Problems in Classifying Cacti

Cacti are more difficult to study than many other plants. They grow in areas that are often hard to reach and inhospitable. Their slow growth presents a problem in studying them throughout an entire cycle of their lives. Cacti may grow for years before they flower, and flowers, fruits, and seeds have many of the characters that are useful in identifying them and comparing them with other cacti.

Because of individual variation, it is important to examine as many individuals as possible to decide where to draw the boundaries, geographical and morphological, between taxa. In addition to flowers, fruits, and seeds, leaves are also an important source of taxonomic characters in many plants. Most cacti are leafless in the normal sense, usually having highly modified leaves in the form of spines. In addition to variation between individuals, spines on cacti may vary considerably in color, size, shape, and number on single individuals, even more so than leaves on a tree or shrub. Variation in other vegetative characters is also common in cacti, including the size of the plant and number of ribs, for example, in addition to variation in spination.

The study of such variation, from as many individual plants over as wide a geographical range as possible, is done through field work and through the use of pressed, dried plants preserved as herbarium specimens, which are stored in herbaria for use in research, now and in the future, just as books are preserved in scholarly libraries. A number of individuals of small, herbaceous plants may be preserved on a single herbarium sheet. Clippings from even very large plants such as trees may adequately represent the variation to be seen in an individual if the clippings are carefully selected for a single herbarium sheet. But some plants such as palms are difficult to preserve as herbarium specimens. Cacti and other succulents also present special problems. Because of their succulence, parts of cacti must be cut open and thoroughly dried—something they have evolved special adaptations to prevent—before they can be preserved. Their spines and thickness make them difficult to attach to herbarium sheets.

TYPE SPECIMENS & NOMENCLATURE It is difficult to make a good herbarium specimen of a barrel cactus or cacti such as Ariocarpus. Thus, unfortunately, many early botanists and amateurs failed to document their work with permanent herbarium sheets or specimens preserved in alcohol. So, poor record keeping is another problem that plagues the study of many cacti that have been described. Some people even failed to recognize the importance of creating type specimens. The type specimen is not necessarily typical of the taxon. It is simply the herbarium specimen with which a name is always to be associated and upon which the original description was based (Rowley 1997, 94,110— 111). The type specimen is no more important than any other specimen in understanding variation of species; its importance is nomenclatural. If two type specimens are judged to be different individuals of the same species, for example, then the correct name of that taxon, that species, is the name that was published first. Katharine Brandegee (1900), an early botanist of the American West noted for her spirited criticism of poor taxonomic practice, lamented:

Students of other orders than the so-called "succulents", especially Cacti, have not usually a clear idea of the confusion existing in their nomenclature. It would be possible, perhaps, if all the species of Astragalus or Senecio [two very large genera] were described from specimens without flowers or fruit, and the types preserved, to identify all or nearly all of them with living forms, though the labor required would be immense. If, however, these imperfect types were thrown away and botanists required to identify plants with only such descriptions, the identification could but be a series of guesses. In this predicament are students of the Cactaceae____

Cacti, as is well known, are in trade to a considerable extent. Large collections are made in their native places and shipped to dealers, mostly in Europe, who find numbers of plants that answer to none of the spine descriptions in their manuals. They must have names, being unsalable without, and the honest dealer has recourse to a specialist, and waits with what patience he may, while the botanist racks his head over a mass of descriptions which do not describe and struggles to find by a process of exclusion whether the plants have possibly been described by some one who threw away the type after inditing a diagnosis that, but for the generic name, might stand for a sea urchin. In the meantime the unscrupulous dealer prints his list bespangled with "new" species to which he attaches his name, disseminates them to the four quarters, mixing them from time to time as the exigencies of trade demand; the next monographer pours all these names into the turgid synonymy, and so the process continues.

