Cactaceae are strongly xerophytic herbs or shrubs, rarely trees; roots elongated, slender, fleshy, occasionally tuberosus, often a well-developed tap-root occurs; stem fleshy, usually modified into phylloclade’s of various shapes, flat or cylindric, angular, ribbed, often globose, provided with spines or thorns, rarely bearing normal leaves.
Flowers are solitary or in clusters, rarely paniculate or spikate, usually sessile, hermaphrodite, actinomorphic or zygomorphic, epigynous, on or near the areoles or on the axils of tubercles, often clothed with stiff hairs or spinous structures.
The receptacle is a cup-shaped structure on which numerous perianth segments are spirally arranged; the receptacle with bases of perianth segments forms the hypanthium; the perianth segments gradually change from sepals to petals, often spinous at the tips.
Stamens numerous, spirally arranged on the inner face of the hypanthium; filamens free, slender, sometimes much exserted, often united to form a column or flat; anthers 2-celled, basi- or dorsifixed, dehiscing longitudinally; pollens minute, globular with finely warted exine.
Carpels 2-∞, syncarpous, forming a unilocular inferior ovary; ovules numerous, campylotropous or anatropous, with long funicles, on parietal placentas; style simple, rarely branched, stigma capitate, rarely lobed. Fruit a berry, spiny or bristly or glochidiate, often dry. Seeds numerous, often finely pitted or with smooth testa, rarely winged and with corky aril; endosperm scanty or absent.
The spines are regarded as modified leaves of an axillary branch which is represented by a small tuburcle. The tubercle with the group of spines is termed as the areole. The areoles are often furnished with glochidia or barbed bristles. The areoles are arranged in a definite phyllotaxy. In globose or cylindric species the areoles at regular intervals form the ribs by the confluence of the leaf bases in a vertical row.
Broad flat succulent leaves occur in Pereskia and Pereskiopsis. Fleshy leaves appear also in Opuntia which soon fall off. Carnegiea gigantea is the tree cactus of Mexico attaining a height of 17-18 mtr. Pereskia, Pereskiopsis and Quiabentia are shrubs or small trees. Epiphyllum has unarmed flat leaf-like phylloclades that form a jointed ribon-like stem. Cereus is epiphytic and C. giganteus attains a great length.
The cell-sap is usually mucilaginous. The internal tissues are mostly parenchymatous where water is stored. Epidermis is thick and heavily cuticularised. Mechanical tissue is present on the ridges and the stomatas are situated on the furrows.
Vessles are small, solitary or in multiples, with simple perforations; intravascular pitting alternate or opposite. Fibres are septate with simple pits. Wood parenchyma is paratracheal. Vessels are lacking in some genera due to secondary reduction; here the vessels are replaced by vascular tracheids.
Engler placed Cactaceae in the monotypic order Opuntiales and considered this order to have been derived from Parietales. From serological evidence as well as from floral morphology Mez considered that Cactaceae was allied to Loasaceae and expressed his opinion that Cactaceae should be included in Parietales.
Bessey and Hutchinson considered that Cactaceae was allied to Cucurbitaceae while Wettstein, Hallier, Buxbaum, Maheshwari and Martin suggested on the evidence of embryology, floral morphology and anatomy that Cactaceae was allied to the families included in Centrospermae (or Caryophyllales).
Takhtajan and Cronquist also hold the same view. Buxbaum also states that Cactaceae has been derived from Phytolaccaceae parallelly evolving with Aizoaceae.
Britton and Rose divided the family into 3 tribes as:
1. Pereskieae:
Leaves broad, flat; trees or shrubs with terete stem; glochidia absent; flowers pedicelled, usually clustered.
2. Opuntieae:
Succulent herbs with jointed flat or columnar stem; leaves small falling off very soon or modified into spines; glochidia present; flowers sessile, rotate.
3. Cacteae (or Cerecae):
Succulents, often epiphytic climbers; leaves rudimentary; glochidia absent; flowers sessile, funnel-shaped or salver-shaped. The family comprises about 2000 species under 50 genera (or much more genera according to some) occurring in the tropical countries of N. and S. America. Rhipsalis grows wild in Africa and Ceylon but is considered to have migrated from Trop.
America in the remote past. In India a few species of Opuntia have been naturalised and grow wild in the plains, e.g. 0. dillemi Haw (=O. elatior Mill) and 0. vulgaris Mill. Cereus hexagonus (Linn.) Mill is also met with in gardens and also as an escape in villages.
The plants of this family are cultivated for the large showy flowers and for the peculiar shape of the plants. Fruits of different species of Opuntia, especially of O. ficus- indica Mill, are sweet and edible. This species is cultivated in southern part of Europe as a fruit tree. In drier areas Opuntia-plants are used as cattle-fodder.
The cochineal insects that yield a red dye are reared on different cactii-plants, viz. Nopalea coccinellifera Solm-Dyke, and Opuntia tuna Mill. Anhalonium species are highly poisonous containing saponine and other alkaloids. Cereus grandiflorus Mill, and some species of Opuntia have medicinal properties. Species of Lophophora are used for preparing an intoxicating drink by the American Inidians on religious festivals.