Mexico has the greatest floristic diversity of cacti of any country in the world. Cacti are particularly plentiful in Mexico's arid, warm, dry regions. Over the last several decades, important floristic, systematic and ecological studies that investigate the sustainable management of natural resources, have made cacti one of their research priorities. In addition, cacti can be used for decoration due to their strange shapes and colourful flowers. This has made them increasingly important on the growing ornamental plant market. Cacti are distinguished from other groups of plants or botanic families by the morphology of their stems and flowers. Without going into great technical detail, they are practically without leaves, although there are some notable exceptions like the Pereskia lychnidiflora (matiare) and the Pereskiopsis aquosa (water prickly pear). They usually have spines (there are exceptions to this, such as the Lophophora williamsii (peyotel), and their fleshy stocks take many different shapes. For example, some are flat and shaped like tennis rackets, such as the Opmtia (nopal), a common Mexican cactus which is eaten grilled or boiled and chopped up in a salad. Others have cylindrical stocks of different widths and heights, like the Cephalocereus semilis (the viejito, or "old man"), which can grow to as high as 8 meters and is typically shaped like a column. Another cylindrical cacti is the Ecinocereus schmollii (known as the "sheep’s tail"), which grows no larger than 10 centimetres and is just a tiny reed no thicker than a pencil. Several kinds of cacti, like the Echinocactus (the ‘burro biznaga) end the Mammillaria (the "little biznaga"), are spherical. However, the most distinctive feature of cacti is that they have little pads covered with tiny, light-coloured hairs called areolas. The spines and flowers sprout from the areolas: they are structures similar to lateral buds in other flowering plants. "Cactus" comes from the Latin word for ‘thistle’ it is a broad term applied to any spiny plant. Today, however, the Latin word "cactus" (plural ‘cacti,') applies only to the genu, we have described above. The cactus family (Cactaceae) includes more than 1,500 species. Taxonomically, it is divided into three sub-Families: the Pereskioideae, with only two geniuses, one of which have leaves in the shape of sheets; Opuntioideae, with five geniuses and over 200 species, including nopales, chollas and Cactoideae, the largest group, which is found throughout the Americas and includes tree- and bush-like plants, vines and epiphytes with cylindrical, spherical or flat stocks. Arid and semi-arid zones are characterised by minimum and irregular annual availability of humidity, low atmospheric humidity (with a few exceptions, like the arid region of Baja California) high daytime air temperatures and abundant sunlight, which can raise the ground temperature as high as 60® C. These physical characteristics make it easy to support that, to adapt plants and animals native to these areas must make efficient and economical use of water. Cacti’s thickened stems, roots or leaves do just that: they store water to survive during dry periods. Thickened plants are known as succulents, of which there are wide variety by practically all species of cacti are included. The evolution into cacti of some particularly succulent South America plants was basically the result of a process of adaptation to seasonal periods of drought. Today, cacti grow in all the main natural regions of the Americas, from damp tropical forests to the bush area of the arid zones. In tropical forests, the only cacti species are creepers or epiphytes on the trunks of large trees, in arid zone, they are trees, bushes or vines. However, most of the species grow in intermediary areas, in what are called dry tropical forests, with both a dry and rainy season. They flourish in these areas because they specialise in absorbing, storing and conserving humidity with maximum efficiency developed during gradual, profound changes in their anatomy, morphology and physiology. The change and reduction of the leaves are part of this evolution. At first, primitive cacti tended to increase storage of water in their leaves and reduce the vascular system to minimum functional structures. However, a new evolutionary phase was needed to reduce transpiration to a minimum: the surface of the leaves gradually diminished to microscope size. Today, many cacti have only this kind of “leaves” and only during the earliest stages of growth. Simultaneously, the stocks also had to change to take on the functions of storing water, breathing and photosynthesis. Besides becoming a succulent, the stock has taken on whimsical forms that give cacti minimal volume and surface area: they are flat, cylindrical, column-shaped and spherical. For the cactus to