Seen in a Botanical Garden - which species is it?
That is Senecio rowleyanus, commonly called a string-of-pearls plant. It is a perennial succulent native to southern Africa, but commonly grown in gardens, or as a house-plant in climates too cool for it to live outdoors. They propagate easily from cuttings. It is tolerant of shady conditions and requires minimal watering. It's somewhat toxic to humans and animals if eaten, and there is potential for skin irritation from contact with the sap, so wash your hands if you get any on you.