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Three series, Kalanchoe ser. Raveta, K. ser. Longiflorae, and K. ser. Rotundifoliae, are described in Kalanchoe subg. Kalanchoe sect. Raveta (Crassulaceae subfam. Kalanchooideae). The type species of all three series are indigenous to southern Africa. The first series, K. ser. Raveta, includes the robust, white-waxy kalanchoes with soup plate-sized leaves, dense-flowered, club-shaped inflorescences, and variously coloured (yellowish, whitish, greenish, and pinkish red) flowers. The second series, K. ser. Longiflorae, includes medium-sized to rather large shrubs with yellow flowers that generally do not, or hardly, show diurnal movement and do not become tightly twisted post-anthesis. The third series, K. ser. Rotundifoliae, includes small, generally low-growing, shrubby species with variously chromatic flowers (different shades of red, orange, and yellow, never white) that show distinct diurnal movement and with the flowers becoming tightly twisted post-anthesis.
Gideon F. Smith (Thu,) studied this question.