It is the branch of chemistry concerned with plants and plant products. Specifically, it describes the large number of secondarymetabolic compounds found in plants. Many of these are known to provide protection against insect attacks and plant diseases. They also exhibit a number of protective functions for human consumers. 

Related Journals of Phytochemistry 
Journal of Pharmacognosy and Phytotherapy, International Journal of Phytotherapy Research, International Journal of Phytotherapy and Ethnobotany,Journal of phytotherapy and pharmacology