Classifying an organism lists the different groups to which it belongs. The first category is the kingdom, followed by the phylum, division, class, subclass, order, family, genus and species. With each subsequent category, the characteristics of the organism become more specific and defined. Cacti, as plants, are members of the kingdom Plantae.
Phylum and Division
Cacti belong to the phylum Tracheophyta. These are vascular plants. Cacti belong to the division Anthophyta, which are angiosperms or flowering plants.
Class and Subclass
Cacti are part of the class Magnoliopsida, also called dicots, and subclass Caryophyllidae. Plants in this class and subclass are generally insect pollinated. The seed has two cotyledons.
Order and Family
Cacti belong to the order Caryophyllales and the family Cactaceae. Caryophyllales is a large group of 26 families of flowering plants with about 12,500 species. These include the succulent plant families Aizoaceae, Cactaceae (cacti), Didieraceae and Portulacaceae (purslanes) and the carnivorous plant families Droseraceae and Nepenthes. The Caryophyllales also includes important horticultural and agricultural crops: beet, bougainvillea, Caryophyllaceae, which are carnations and pinks, plumbago, rhubarb and spinach.
There are approximately 150 genera in the Cactaceae family. They are further divided into three subgroups or tribes. These are Pereskieae, Opunitieae and Cactoidea. Most cacti fall into the Cactoidea subgroup. Two genera, Pereskia and Maihueniahe Persicae make up the subgroup. Cacti in these genera have woody stems and persistent or semideciduous leaves. The areoles are spiny and woolly at the leaf axils. The cacti also have pendulous flowers and fleshy fruit. The Opunitieae are the prickly pear varieties -- Pereskiopsis, Pterocactus, Quiabentia, Tacinga. These varieties have fleshy, branches; the areoles always have fine hairs and the leaves are flat or cylindrical. The fruit is a seedy berry.