Hi Stuart,One thing that puzzles me is the evolutionary route the various cephalium-bearing genera have taken - both the columnar ones and Melos/Discos. What is the purpose of them, and/or how did they come about, do we know?
Stuart Estell
BCSS Birmingham Branch - stuartestell.co.uk
How and why the cephalium evolved are indeed quite interesting questions. This structure evolved several times in the family, in many independent groups both in North America and in South America. Thus, there must be a very good reason for so many different plants have evolved similar structures.
With a few exceptions, each areole of all cacti produce a single flower, and that is it - after this flower is produced, the areole become inactive. Exceptions to this rule are Myrtillocactus in North America, which can produce several flowers per areole:
Myrtillocactus geometrizans. Photo: Gerard Ardisson's Cactus & Succulents Photo Gallery
Neoraimondia gigantea. Photo: Mauseth's Lab
Many cacti devised clever ways of minimizing the amount of energy spent for reproduction by producing more areoles in a smaller area, thus producing more flowers without having to grow significant amounts of new stem. Take for example Astrophytum myriostigma. Young plants of this species have areoles well-spaced along the ribs:
Photo: http://www.succulente.info/
Photo: Wikipedia Astrophytum page
Another example is the Senita Cactus, Lophocereus schottii, which is a columnar cactus species from northweastern Mexico and southern Arizona, US:
Photo: http://cactiguide.com/
Photo: http://cactiguide.com/
Photo: http://cactiguide.com/
apical cephalia like those produced by Melocactus:
Melocactus zehntneri. Photo: Marlon Machado
Discocactus placentiformis. Photo: Marlon Machado
Backebergia militaris. Photo: Mauseth's Lab
Cephalocereus senilis. Photo: http://www.astrobase.de/
Espostoa guentheri. Photo: Philippe Corman, http://www.cactuspro.com/
Coleocephalocereus aureus. Photo: Marlon Machado
Coleocephalocereus purpureus. Photo: Marlon Machado
Coleocephalocereus buxbaumianus. Photo: Marlon Machado
Gerard Delanoy with Coleocephalocereus goebelianus. Photo: Marlon Machado
Arrojadoa dinae. Photo: Marlon Machado
Arrojadoa marylanae. Photo: Marlon Machado
Arrojadoa rhodantha. Photo: Marlon Machado
Gerard Delanoy with Stephanocereus leucostele. Photo: Marlon Machado
Stephanocereus leucostele in fruit. Photo: Marlon Machado
This is a very interesting subject indeed, and a good place to find more information about the cephalium is the website of Prof. Dr. James D. Mauseth. His text is very instructive, but beware that he uses a lot of technical terms. And, if you still wants to learn more, there is an excellent review also published by Mauseth:
Mauseth, J.D. 2006. Structure?Function Relationships in Highly Modified Shoots of Cactaceae. Annals of Botany 98: 901?926.
Cheers,