change its volume without affecting its internal form and structure, the stock had to change: the base of its leaf pads became tubers which expand when water is available or contract in the dry season. Geniuses like Mammillaria and Coryphantha have this kind of tuber. In other cacti, these tubers are fused in the form of lengthwise ribs, forming a sort of accordion which expands or contracts depending on the amount of water stored. The Pachycereus, the Ferocactus and the Acanthocereus, among others, have this kind of structure. Another important part of the survival of cacti in arid and semi-arid regions is their relationship with animals and other plants. One important form of biotic interaction in these areas is the interdependency during the first stages of seedling development, particularly in the phase of initial formation (germination and early survival). Some ecological analyses point to the importance of trees and bushes which beneath their canopies provide a micro-habitat favourable to the growth of cactus seedlings and other vegetation. The longevity of some tree- or column-like cacti is significant for these biological systems. In Mexico there are almost 60 species of this sort. Among those already researched are the Neobuxbaumia tetetzo (teteche) in the Tehuacán Valley in the state of Puebla, reported to live up to 400 years, and the Carnegiea gigantea (saguaro) from the desert of Sonora, estimated to be up to 500 years of age. Mexico has the greatest floristic diversity of cacti of any country in the world as it boasts approximately 850 species of the slightly more than 1,500 reported in recent literature about the Americas cacti. It is estimated that almost 700 species (nearly 80 percent of all the cacti in the country) are endemic and originate in Mexico. Whereas, in Brazil and the United States, for example, floristic inventories on cacti show less than half the number of species to be found in Mexico. In addition, Mexico has been the seat of the evolution of some taxonomic groups of cacti. For example, the column - and tree - like cacti (Pachycereeae group) and the spherical ones, better known colloquially in Mexico as biznagas (Cactae group) originated and diversified in Mexico. A reason for this wealth of vegetation is due to the geographical location of Mexico as it has led to the diversification and development of a group of creepers and epiphytes (Hylocereeae group). Today, sustainable use of natural resources is a priority above all in countries with high biodiversity like Mexico which also has a considerable gamut of economically exploitable flora. These plants were used by men as far as back as the first inhabitant of the Americas. Anthropological evidence of their use as food by primitive man has been found at archaeological digs in Tehuacán, Puebla and in the state of Tamaulipas. Cactus stocks, fruit and seed were part of the diet of the people who inhabited these areas as far back as 6,500 as. C. During the rise of the great Mesoamerican cultures some cacti were already an important part of the diet and culture. Today, ethnobotany has proved to be valuable to different human groups in Mexico. For example, the Seris, an indigenous group which lives in Mexico’s northern state of Sonora, use 12 species of cacti for food, herbal medicine, construction, hunting and religious rites. Another important use of cacti has been the selection and cultivation of their edible fruit, harvested both from wild and semi-cultivated plants. This is the case of the pitayo (Stenocereus pruinous, S.queretaroensis), garambullo (Mytillocactus geometrizans) pitahaya (Hylocereus undatus) jiotilla (Escontria chiotilla) and above all the prickly pear cacti (Opuntia amyclaea, O. hyptiancantha Opuntia megantha and a great many hybrids). Among the cultivated edible stocks, the most important is the nopal (opuntia ficus-indica, Opuntia megacantha and many hybrids). Other ways of utilising these plants include: Medical uses: as a diuretic, a laxative, antispasmodic and to treat fevers, ulcers and hypoglycaemia, ads roughage; as a second food source (for cool or fermented drinks, a condiment or snack and can be grounded into flour); and a colour additive (nopal cochineal seeds etc.) One of the best-known uses of cacti is ornamental, particularly the rarer species with limited distribution like the majority of those endemic to Mexico. The largest markets are the United States, Europe and Japan where cacti are intensively cultivated and marketed. Integral research on cacti is clearly central to their conservation and the management of their natural and cultural diversity. It is a way to define strategies for their preservation at the time when “the crisis of biodiversity” – the disappearance of animal and plant species – is of great concern and interest in countries like Mexico.