In $Opuntia$,the stem is modified into a flattened green structure to perform the function of leaves (i.e.,photosynthesis). Cite some other examples of modifications of plant parts for the purpose of photosynthesis.

(N/A) $\rightarrow$ $Opuntia$ is a xerophytic plant,in which leaves are modified into spines to reduce the rate of transpiration,and thus they cannot perform photosynthesis. Therefore,photosynthesis is performed by the stem in $Opuntia$,which is a thick,fleshy,and flat structure. It possesses chlorophyll and stores food. Such a structure is called a phylloclade.
$\rightarrow$ Sometimes,stems are modified into internode-like structures that perform photosynthesis,e.g.,$Asparagus$. These are at the level of scaly leaves,while true leaves are modified into either scaly leaves or spines.
$\rightarrow$ Similarly,in some plants,there are assimilatory roots,e.g.,$Trapa$ and $Tinospora$.
$\rightarrow$ These roots develop outside the soil,contain chloroplasts,and perform the function of photosynthesis.