“String of pearls” is a very good name, although to me its looks more like a string of peas. The spheres are modified leaves, and they grow on trailing stems that seem to go on forever. The leaves are round to reduce the surface area exposed to the hot sun and dry air, thus minimizing the loss of precious water through evaporation.
In its natural habitat in southwestern Africa, the stems trail on the ground, rooting where they touch and forming dense mats. In cultivation, Senecio rowleyanus is almost always grown as a hanging plant to display the trailing stems and curious leaves to maximum effect.