In submergent plants, air passages not only provide buoyancy but also allow for oxygen storage, to supply oxygen at night. Since submergent plants must obtain oxygen from the water, their leaves are very thin and extensively partitioned, allowing for gas exchange directly through the cell walls. Water marigold has this type of leaf below the surface, but the leaves above water exchange gases through stomata's. A variety of reproductive strategies are found among aquatic plants. In many cases, plants reproduce vegetatively some or all of the time.
Some plants are perennial.
Even among submergent plants, pollination and seed dispersal usually take place above the water's surface. Wild celery produces male and female flowers. The female flowers are suspended on long stalks extending to the surface. Male flowers develop underwater, float to the surface, open, and drift until they contact a female flower. The female flower is pollinated and then coils down into the water, where the seeds develop.