Opuntia is a xerophytic plant, in which leaves are modified into spine to reduce the rate of transpiration and they do not perform the photosynthesis at all.
So function of photosynthesis in Opuntia plant is performed by stem which is thick fleshy and flattened structure containing chlorophyll and stores food. It is known as phylloclade.
Sometimes, the stem, i.e., about one internode long modifies into a leaf like structure to carry out photosynthesis, as in Asparagus. They are present in axil of scale leaves while true leaves are reduced to scales or spines.
Similarly in some plants, roots become assimilatory as in the case of Trapa and Tinospora. These roots grow outside the soil, develop chlorophyll in them and perform photosynthesis.
Pneumatophores Roots are the organs 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 $\mathrm{O}_2$ is available to them.
In such cases, roots come out of the soil shows negative geotropism and breathe after coming in contact with air, e.g., Rhizophora. Such roots are called pneumatophores respiratory roots.
In Pistia and Eichhornia, in these floating plantes, the stem is like a runner where it branches to form leaves at the apex and roots below. The roots are found near the surface of water as both the plants are hydrophytes.
