The best answer is pneumatophores.
In the environment where mangroves grow, oxygen is very limited. This necessitates mangrove root system to take up oxygen from the atmosphere. For this purpose, mangrove species have specialized above ground roots called pneumatophores or breathing roots.
In some species, these roots are pencil sized and peg-like while in others they look like a knee. These roots have numerous pores through which oxygen enters into the underground tissues. In some plants, buttress roots function as breathing roots and also provide mechanical support.
