Industrial developers often contain the active ingredients metol and hydroquinone. These two chemicals are superadditive, they have a synergistic effect. Both can be classified as phenols, a group of chemicals containing one or two electron rich groups of atoms, which provide the necessary electron to initiate development. Coffee contains several phenols making it suitable as a developer, especially in combination with vitamin C which provides the superadditivity similar to industrial products. This occurrence of phenols in coffee is the basis of Caffenol developer.
Many plants also contain some sort of phenol, polyphenol or terpenoid (another reactive group of chemicals), especially in spring when the plant grows fast. By soaking the plant in the soda and vitamin C solution the phenol, polyphenol or terpenoid is released making the plant suitable as a developing agent. Still, this is a relatively weak developer, exposure to sunlight will speed up the developing process dramatically. The area were the plant and the photosensitive emulsion touch the chemical process is set in motion. A careful application of a leaf or petal will result in a detailed image. Sunlight that filters through the leaf or petal influences the process, resulting in variations in tone revealing the internal structure of the plant in detail.
In addition some plants release chemical compounds that can result in subtle variations in colour (brown, sepia, green). Also bright colours (red, purple) occur around the plants on the area of the paper or film that is fully exposed to the ultraviolet rays of the sun. These colour effects are dependent on the type of film emulsion used and the expiry date of the material. It is much more difficult to get the same colour effects when artificial light is used.
Phytogram 