Neptune's grass (posidonia oceanica) is a vascular plant, so contrary to the seaweed, it has leaves, stems and roots, and it produces flowers and fruit, but it is submerged. These plants make meadows, more or less big clusters which has leaves that grow up to the sediments. Leaves are in the shape of flat stripes that are 1 cm wide and from 10 to 150 cm long, depending on the season. Neptune's grass clusters make large underwater meadows, which are stable and long-lasting, but which can sometimes be affected by a delicate environmental balance. Posidonia oceanica is very sensitive to pollution, so it is thought to be a good indicator of water cleanliness. The increase in contamination levels on the coast has resulted in a decline of meadows. The building of purification systems and the quality control measures are improving the situation, but the meadows grow slowly and take decades to recover.