This remarkable statement came years before cacti suffered some of the most flagrant violations of the International Code of Botanical Nomenclature, the rules of which govern the naming of plants. For example, the German horticulturist Curt Backeberg was once asked by Lyman Benson (pers. comm.) for the location of some of the type specimens for new species he had described. Backeberg replied, "Why, they are in my greenhouse." Backeberg (1949) also described Or-eocereus crassiniveus as a new species, from a moving train as he passed through the mountains of Bolivia. Of course, no type specimen was cited, nor were any photographs included. Yet he did include a Latin description. Thus for many of Backeberg's names, as well as those of some other workers, no permanent specimens exist. Hence we do not really know to what plants the names refer, making those names and the published information about the cacti essentially worthless.

Historically, in many cases a new species was described from one or only a few plants growing in a greenhouse. Variation patterns could not be studied from such a small number of individuals, so variant forms were often described as species or varieties when, in fact, they were simply a few of many slightly different forms of a single population. Workers tended to look for differences rather than similarities. Studies of cacti in the field are important because observations of natural populations allow researchers to understand the range and scope of variation. I recall looking at one clump of peyote, Lophophora williamsii, in Mexico and seeing cactus heads that represented what had been described as five different species.

Newer techniques such as the analysis of DNA sequences from cacti (Wallace 1995,1996) also shed light on cactus relationships. This information is helping to clarify relationships at the level of species, and higher. It is resulting in new interpretations of the boundaries of genera and of subtribes and tribes of cacti. Thus classifications—and names of taxa— change as our understanding of cacti becomes more profound. Many people are frustrated and at times resent changes in names of well-known plants, but it is not the aim of botanists to confuse or needlessly change the classification of cacti. Rather, the need for change is usually the result of new evidence or new interpretations of data. The whole process is, indeed, dynamic and constantly changing. No final, definitive classification of cacti—or any other organism—will ever be produced by any individual or group of scientists because new information will continue to be found. This is one of the exciting aspects of the study of living beings.

The goal of nomenclature is to give each plant a unique name. No other plant species can have the same binomial, which is the combination of the name of the genus followed by the name of the species. A complete binomial will also have the name of the person who described the plant. For example, the fishhook barrel cactus of the southwestern United States and Mexico was originally named Echinocactus wislizeni Engelmann. Engelmann refers to George Engelmann, who first described the plant, placing it in the genus Echinocactus and giving it the epithet wislizeni. Nathaniel Lord Britton and Joseph Nelson Rose later placed that species in their newly created genus Ferocactus. Thus the present binomial is Ferocactus wislizeni (Engelmann) Britton & Rose, and E. wislizeni is a synonym of F. wislizeni.

Well more than 15,000 scientific names for cacti have been published but many are no longer relevant. In deciding which names to include in the synonymies and in the Index of Sci entific Names, I benefited from the collective wisdom of the International Cactaceae Systematics Group, and from David Hunt in the preparation of the cites Cactaceae Checklist editions of 1992 and 1999 and his planned cactus lexicon.


Hybridization between cacti in the wild presents a number of challenges to taxonomists. In some cacti, crosses may occur between species, even between species in different genera. For example, Echinocereus xroetteri (syn. E. lloydii) of the southwestern United States is a natural hybrid between E. coccineus and E. dasyacanthus (Powell et al. 1991). Several genera are of hybrid origin, examples of which are xHaages-postoa, xMyrtgerocactus, and xPacherocactus. It has also been suggested that the more recently described Mexican genus Geohintonia may have originated from an ancient hybrid cross (Robert S. Wallace, pers. comm.). Many natural bi-generic hybrids have been recognized and are discussed by Rowley (1982,1994).

As mentioned in Chapter 1, under Dispersal, vegetative reproduction combined with crossing can cause confusing situations for taxonomists. Unusual variations resulting from a cross between very different individuals of a single species or between individuals of different species may be perpetuated by asexual, vegetative reproduction even though the hybrids are incapable of flowering and fruiting. As an additional complication, among the prickly pears (Opuntia) and chollas (Cylindropuntia) of North America, for example, there are many such crosses as well as the development of polyploidy and further backcrossing. This has led to swarms of cacti whose parentage is difficult to determine (Pinkava 2001). Even the well-known Indian fig or mission cactus, O. ficus-indica, is believed to have hybridized with native Cali-fornian prickly pears (Benson and Walkington 1965).

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