$\rightarrow$ Opuntia is a xerophytic plant. Due to dry atmosphere, less water is available from soil. So to prevent waste of water, leaves are shed or became small or get converted into stipule/spine. Thus in absence of leaves to perform photosynthesis, stem becomes broad, flat green and fused with chlorophyll that acts as a leaf. Thus the modification of stem to leaf like strucutre is called phylloclade.