Pneumatophores Roots are meant for the absorption of water and minerals from the soil. Cells of roots require $\mathrm{O}_2$ to respire. In swampy areas, soil does not have air, so no $\mathrm{O}_2^2$ is available to them.
In such cases, roots come out of the soil showing negative geotropism and breathe after coming in contact with air, e.g., Rhizophora. Such roots are called pneumatophores or respiratory roots.