Small succulent plants that can reach 10 to 20 cm. in height. Their stems are easily recognizable by 4 vertical ribs, very pronounced and covered with small thorns. Much appreciated and sought after by lovers of succulents for their very peculiar flowers ranging from yellow, to red, to violet, often with very lively and showy streaks. However, they have an unpleasant smell which is used to attract pollinating insects.