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 or polyphenol, especially in spring when the plant grows fast. By soaking the plant in the soda and vitamin C solution the phenol or polyphenol is released making the plant suitable as a developing agent. Still, this is a relatively weak developer, but by adding sunlight (or artificial light) the developing process is speeded up dramatically. The area were the plant makes contact with the photosensitive emulsion will darken and a careful application of a leaf or petal will result in a stain with a similar form. Sunlight that filters through the leaf or petal influences the process of staining, resulting in variations in tone revealing the internal structure of the plant in detail (thanks to Kevin Rice for getting this explanation straight).
In addition some plants release other chemical compounds that can result in subtle variations in colour (brown, sepia, green) more rarely also bright colours occur (red, purple). These colour effects are dependent on the type of film emulsion used. The exact mechanism driving these colour variations is not entirely clear to me but seems to be influenced by the sensitivity of the film, the fineness of the film-grain, the plant species used and the amount of available sunlight. I hope to discuss this with a person who has understanding of both phytochemistry and photochemistry to gain further understanding of these effects.