There are about 15 species in this genus which consists mainly of biennials. The attraction is mainly due to the structure of the plant. It is a tall, upright, open-branched plant which has a stately architectural quality about it. The stems and leaves are covered in stout prickles and the large, opposite leaves join at the stem forming a large cup which is usually filled with water. H 2m (6ft) S 1m (3ft).