This Christmas cactus, Schlumbergera truncata, an epiphyte from South America, has flattened (two-ribbed) succulent stems. Each segment, or joint, forms as a growth increment. Very fine, short spines arise from areoles, modified axillary buds positioned along the two ribs. Succulent cactus epiphytes have CAM, whereby carbon dioxide diffuses into the stems at nighttime through open stomates but sugars are manufactured during daylight, while the stomates are closed.