Structural organization and functional activity of the photosynthesizing system of three aquatic plants, dependent to a variable degree on the aquatic environment (a helophyte Alisma plantago-aquatica L., a pleustophyte Potamogeton natans L., and a hydatophyte Potamogeton lucens L.), were studied during leaf development. The structure of the photosynthetic apparatus of floating and aerial leaves, which grew at the initial stages of development in the aquatic environment, was similar to that of terrestrial sun plants. Submerged leaves had hydromorphic features which resembled those of shade plants. At the initial phases of growth, the formation of the pigment complex did not differ significantly in the chloroplasts of the three hydrophyte species. When floating and surface leaves emerged from the aquatic environment into the air, the pigment content, as calculated per leaf area unit or per chloroplast, was similar to that of mesophytic plants. Photosynthetic capacity was dependent on the activity and content of Rubisco. The lowest photosynthetic capacity (3 mg/(dm2 h)) and the lowest Rubisco content (8 mg/dm2 and 30 mg/109 chloroplasts) were found in submerged leaves. The rate of photosynthesis reached 20-30 mg/(dm2 h) in floating and aerial leaves. These leaves contained 60 mg Rubisco/dm2 or 65 mg/109 chloroplasts. It was concluded that, at early developmental stages of floating and aerial leaves, the aquatic environment mainly affected the functional parameters of the photosynthetic apparatus, i.e., the pigment complex, photosynthetic capacity, and the content and activity of Rubisco.