Anatomy and Structure
The distinction between the Cactaceae and the order succulents is that the Cactaceae contain areoles. An areole is a structure that bears spines, the buds that eventually turn into flowers and then fruit, and sometimes the areoles bear glochids. On most cacti, the areole is located on the tubercle or elsewhere on the rib region. Only several species in the Opuntia genus contain glochids (located on the areole). The rest of the order succulents (not including the Cactaceae) contain spines that grow not from the areoles, but directly from the succulent tissue. The adaptation of areoles in the cactus family is essential, thus if the spine of a succulent is removed, the tissue surrounding the spine is damaged. Rather when a spine is removed from an areole, the tissue is not damaged because the areole bears the spine, the tissue does not bear the spine on a cactus.
A spine is termed as a structure developed from a leaf. All spines in the Cactaceae are located on an areole. Spines are identified into two groups: the central spines and the radial spines. The central spines are usually located on the center of the areole; the radial spines are located around the margin of the areole. The adaptation of spines is used for several purposes. The main purpose of spines is to protect the plant from predators seeking food or water from the cactus. Another purpose of the spine is when atmospheric water (mists) falls onto the spine, the structural shape of the spine concentrates the atmospheric water into water droplets, thus falling onto the soil and the water is absorbed by the shallow root system of the cactus.
A cactus flower is very similar to other flowering plants. Its colorful blooms and scents attract pollinators. The flowers that are located in arid regions that will receive prolong periods of drought are designed to open as soon as it rains, thus leaving its dormant stage to flower. When they are pollinated, some can produce seeds in less than one month. Most desert flowers bloom during the day and produce colorful, unscented blooms that attract mainly flying insects. Many jungle species have large blooms that open at dusk; these are pale and richly perfumed so that pollinating moths can easily locate them. By contrast, some flowers are nighttime-flowering that have foul-smelling, fleshy blooms that attract bats.
Cacti have an extremely shallow and wide root system, enabling them to absorb water quickly, especially during dry periods when water will evaporate quickly. For example, Cannon (1911) found the roots of a large barrel cactus (Ferocactus wislizenii) to occur at an average depth of 3 cm. Even during long periods of drought, distal portions of the roots remain receptive to some water and rapid growth of the root hairs makes much more available. The fallen joints of chollas, prickly pears, fragments or uprooted stems of cacti can root rapidly. In certain genera this may result in asexual reproduction, thus during periods of unfavorable conditions, branches or even fruits of some species may root and grow.