What Is a Cactus Cephalium? A Detailed Explanation of Flowering Characteristics
The primitive cactus genus Pereskia bears branching inflorescences, meaning each flower has its own individual pedicel. The flowers of other genera are borne directly on the stem. Sometimes several flowers emerge from the same areole, as in the genera Myrtillocactus, Rhipsalis and Schlumbergera. Many cacti in the family Cactaceae produce enormous flowers, and the blooms of some species are among the largest in the world. For example, the flowers of the Selenicereus macdonaldiae cultivar "Queen of the Night" can exceed 30 cm in diameter. The flowers of the genus Mammillaria, by contrast, are usually very small, forming dense rings around the stem tip, yet they are quite eye-catching as well.
Pictured: cactus flowers being borne
Whether solitary and huge or densely clustered and small, these flowers are all means by which the plant attracts insects to achieve pollination.
The flowers of most columnar cacti are borne on the sides of the stem. Small globular species also flower, such as those in the genera Echinocereus and Rebutia, whose flowers are even often borne near the base. But the flowers of most globular species are borne at the apex or at young areoles near the apex.
Flowers generally emerge from ordinary areoles, which beforehand look no different from any other areole. However, on many columnar species and two globular genera - Melocactus and Discocactus - the plants bear a special organ, the "cephalium," from which the flowers emerge. The cephalium is often densely covered in woolly hairs, and the spines borne on the cephalium differ greatly in form from those of the areoles. People once regarded the cephalium as an important characteristic of these plants, but later it was found that cephalia differ greatly from one another. For example, the botanist Werdermann discovered that some cephalia already exist before flowering, the so-called true cephalia, while others appear together with the flowers, the so-called false cephalia. Later, sunken and cap-shaped cephalia were also discovered. Finally, cephalia that form only after flowering were found; such a cephalium sits over the flower bud like a ring studded with spines or hairs, and is very striking. The transitional types discovered afterwards blurred the boundary between cephalia and areoles; for instance, some flowering areoles differ from non-flowering areoles in the form of the spines or hairs they bear. The cephalia of Melocactus and Discocactus are sharply defined on the plant body and are the most typical cephalia, yet these two genera are not closely related. It can thus be seen that in different genera and species, the formation of the cephalium must have arisen independently.
Therefore, people no longer treat the cephalium as an essential marker for classifying genera and species. Even a non-flowering cephalium attracts attention because it bears long white hairs. The flowers, densely born among the tufts of woolly hairs, on the contrary often go unnoticed. Sheltered by the thick woolly covering, the flower buds are well protected, and the young fruits also often ripen gradually within the woolly tufts.
Likewise, if the flowers are borne in deep recesses on the plant body, they are also well protected. Some tubercled cacti of North America have this characteristic. For example, in the genus Thelocactus a groove arises from the areole, while in the genera Ananas and Strombocactus, as well as the genus Neolloydia, the groove extends all the way to the base of the tubercle, that is, the axil of the tubercle. The flowers are also borne in the tubercle axil, so the flower buds can grow safely among the tubercles. In many species the tubercle axils bear bristles or dense tufts of hairs, in which the flower buds hide unnoticed.