Water plants and land plants require far different adaptations to survive in their respective environments. Most scientists believe that plant life on land can be traced back to plant life in water. As plants adapted to growth on land, they changed to deal several important factors that made it difficult for them to grow. Aquatic plants keep various adaptations that allow them to survive in water. This makes the two plant types different from either in several ways.
Soil actually presents a problem for land plants. While it does allow them to anchor into the ground firmly with their root system, it is more difficult for them to get nutrients from the soil than the water, which presents nutrients everywhere in a liquid medium. As a result, aquatic plants absorb compounds straight into their tissues, while land plants produce tiny root filaments and leaf pores to taken in their nutrients.
Plants in the water do not need to worry about gravity. They move with the water and need relatively little support structure. Plants on land, however, need strong stems and woody cells to withstand the force of gravity pushing them down. This is why plants on land have more complicated growth structures and produce starchy compounds to thicken their cell walls.
Air does offer some advantages to land plants. In the water, especially salt water, aquatic plants must develop ways to protect themselves from unwanted compounds like salt. Land plants do not need to worry about the presence of these particles or the chance of them seeping unwanted into their leaves, although roots may still be a problem.
Water plants tend to create pollen and seeds that can float across the water in order to survive. Waterlogged plants like mosses also do this. Land plants, however, cannot spread pollen or seeds in such a manner (except for certain species like coconut trees). Land plants reproduce using the wind, animals, and the buoyancy of the air itself to spread their seeds.