Mesophyll is the tissue between the upper and lower epidermis of a leaf. It contains a high concentration of chloroplasts, the cells responsible for photosynthesis. The chloroplasts are clustered together vertically in the upper layer of the epidermis. The chloroplasts in mesophyll are very close to the surface of leaf. These specialized cells use energy to convert carbon dioxide into energy. Chloroplasts contain chlorophyll, a unique chemical element that absorbs, stores, and converts carbon dioxide. This substance absorbs all wavelengths of color except green. Instead of absorbing the wavelength corresponding to green, the substance reflects that wavelength. Thus, mesophyll appears green to us.
mesophyll is green coz it contains a green coloured pigment called 'CHLOROPHYLL'.it is this pigment which plays a key role in the process of photosynthesis in plants.