Graptopetalum
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Genus Graptopetalum

Genus Graptopetalum is an odd genus in that it contains some greatly differing species. And also rather strangely, all species may fit within the definition of what is a Sedum (as genus Sedum happens to be broadly defined). This may show that the old Sedum genus definition needs some rewording, but this is unlikely to ever be done as it was Linnaeus who wrote it. The genus definition for Graptopetalum is more specialized, and does exclude all other Sedums. Rose (of the famous team of Britton & Rose) wrote the genus definition for genus Graptopetalum. It is as follows:


“Stamens twice as many as petals, (petals) free, becoming recurved when stigmas are receptive: petals 5 or 6, not appendaged within, connate to the middle, radially spreading above. More or less distinctly marked with dots or bands; sepals above as long as corolla tube.”


The “type species” was designated as being G. pusillum. Take special notice of the first line of the genus definition for a unique and distinct point for identification of species within this genus. It tells us that the stamens curve outward and downward between the petals shortly after the flower opens (when the stigmas become receptive). This is typical of Graptopetalum behavior only.

The Latin genus name of “Graptopetalum” refers to a characteristic which defines the group well (petals often appear to be “engraved” by dark, red-brown dots or lines of surface cells); but in reality it must be pointed out that some species do not posses this unique feature! [And a few Sedums do possess it!]. This is a diverse group: they are so varied that they do not make a very coherent grouping. The approximately 15 species range from low plants with rosetted leaves to branching shrubs and hanging vines; from tiny plants to rather large ones. There is no doubt that they are indeed closely related to Sedums, but close study of them does indicate they belong together. (The study reveals that even those few species which do not have petals that can be described by the Latin word “graptopetalum” probably are descended from of some earlier species that did have this feature; thus the close relation would remain valid botanically in any case. To place them within genus Sedum it would result in enlarging this genus even more, and it is already far too large). Most of these 15 species just do not look much like what we think of as Sedums, so it would also not be comfortable to have them there. One thing we usually expect of different genera is that their species will not be fertile to each other, and this holds well in most genera in other families of the plant kingdom. It does not hold for this genus (and many other Mexican genera): Graptopetalum is fertile to several species (probably all are Mexican) in other genera of this rather unique family Crassulaceae. This could bother us a lot except for the fact that the other Mexican genera of this family include many other similar exceptions to this rule, so botanists feel rather free to ignore it for all of these abnormal Mexican genera.

The genus was erected in 1903 by botanist Joseph Rose of the famous team of Britton and Rose. It was intended to handle certain “difficult to classify” plants, then mostly classified within genus Sedum. One characteristic that is common to almost all of these species concerns markings on the petals of the flowers. This consists of red-brown dots or tiny lines seen mostly on the topsides of inner, lower parts of each of the petals. Otherwise, the flowers are very Sedum-like indeed. They have 5 pointed petals like most Sedums, rarely more. Petal markings are used to justify the Latin name for this genus which is “Graptopetalum” and means “engraved petal”; but it should be pointed out that this neat idea is confused by two facts. The first is the fact that such petal markings are not confined to this genus. They can appear in some Sedum species of the Sedum allantoides group, etc. (but even in this group it does not always appear). The second fact is that it does not appear in all Graptopetalum species all of the time. The basic color range of the petals of Graptopetalum is from “off-whites” to dull yellows and oranges. The dot and line markings, when present, normally appear toward the base of these petals as dots or transverse lines. But in those with total dark coloring it seems that the “markings” cover the entire petal solidly, leaving no light color at all. These facts about markings can be confusing for any amateur trying to identify species, so it is not advised for anyone to use it for genus identification purposes.

A species closely related to the Graptopetalums is Tacitus Bellus. At first it was assumed to be just another Graptopetalum. But closer studies proved it was just different enough to cause problems should it be included (because including it would broaden the genus concept too much). Another way of looking at this is to say that its evolution proceeded just a bit too far. So, most authorities now agree to keep it separate (in a “monotypic” genus of its own). But the line of its evolution is clearly very close to Graptopetalum, probably this species evolved from some earlier species of Graptopetalum. It seems to be one that merely went too far in its evolution, placing it marginally outside the main group. In this list it will be included, but given a “monotypic” genus listing in accordance with present understandings.

Some, and perhaps all, species of Graptopetalum are fertile to species in some other Mexican genera of this family. This includes some Sedums and Echeverias. For one thing, this demonstrates their close relationship. In fact, the concept of genus here is rather loose: this happens more in the Crassulaceae than within other families, as explained above. These crosses produce what we call “intergeneric hybrids”. These are usually identified by a “genus name” which is just a combination of syllables of the two parent genera involved (such as “Graptoveria” for a cross of a species from this genus and a species of Echeveria). Some such hybrids can be quite beautiful, and many are popular, and are important in the nursery industry, but are far too numerous to include here, and more are constantly being prepared by horticulturists. This is done out of curiosity, or to try to find useful new hybrid “species” to sell.

In fact, this ease of species and genus mixing can become a problem, as it is sometimes hard to know if a particular plant found in cultivation is a pure species, or is perhaps “tainted” by some degree of mixing. Collectors should keep this in mind. Confusing this picture even more is the fact that some of the Mexican Crassulaceae have high chromosome counts (due to multiple sets of chromosomes: polypolidity). One notable result of this is that a cross involving a plant with a high count number being crossed with one with a low number, will tend to show the characteristics of the plant with the higher chromosome number. Thus, please be forewarned that intergeneric mixing cannot be determined from any study of morphology (looking at the physical characteristics of these plants). Since it is almost impossible to tell, after the mixing is done, if any mixing has occurred, serious growers (or “purists”) should “bend over backwards” to obtain pure strains, and to not allow any later mixing of plants they are growing. Leave this mixing work to specialists who will select the parents to get best possible results.

Obtaining these plants for a collection can be a problem. As with all other American members of Crassulaceae, the few well-known members are usually available from specialty nurseries and catalogs, but the more unusual ones can be a big problem to obtain. The best idea seems to be for all serious students and growers to try to get to know each other and to keep in contact. In this way, at least it will be easier to exchange cuttings, and ideas. The Internet may be the best way to keep in contact for such activities. It is still possible for new species to be discovered, but by now no doubt most have been found.

 




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