Lignin
It is a complex organic polymer that form an important structural materials in the support system of vascular plants and can also be found in some algae. This is an important molecule in the structural composition of cell wall, such as in wood and bark. This also allow woods and barks to have strong, rigid which allows them to no break easily. Lignin can also be found inserted in the cell wall of this plants.
A lemon has no colour, but when one see a lemon, the brain process the information and make one to perceive that the lemon has yellow colour.
The mental process used by the brain to do this is divided into two stages, the first stage is reflection. When light is incident on the lemon, it reflects the light energy into one's eyes, where it is transformed into neural message. The eye is made up of three different cones which are sensitive to different frequencies of light. These cones process the color and the colour perceived by the observer depend on the type of sensitive opponent cells that are stimulated. A yellow color is perceived when the yellow sensitive opponent cells are stimulated.
