Mammillaria is one of the largest and most diverse subgroups of the plant family Cactaceae, commonly known as the cactus family. The genus contains perhaps as many as 300 different species of flowering cactus. Its name comes from the Latin word for nipple, "mammilla," a somewhat humorous description of the fleshy bumps called tubercles that typically cover the cacti. In fact, a common name for cacti from the genus Mammillaria is nipple cactus; the plants are also referred to by the common names fishhook cactus and pincushion. Most species of Mammillaria are very easy to grow and maintain, making cacti from this genus popular as potted or outdoor plants.
Since Mammillaria contains so many unique species, many species look very different from one another. Most species are small to medium in size and are covered in the protrusions that give the plant its name. However, the shape of the plants and their needles often vary greatly. Mammillaria cacti may be round, oval, or shaped like cylinders or a group of cylinders. Their spines will vary in length and texture, with some having finer, softer spines that resemble hair, and others having sharper, bristlier spines. Some nipple cacti do not have spines at all.
These cacti are native to the Americas, and a number of species can be found in great abundance in Mexico. In their native desert environment, Mammillaria cacti are accustomed to dry or sandy soil and hot daytime temperatures that can plummet sharply at night. The flowers of these cacti are usually only visible during the day when it is warm. Like so many of the plants' other features, these flowers vary in shape and color and may be yellow, red, pink, or white. The cacti also have fruits that follow the flowers, which can be an abundant source of seed for cactus hobbyists wishing to grow and breed their own cacti.
Most Mammillaria cacti are low maintenance plants that can thrive as potted plants either indoors or outdoors. For the most part, the cacti prefer warm conditions, although they will tolerate temperatures as low as about 50 degrees Fahrenheit (10 degrees Celsius), particularly at night. During the summer months, the plants may be kept outdoors in full sunlight, and then moved indoors when colder weather comes. Like other cacti species, these plants are succulents that generally require little watering. Beginning in the fall, the plant can be watered less and less to encourage it go dormant for the winter.