You are referring to the green pigment known as chlorophyll. It is found in cell organelles called chloroplasts. These are located in the cytoplasm of plant cells. These organelles are classified as plastids. Plastids are responsible for manufacturing and storing important chemical compounds that will be used by the plant cell. Chlorophyll can absorb sunlight to be used in photosynthetic reactions. Chlorophyll appears green because the green wavelengths of light from the visible spectrum are reflected, while the wavelengths for blue and red are absorbed the most by chlorophyll. Plants use this light energy to carry out photosynthesis, which occurs in the green parts of plants--stems and leaves which contain cells with many chloroplasts.