There are many classification systems for crops. For example, according to the growing year, there are annual, biennial (or perennial) and perennial crops. According to the growth conditions, there are dryland crops and paddy crops. According to the use, it can be divided into food crops, cash crops, fruit trees, vegetables, feed and green manure crops, trees, flowers, medicinal crops and so on. But all kinds of crops are inseparable from the most fundamental is botanical classification.

The commonly used taxonomic units in the plant kingdom are: department, class, order, family, genus and species. Taxonomic units at all levels are sometimes too large to fully include their characteristics or systematic relationships. When it is necessary to add another level, the word "sub" should be added before each level, such as subfamily, subgenus and subspecies. In addition to subfamily, the following families sometimes combine similar genera into one family; In addition to subgenus, sometimes similar species merge into a group or series. In botany, the following species classification is often divided into varieties, forms or races.

Classical plant classification can be said to have started from18th century. Linnaeus (C.Linnaeus, 1735) proposed a classification based on the difference of sexual organs. In his book Natural System, he divided the plant kingdom into 24 categories according to the number and characteristics of stamens and their relationship with pistils. Then he listed 63 orders in the factory (1738). In the19th century, de Candolle and his son divided plants into 135 orders (families) according to their similarity, and then developed into 2 13 families. Since the publication of Darwin's Origin of Species in 1859, plant classification has gradually changed from natural classification to phylogenetic classification. Darwin's theory has three influences: ① "species" is not specially created, but evolved from another species in the long river of life, and it has been evolving; (2) True natural classification must be based on genealogy, that is, all species come from a common ancestor; ③ "Species" are not displayed by "patterns", but are composed of constantly changing populations (Wu Zhengyi et al., 2003). The scientific botanical classification system is a systematic classification system, that is, it should objectively reflect the genetic relationship and evolution of natural organisms, so now the generalized taxonomy is also called systematics. In recent decades, plant taxonomy has applied all kinds of modern science and technology, and derived research basins such as experimental taxonomy, chemical taxonomy, cytotaxonomy and numerical taxonomy, especially the development of biochemistry and molecular biology, which greatly promoted the development of classical taxonomy from description stage to objective experimental science.