Calymmanthium is a primitive tree-like cactus from Northern Peru and Southern America. It comes from a tropical area and is very tender in cultivation. Its floral structure is very odd. The perianth is completely enclosed in a spiny short shoot until expansion.

Calymmanthium flower develops inside a pouch of vegetative tissue -- when the flower is ready to open, it actually has to tear the vegetative tissues apart before the petals can emerge.

In Calymmanthium plant, the vegetative tissues extend to the very rim of the tube as expected of cacti, but the rim itself does not expand much during flower development. The rim remains a microscopically narrow hole that is the same diameter as when it was formed by the floral apical meristem. In all cacti the rim grows along with the rest of the flower, forming a wide rim that bears many petals.

The petals of most cactus flowers begin to be inserted at the top of the flower tube; the rim continues to be stem tissue in Calymmanthium. Because all flower parts begin as microscopically small primordia, when the tip of this tube was formed, it too was microscopically small -- and it stays that way. It does not grow into the large opening like other flowers.

Calymmanthium plants are big, rambling bush-like cacti.

 Did you know?

• The genus name comes from the Greek kalumma, meaning veiled and anthos, meaning flower.