Living Rocks of Mexico
 A Study of the Proposed Genus Roseocactus (1)
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This article is reproduced form The Cactus and Succulent Journal (U.S.), Vol. 33 (1961). No .4, p122-127,. with the kind permission of the Editor and Edward Anderson.



POMONA COLLEGE Claremont, California

ARIOCARPUS Scheidweiler, Bull. Acad. So Brux 5: 491-492 1838.

  • Subgenus 1. ARIOCARPUS

  • Subgenus 2. ROSEOCACTUS (Berger) W T Marshall, Cact. Succ. Jour. 18, 55, 1946.

  • Roseocactus. as genus, Berger, Jour. Wash. Acad Sci. 15, 43, 1925.

  • Roseocactus, as subgenus of Ariocarpus, W T Marshall, Cact Succ. Jour. 18, 55, 1946.

In 1925, Alwin Berger, after an investigation of plants and herbarium specimens of the genus Ariocarpus, published an article proposing a new genus in the Cactaceae. He named this genus Roseocactus in honor of Dr J N Rose, co-author of The Cactaceae. It was Berger's belief that some of the species in the genus Ariocarpus differed sufficiently to justify placing them in a separate genus The plants possessing a longitudinal areolar groove, A. fissuratus (fig.2, fig 17), A. kotschoubeyanus (fig.12), and A. lloydii were placed in the new genus, while a single species, A. retusus (fig 1, fig 16), was kept in Ariocarpus. He did not mention A. trigonus (fig. 5)

Berger (1925) concluded that the two genera were different on the basis of two characters:

"I finally have come to the conclusion that there are indeed two distinct genera, differing not only in the nature of the tubercles but also, and chiefly, in the origin of the flowers"

He went on to add the following about the areoles:

"If we break off an old tubercle of the Texan plant [A. fissuratus) or of Ariocarpus lloydii and cut it longitudinally, we shall see that the fissure, filled with grayish hairy wool, corresponds to an areole and we shall discover in its lower part the remains of the fruit and perhaps even a few seeds. The tiny areole at the top of the tubercles of the true Ariocarpus of Scheidweiler [A. retusus] is wholly different. It does not show any sign of having ever produced flowers. In fact, if we break off one of the tubercles on a dried specimen, we shall detect in the axil between the tubercles, at the base of the tubercle but rising from the axis, the remains of the old fruit with numerous seeds, which have been hidden there for years".

Berger's second criterion for creating Roseocactus was flower origin. He believed that the flower originated from within the lower part of the areolar groove of A, fissuratus and from the axil between the tubercles in A, retusus.

Berger's decision to create a new genus was strictly artificial, that is, he looked for differences between the two groups and ignored the similarities. I have strived, in this study, to approach the group naturally, trying to note the similarities and weighing these against the differences. By this method a grouping may be defined based more accurately on all aspects of botanical analysis.

Two field trips to Mexico were made (Anderson, 1958) to gather material and to study the plants in their native state. Numerous photographs were taken and several hundred plants were brought back to California and placed in pots. Experiments were carried out on living material, and herbarium specimens were made Specimens from other herbaria were borrowed in connection with the materials collected on the two field trips, and variations of these specimens (including some type material) were recorded

The groups were studied from the standpoint of both similarities and differences, and conclusions were then drawn at the completion of the study The similarities between Ariocarpus and Roseocactus are as follows


Ariocarpus and Roseocactus are found only in the Chihuahuan Desert of Texas and Mexico, and are restricted to calcium bearing soils. In most localities occurrence on limestone is obvious, but in some areas a 10% hydrochloric acid reaction test was necessary for conclusive proof The soil in which these plants grow is also neutral or slightly alkaline. Most species usually grow near the summits of low hills; rarely are they found on flat, poorly drained soil Plant associations are similar for the two groups, with variation being accounted for by difference in rainfall and temperature Plants commonly found with Ariocarpus and Roseocactus are Agave lecheguilla. Jatropha spathulata. Larrea divaricata, Opuntia spp. Mammillaria spp., Echinocereus spp., and Yucca spp. No differences could be noted in the ecological requirements of the two groups (fig.3 and fig.4)

