Cactus
Morphology
Growth habits of cacti Treelike habit (Pereskia aculeata) Tall treelike habit (Pachycereus pringlei) Tall unbranched columnar habit (Cephalocereus) Shorter clustered columnar habit (Ferocactus pilosus) Solitary globular habit (Ferocactus echidne) Clustered globular habit (Rebutia species) Epiphytic cactus (Rhipsalis paradoxa) The leafless, spiny stem is the characteristic feature of the majority of cacti (and all of those belonging to the largest subfamily, the Cactoideae). The stem is typically succulent, meaning it is adapted to store water. The surface of the stem may be smooth (as in some species of Opuntia) or covered with protuberances of various kinds, which are usually called tubercles. These vary from small "bumps" to prominent, nipple-like shapes in the genus Mammillaria and outgrowths almost like leaves in Ariocarpus species. The stem may also be ribbed or fluted in shape. The prominence of these ribs depends on how much water the stem is storing: when full (up to 90% of the mass of a cactus may be water), the ribs may be almost invisible on the swollen stem, whereas when the cactus is short of water and the stems shrink, the ribs may be very visible.
Cactus-13