cuticle on both surfaces. Many are also amphistomatic (stomates on both surfaces and in nearly equal densities) and have well-developed leaf mesophyll on both sides, to take advantage of the abundant light.