Development of seedlings

Buxbaum (1950) compared seedlings of Ariocarpus, Roseocactus, Encephalocarpus, Obregonia, and Leuchtenbergia, and noted several important "evolutionary trends". He also stated that Ariocarpus retusus, Roseocactus fissuratus, and Leuchtenbergia were alike in many ways in seedling development. My observations were that seedlings of Ariocarpus and Roseocactus are similar from the time of their emergence from the seed coat through the first year of development (fig.6 and fig.7)

Presence of mucilage canals and reservoirs

The extensive system of mucilage canals and the large central reservoir in Ariocarpus and Roseocactus are unknown in other genera of the Cactaceae, and development of this mucilage system is identical in both genera. Canals form near the vascular bundles and extend into the shortened stem area in which a reservoir is located. These canals are as much as 2mm in diameter (fig. 8 and fig.19), and the reservoirs have been found up to 20mm in diameter (fig. 9 and fig.20). As mucilage canals are probably unique to these two groups, this common feature strengthens the proposition of recombining them into a single genus.

Seed structure

A recent study of the seeds of the Cactaceae was by Buxbaum (1953), who published numerous drawings and discussed the "taxonomic value" of seeds. Craig (1945) and Boke (1959 and oral communication) have also found seeds important in taxonomic work. Buxbaum states that there are several characters of the seed, which showing variations, are of taxonomic value. They are the hilum and strophiole, the shape, the color of the integument, and the structure of the outer testa. On the outer surface (testa) the size and density of the nipples are said to be of significance. The application of the above characters to taxon analysis is dependent upon the uniformity of the characters within the lower taxon.

The seeds of both Ariocarpus and Roseocactus were studied. Small differences in shape were noted in some seeds, but the overall shape and structure were not variable (fig 10 and fig 11). The two groups are similar in all the diagnostic characters of seed structure as outlined by Buxbaum

Fruiting habit

Ariocarpus and Roseocactus have similar fruits and means of seed dispersal (fig.12) Descriptions of the fruits of the two groups have been omitted from many works because they are seldom seen. The masses of wool in the centers of the plants usually hide the fruits at maturity; in fact, the fruits often develop, mature, and decompose while hidden among the wool. They persist there until water or some other agent removes them.

Origin and structure of flowers

One of Berger's (1925) two main criteria for establishing Roseocactus was the location of flower origin, but evidently he did not dissect living material I studied living specimens of both groups while in flower and found little difference in the point of origin of the floral organs. The floral parts develop from the bases of the tubercles (figs.13 and 14).

Other floral characters are similar within the two groups. The naked ovary has been a diagnostic character used in descriptions and keys by Britton and Rose (1923), by Bravo (1937), and by Marshall and Bock (1941).

Pollen grain structure

There has been little intensive work on the structure of pollen grains in the Cactaceae Kurtz (1948) concluded that identification of genera and species of cacti on the basis of pollen is impracticable. Studies of Ariocarpus bear out this opinion, but the remarkable similarity of the pollen between the two groups is of significance. Pollen of Ariocarpus (fig.18) is tricolpate ~the three aperatures are more than twice as long as wide in outline) and the exine is three-layered. Contrary to Kurtz's observations on the cacti, the exine is not pitted. The resemblance to pitting is actually a reticulum formed by knob-like pila which are an outward extension of a layer of the exine. Pollen grain details may be seen in figure 18. Pollen grain size within the genus varies slightly, from 60-85m.

Chromosome number

Most cacti are reported as having a chromosome number of 2n = 22. Root tips of Ariocarpus were studied and several counts were made, all being 2n = 22 (fig 21 and fig 22) This number is different from that found by Takagi in 1938 (reported by Darlington and Wylie, 1955) in R. fissuratus, which was n = 38. However, this number is questionable in the light of other work done on the Cactaceae. No evidence of polyploidy was found.

Trichome development

Only a few genera in the Cactaceae have the degree of trichome development found in Ariocarpus and Roseocactus. The development, structure, and appearance of trichomes in both groups are similar. The 'wool' develops abundantly in the floral areolar regions at the bases of the tubercles and also in the areolar grooves or areolar regions on the tips of the tubercles when these structures are present. The trichomes are multicellular and uniseriate, and they develop from the epidermal layers in the areolar regions.

Tubercle structure

Microscopic analysis of the internal structure of the tubercles of both Ariocarpus and Roseocactus shows that the layers of cutin and the epidermis are analogous in structure and appearance. Some cactus genera have uniquely-shaped epidermal cells, but such is not the case with the two groups under consideration.

