A Cactus, in botany, is a member of the family "Cactaceae". This family include more or less 120 genus and 3000 species and is part of the order "Caryophyllales".
Within the family we can find plants of very different sizes: the smallest cactus is "Blossfeldia liliputana", with a maximum diameter of 1 centimeter, and the biggest is Pachycereus pringlei, with a maximum height of 18-19 meters.
The word "Cactus" comes from a greek word, "Kàktos". Even if the ancient greeks couldn't know Cacti, which are mainly native from America, this word has been used by Theophrastus in his book, "Historia Plantarum", to describe an unknown, spiny plant. Then the term "Cactus" was adopted by the famous botanist, Linneo, to define a genus of thorny american plants. The following botanical reviews actually abolished this term. Nowadays, the word "Cactus" is colloquially used to define plants belonging to the family of Cactaceae, but there is a confusion between Cacti and Succulents. Succulents is a more generic term to describe plants which have developed systems to store water in arid environments, while "Cactus" means "plant belonging to the Cactaceae family".
Cactis have generally unusual shapes, colours, form and dimentions of their flowers, which gives them the capacity of adaptament to extreme climates and also make them really appreciated as ornamental plants.
Cactaceas are mainly widespread in North and South America. Anyway, due to the exchanges with America, Cacti have been introduced also in Europe and in the other continents. The genus Rhipsalis is also present in tropical Africa and different species of Opuntia have been introduced in Africa, Australia and India. In Italy we can find many spontaneous Opuntias and a few spontaneous Cereuses in the Southern regions.
Their habitats are generally deserts or anyway, very arid climates. Few cactis grow in tropical forest as epiphytes, which means that they grow upon other plants.
Cactaceae are weed or trees with thorny stems, generally differentiated in macroblasts usually cilyndrical, conic, globose or flattened, generally succulent. They can have photosyntetical leaves or not.
The most common shape of cacti is the cilyndrical one (such as in the genuses of Espostoa, Cereus, Cleistocactus), simple or branched, which, over the years, can become columnar and arboreal, for exemple in the case of the mexican Saguaro (Carnegiea gigantea), probably the most famous Cactus in the world, it can become 20 meters tall and its flowers are the symbol of Arizona.
There are also several short and globose-shaped species. This is a kind of adaptation to arid condition: the form of these cacti garantees a low rate surface/volume: this means that the surface is reduced, proportionally to the Volume. This minimizes water loss through transpiration (water is always expelled through the surface of the plant). The most famous globose cactus is the "Mother-in-law's pillow" (Echinocactus grusonii) an almost sphearical cactus, also called "Golden ball".
Another existent stem form is the flattened one, typical of the stems of Opuntias. In Opuntias we have peculiar flattened segments, the Cladodes, of various size depending on the species.
Leaves, when present, are generally reduced and prematurely caduceous. The only exception is in the genus Pereskia, considered as the link between cacti and all the other plants. It has been included in the Cactaceae family because it possues Areoles.
Stems in Cactaceae are generally greyish-green and fleshy. They fulfil the function of photosynthesis, normally held by leaves, which are generally reduced or either absent in this family.
These particular stems have the peculiarity of being divided in typical ribs, to expand and contract like an accordion depending on the quantity of water stored: if it'abundant, they expand and become more fleshy; if it's scarce, they contract and become more wrinkled and dried out.
Upon the stems there are the Areoles, typical feature of this family, which are kinds of bumps in which develop thorns, hair, bristles, buds and flowers. Areoles take on the function of the nodes in other plants. Even if it looks like a tree and its trunk is hard and apparently lignified, its tissues are spongy as in the other cactis.
Cacti's spines have the feature of coming off easily if they penetrate the skin, unlike the ones of Euphorbiaceae, which cannot be detached without hurt.
Spines are solitary or in groups and they always develop from the areoles.