What makes a columnar cactus?  I’d say something that can stand up tall, while holding onto the shape of a pillar. Elongated in general instead squat, globular, and round.   The fascination with these forms initially began with the desire to use them for landscape.  But as we soon discovered, the ability to collect these rare forms could encompass a whole yard of potted columns.  So many genera of tall Cereus including my favorites: Trichocereus, Pilosocereus, Cleistocactus, Borzicactus, Stenocereus, Eulychnia, Micranthocereus, Weberbauerocereus, Haageocereus and on and on.