(biochemical taxonomy) The application of the principles and procedures of chemical analysis and the results obtained to the classification of plants. Although the science has diverse and ancient origins (e.g. the medicinal and ethnic interest in the chemical constituents of plants) it has only made significant growth in recent decades. Plant product analysis has been simplified by the development of chromatography and electrophoresis, which with other new techniques, make routine analysis of a representative selection of material a possibility that was not once available. Many different compounds are potentially of taxonomic value and three main groups are recognized; primary and secondary metabolites and *semantides
. Primary metabolites are those involved in the essential biochemical pathways of the plant. As such they are virtually ubiquitous through the plant kingdom and are rarely useful taxonomically. Secondary metabolites are those substances accumulated by plants that have traditionally been regarded as waste products, e.g. terpenoids, alkaloids, phenolics, etc. Often distinct discontinuities are found in the distribution of secondary metabolites, which have been used to delimit taxa.