Final answer:
The mesophyll layer, specifically the palisade and spongy mesophyll, is responsible for most photosynthesis and carbon dioxide absorption in a dicot leaf.
Step-by-step explanation:
In a dicot leaf, the layer of tissue responsible for most photosynthesis and carbon dioxide absorption is the mesophyll. This layer is further divided into two parts: the palisade mesophyll, which is made up of column-shaped, tightly packed cells located beneath the upper epidermis, and the spongy mesophyll,
which has irregularly shaped, loosely arranged cells beneath the palisade mesophyll. The majority of chloroplasts, the structures in which photosynthesis takes place, are found within these mesophyll layers. Carbon dioxide enters the leaf through openings known as stomata, which are found mostly on the underside of the leaf and regulated by guard cells.