Although the first plants grew in water, their modern descendants have evolved again and again as the earth and its atmosphere have changed; if plants are to survive, they must adapt. The plants we place in our wet gardens, lakes and ponds reflect the adaptations that helped mold our environment and began all plant life on earth.
According to paleobotanists including Robert Blankenship at Arizona State University, aquatic plants may be responsible for changing the dominant gas in Earth’s atmosphere from nitrogen to oxygen and the development of the protective ozone layer as many as 3500 million years ago. The development of bacteria and photosynthesis began a process of adaptation that included the development of roots, branches, flowers and sexual reproduction.
Plants adapted to the land and to environments created by glaciers, meteor strikes and geological shifts. During this process, some plants turned back and re-adapted to the aquatic environment, providing a wide variety of plants to choose for wet gardens.
Aquatic plants succeed because they are able to adapt to a low-oxygen and lack of infrared and ultraviolet light. Other factors that may never pose problems for land-dwelling plants—salinity, algae and barriers to sexual reproduction—force adaptation in water-dwellers that determine their place as emergent, floating or submergent plants in the aquatic environment of ponds or lakes.
Plants returning to water had to adapt to the lower availability of oxygen. Emergent plants like cattails and grasses and floating plants like water lilies and lotus adapted by developing more efficient cell structures with spaces between cells, called aerenchyma, to collect carbon dioxide (a molecule containing one carbon and two oxygen atoms). Photosynthesis moved it to the roots where it combined with other nutrients to produce root tissue, ethylene and more aerenchyma tissue. Roots also grew deeper and wider to anchor plants.
In submergent plants like anarchis, fanwort and red ludwigia, the space between cells increased to take more than half the space in the plant, forming channels throughout the plant called lacunae. Some submerged plants formed “air roots”, fibrous roots along stems and branches that absorb oxygen. Some collected carbon dioxide through holes at the ends of branches.
Red and blue light waves are the most important light for photosynthesis, the process that provides energy to combine carbon dioxide and nutrients to form new plant tissue. Green light is reflected, giving plants their distinctive coloring. As water depth increases, more red and blue light is filtered out, leaving a higher percentage of useless green light. Algae blooms can block red and blue light entirely, causing submerged plants to die of starvation. Wetland and aquatic plants adapted by developing increased sensitivity to light and abilities to use carbon dioxide more efficiently. As with terrestrial plants, excess oxygen was discarded, earning submergent plants their reputation as “oxygenators."
Considerations for Wet Gardens
Adaptations of aquatic plants success may require special care in the wet garden. Special barriers and organs that allow plants to live in seawater may strangle them in freshwater lakes. Some, like the Chinese lotus, can remain dormant for years, waiting for optimal conditions. Greater reliance on vegetative reproduction, seeds that float or germinate while still on the flower and the need for dry and wet seasons may control where wetland plants can be used.