Seagrass fills numerous important roles in the ocean habitat. It is a nursery for young animals, a food source, an oxygenator, a home and a hiding place for many species. The ocean currents help fertilize the plants and then carry the seed to an appropriate growing site. Seagrass spends its entire life underwater and is found in a variety of species. Seagrasses are not truly grasses but carry the name due to their long flowing blades that resemble grassy leaves. The many varieties of seagrass provide important organic detritus to the sea floor and uncounted benefits to sea life.
Seagrasses evolved 100 million years ago. Seagrass is an angiosperm that reproduces by flowering and seed. It is characterized just like any vascular plant by its roots, stems, reproductive organs and leaves. Seagrasses have veins that perform the essential function of moving plant sugars and nutrients up and down the body of the plant. They perform photosynthesis like any terrestrial plant and reproduce in much the same way. It is thought they developed from wind-pollinated plants and then became hydrophylous due to opportunity. The fact that they evolved from terrestrial flowering plants is undisputed, but then the plant had to develop a complex anchoring and uptake root system to survive underwater. Reproductive processes became different due to the nature of the environment.
Simple plants began in water and then moved to a terrestrial state where they evolved into vascular organisms. The next step in evolution was the freshwater plant. These plants spread from seed and animal intervention into saltwater, and the offspring adapted. Seagrasses had to develop a tolerance for saline conditions and reduced sunlight for photosynthesis. Not all seagrasses resemble grass but most retained the long leaves and developed wider blades to capture maximum light. The veinous system is very complex in seagrasses, in order to provide food to a long plant with limited food resources.
Seagrasses produce flowers that are pollinated by sealife and coastal tides. They then produce fruits with seeds that open when ripe and disperse the seed to appropriate resting sites via wave action. Seagrasses use their root system to trap sediment and create nutritive beds for germination. In order to increase their reproductive chances, seagrasses can also reproduce vegetatively. Many species form rhizomes or underground storage organs, which can multiply and produce new shoots of the parent plant. They can also be broken off by wave action or animals and ride the tide until coming to rest in appropriate sediment where they will produce a new plant.
The plant had to develop a method of anchoring itself in water with tides and waves pulling and tugging at its foundation. The roots of seagrass are similar to any terrestrial grass but are actually smaller and have a lacunar system, which helps transfer gases to the plant vascular system. The roots have fine hairs to help increase uptake of nutrients and transfer. The roots hold oxygen in around the sediment at the base of the plant for easy retrieval. The roots and rhizomes grow together to form a dense mat that binds many plants together and creates a nutrient web that captures food for the plants and other animals.
In addition to the broad leaves that many of the seagrasses have evolved, the plants have no stomata, which are the pores that help a plant intake nutrients and air. They are covered with a waxy cuticle instead, which is porous enough to allow gas and food exchange. The extensive vein system is evident in the leaves where Aerenchyma, or thin-walled veins, are used to ferry nutrients up the vascular pathways. Most seagrasses grow in fairly shallow waters to get access to the light, which is crucial for photosynthesis.