Plants comprise one kingdom of living things, the other kingdoms being animals, bacteria, fungi and protists. Scientists classify plants with a series of progressively smaller groups based on shared characteristics or lineages. For the average person, especially the gardener, only from the family designation are plant names and relationships most useful. The manipulation of plants by man finds the plant kingdom increasing in numbers, mainly with the addition of plant varieties called cultivars.
Primary Division of Plants
The plant kingdom is first divided into what is called vascular and non-vascular plants and refers to the presence of a vascular system, like our pulmonary system. More primitive plants, the nonvascular types, are known as mosses and liverworts. Since they lack tissues to transport water and nutrients, they are primarily confined to moist environments, according to the A-Z Encyclopedia of Garden Plants. Vascular plants, which have xylem and phloem cells, include all other plants that survive in a variety of habitats all over the world.
Because the vascular plants are more diverse and have more relevance to the needs of mankind, further classification is warranted. Vascular plants are further categorized as flowering or nonflowering. Flowering plants are called angiosperms and produce seeds in an ovary that are covered with some sort of tissue, such as a cap, fruit flesh or nutshell. The largest family of angiosperms is the orchids. There are more than 250,000 species of flowering plants in the world. Nonflowering plants are known as gymnosperms and develop naked seeds on scales in cones. The largest group of gymnosperms is called conifers and includes plants like pines and spruces. Cycads also are gymnosperms.
Flowering plants are then categorized based on the number of seed leaves—the first leaves that emerge from a germinating seed. Monocots bear only one seed leaf and develop into plants with parallel veins. Dicots develop two seed leaves and become plants with branching, complexly veined organisms.
Although not a core scientific characteristic to differentiate among all plants, the life span or general structure is used to describe plants and categorize them, too. Plants whose life cycle is one growing season are called annuals. Biennials live for two growing seasons, usually maturing and flowering in the second season. Those plants that return year after year are perennials. Perennial plants include soft-stemmed herbaceous types (like a peony or waterlily) and woody types (like stiff-stemmed and barked trees and shrubs). Other categories include water-dwelling plants called aquatics, as well as plants that grow in soil (terrestrial), upon other plants (epiphyte), on rocks (lithophyte) or below the soil surface (geophyte).
Practical Plant Nomenclature
Scientifically, plants are assigned names that assign them to phylum, class, order, division, etc., that encompass many of the features already discussed, as well as similar flowering structure. For the typical person or gardener, the family, genus and species classifications are what is most important, because these three levels distinguish all plants from one another. A family (like the palm family Arecaceae or rose family Rosaceae) is a group of plants that share common features. Within families are genera (plural of genus) that further group plants with similar characteristics more closely. A genus example is Tulipa, which encompasses all tulips. The species is defined as a plant that can breed with each other and create new plants. Localized natural mutations also lead to relevant classification groups called subspecies, varieties, forms. Plants created or selected by man for any number of agricultural or ornamental uses are called cultivars (a conjunction of the words "cultivated" and "variety").