<P> Plants are composed entirely of chemicals of various kinds . Phytochemicals (from Greek phyto, meaning "plant") are chemicals produced by plants through primary or secondary metabolism . They generally have biological activity in the plant host and play a role in plant growth or defense against competitors, pathogens, or predators . </P> <P> Phytochemicals generally are regarded as research compounds rather than essential nutrients because proof of their possible health effects has not been established yet . Phytochemicals under research can be classified into major categories, such as carotenoids and polyphenols, which include phenolic acids, flavonoids, and stilbenes / lignans . Flavonoids can be further divided into groups based on their similar chemical structure, such as anthocyanins, flavones, flavanones, and isoflavones, and flavanols . Flavanols further are classified as catechins, epicatechins, and proanthocyanidins . </P> <P> Phytochemists study phytochemicals by first extracting and isolating compounds from the origin plant, followed by defining their structure or testing in laboratory model systems, such as cell cultures, in vitro experiments, or in vivo studies using laboratory animals . Challenges in that field include isolating specific compounds and determining their structures, which are often complex, and identifying what specific phytochemical is primarily responsible for any given biological activity . </P> <P> Without specific knowledge of their cellular actions or mechanisms, phytochemicals have been used as poison and in traditional medicine . For example, salicin, having anti-inflammatory and pain - relieving properties, was originally extracted from the bark of the white willow tree and later synthetically produced to become the common, over-the - counter drug, aspirin . The tropane alkaloids of A. belladonna were used as poisons, and early humans made poisonous arrows from the plant . In Ancient Rome, it was used as a poison by Agrippina the Younger, wife of Emperor Claudius on advice of Locusta, a lady specialized in poisons, and Livia, who is rumored to have used it to kill her husband Emperor Augustus . </P>

Which of the following is a function of phytochemicals