Alkaloidal properties

Schultes (1937a, 1937b), Reti (1950), and other investigators reported certain alkaloids to be present in Ariocarpus and Roseocactus, but only in R. fissuratus had the alkaloid been identified. Ewell (1896) reported this alkaloid to be anhaline. Several precipitate-producing reagents were tested to determine the presence and location of anhaline. Phosphotungstic acid proved to be the best reagent because of its speed of reaction with the alkaloid and its non-reaction with starch. Live plant materials of all species of the two groups were tested and were found to react, indicating the presence of anhaline. Few other cacti have been reported having this alkaloid , consequently, this provides additional evidence of a close relationship of the two groups.

In the study of the two groups not all characters were found to be alike. The following differences were found between Ariocarpus and Roseocactus.

Flower color

The color of flowers of plants included in Ariocarpus is white in A. retusus and cream (yellow) in A. trigonus. Occasionally both these species show a reddish tint to the midribs of the outer perianth segments. All flowers in Roseocactus are magenta or occasionally white.

Areolar groove

The presence of an areolar groove was one of Bergers two criteria for proposing Roseoactus. Studies indicate that Roseocactus has monomorphic areoles with both floral and spiniferous areolar areas developing within the same meristem and never splitting off. A. retusus, on the other hand, has areole dimorphism with the original single growing point becoming elongated and then dividing into two portions. However, A. trigonus, a species not treated by Berger, differs from both the above groups as the spiniferous growing point never develops from the original meristem. All species begin with an original meristematic area, with differentiation occurring during elongation and maturation of the tubercle. Not always do species develop without variation, for specimens of R. fissuratus have been found which lack the longitudinal areolar furrow (fig.15), and specimens of A. retusus occasionally occur with no rudimentary spiniferous areolar areas at the tips of their tubercles. It is believed that all species have areolar meristematic areas with potential floral and spiniferous areas, but that growth and maturation can occur in one of three places in relation to them. (1) In Roseocactus elongation occurs beneath the spine area (fig.23-1). (2) In A. retusus growth and elongation is between the points of origin of the spines (rudimentary) and flowers, thus producing areolar areas at both ends of the tubercle (fig.23-2). (3) In A. trigonus growth and elongation is distal from both the areolar areas, leaving them at the base of the tubercle (fig.23-3).

Tubercle divergence

It has been noted that the divergence of tubercles is different between Ariocarpus and Roseocactus. The difference is due to the shapes of the tubercles, especially at their bases. If the longer tubercles of Ariocarpus were shortened and rounded at their tips, they would take on generally the same appearance as Roseocactus.

Trichome texture

Trichome length and texture vary according to the habitat. The coarser hairs found on some plants are probably an adaptation for protection from abrasion by wind-blown sand. In cultivation plants tend to have longer and silkier hairs. A. trigonus has shorter and coarser hairs than A. retusus, because the tubercles are more divergent and less compact in the former, allowing abrasive elements such as dust and sand to penetrate


In the foregoing discussion the similarities and the differences of Ariocarpus and Roseocactus have been considered. Evidence is clear that there are many similar features between the two groups, some of which are probably unique among the Cactaceae The areolar groove is variable but is nonetheless a difference. However, with the desire to create a natural grouping based on the many similar characters rather than a few differences, I conclude that Roseocactus should not be considered a separate genus The differences pointed out in this article are excellent characters to use in distinguishing plants on the subgeneric and specific level I believe the genus Ariocarpus should retain the species placed in Roseocactus, and the proposed generic name Roseocactus should be relegated to the formal rank of subgenus as Marshall (1946) proposed earlier.

Literature Cited

ANDERSON, E. F. 1958. A recent field trip in search of Ariocarpus. Cact. Succ. Jour. 30: 171-174.

BERGER, A. 1925. Roseocactus, a new genus of Cactaceae. Jour. Wash. (D.C. ) Acad. Sci. 15: 43-48.

BOKE, N. H. 1959. Endomorphic and ectomorphic. characters in Pelecyphora, and Encephalocarpus. Amer. Jour. Bot. 46: 197-209.

BRAVO H., HELIA. 1937. Las Cactáceas de Mexico. Universidad Nacional de Mexico. Mexico City.

BRITTON, N. L. and J. N. ROSE, 1923. The Cactaceae. Carnegie Inst. