Hi Larry,Hi Marlon, fascinating stuff and thanks for correcting me on Neoraimondia. Now you say that the Pilosocereus that Vic saw doesn't bear a cephalium yet there are some in that genus that do. Now I would have thought that cephalium bearing is a genetic trait i.e. that all plants with cephalia belong to the same group(clade?) and that if one Pilosocereus(=Pilocereus?) develops a cephalium then they all must do to belong in that group.
Yes, the cephalium is a genetic trait but it evolved many times in different groups of cacti, and for that reason it cannot be considered as a decisive characteristic for grouping species together, because in spite of having the same function, they may not have the same origin. For an easy example of what I mean, think of a bird, a bat, and a butterfly: they all have wings and fly, but they are not closely related and belong to quite distinct classes of animals: avian, mammal and insect.
So, not all plants with cephalia belong to the same group.
A clade is a group of organisms that are descendent from the same ancestor organism. It is just a term that biologists use to indicate common ancestry of the members of a group. All cacti form a clade because they all evolved from a single ancestor, but all succulents do not form a clade because succulents is a group of plants which have many different origins. the term clade can be used for any groups of organisms, as long as they have a common origin.
Now to the second part of your doubt, which is if one species of a group has one characteristic, all species should have that characteristic in order to belong in that group. That is not necessarily true. If we were to apply this rule then every species would need to be in its own genus, family, etc. because there are no two species alike. As a matter of fact, no two individuals are the same, unless they are clones of each other; thus, if the same principle were to be applied, each individual would be its own species, genus, family etc.
Of course, we group species in genera or families based in shared characteristics. But the key here is that the characteristics have to be shared, that is, common to all species - provided of course that these species have a common origin (remember the example of the wings). Thus, what brings all cacti together are among others the characteristics of possessing areoles and having flower parts surrounded by stem tissue; what brings all Opuntioideae together are the development of glochids and arillate seeds (seeds enveloped by a tan to brownish woody or corky membrane); and so on.
In the case of Pilosocereus, what brings all the species of this genus together are characteristics of their fruits: all Pilosocereus develop naked depressed-globose fruits, opening by irregular slits at the apex and sides of the fruit, and the fact that the floral remnants are persistent, remaining attached to the fruits until they are ripe, and are deeply inserted in the fruit apex.
Back to the cephalium. Not all species of genera that have cephalium-bearing species does necessarily develop a cephalium. For instance, in the large genus Pachycereus the only species that develop a cephalium is Pachycereus militaris; likewise the only species in the large genus Cereus that develop a cephalium is Cereus mortensenii. In Pilosocereus only two species are know to develop a cephalium: Pilosocereus diersianus and Pilosocereus gounellei - and in the last species the development of a cephalium is quite unstable, just a few populations do it, the majority don't - the cephalium is not yet a fixed characteristic in this species.
Other genera that have plants with and without cephalium are Facheiroa and Micranthocereus. In Facheiroa some species have well developed cephalia, for example Facheiroa ulei:
Facheiroa ulei photographed at Gentio do Ouro, Bahia. Photo: Marlon Machado.
Facheiroa ulei photographed at Gentio do Ouro, Bahia. Photo: Marlon Machado.
Facheiroa squamosa photographed at Ju?, Bahia. Photo: Marlon Machado.
Facheiroa squamosa photographed at Ju?, Bahia. Photo: Marlon Machado.
Facheiroa estevesii photographed at Iui?, Bahia. Photo: Marlon Machado.
Facheiroa estevesii photographed at Iui?, Bahia. Photo: Marlon Machado.
Micranthocereus streckeri photographed near Seabra, Bahia. Photo: Marlon Machado.
Micranthocereus streckeri photographed near Seabra, Bahia. Photo: Marlon Machado.
Micranthocereus auriazureus photographed at Gr?o Mogol, Minas Gerais. Photo: Marlon Machado.
Micranthocereus auriazureus photographed at Gr?o Mogol, Minas Gerais. Photo: Marlon Machado.
Micranthocereus flaviflorus photographed at Morro do Chap?u. Photo: Marlon Machado.
Micranthocereus flaviflorus photographed at Morro do Chap?u. Photo: Marlon Machado.
Micranthocereus polyanthus photographed at Brejinho das Ametistas, Bahia. Photo: Marlon Machado.
Micranthocereus polyanthus photographed at Brejinho das Ametistas, Bahia. Photo: Marlon Machado.
Micranthocereus hofackerianus photographed at Piat?, Bahia. Photo: Marlon Machado.
Micranthocereus hofackerianus photographed at Piat?, Bahia. Photo: Marlon Machado.
Regarding the name Pilocereus, it was an older name used for the plants that now belong to the genus Pilosocereus, however the name Pilocereus was originally described by Lemaire for the plant that we know today as Cephalocereus senilis. Along the years many other columnar plants that had hairy areoles were described in the genus Pilocereus, but these new species were not closely related to the original Pilocereus which was Cephalocereus senilis. Karl Schumman tried to solve the problem and used the name Pilocereus exclusively for the species we recognize today as belonging to Pilosocereus, however this has unfortunate because it was against the rules of botanical nomenclature, and his version of Pilocereus became an illegitimate name. For this reason in 1957 Byles & Rowley described the new genus Pilosocereus to include the species not related to Cephalocereus senilis, and the choice of the name Pilosocereus was to make it as similar as possible to the older but invalid name Pilocereus.
In short, Pilocereus was used by two different authors to indicate two different genera, and for this reason the name became invalid: some species that used to be Pilocereus were retained in the genus Cephalocereus, while the remaining are now Pilosocereus.
Cheers,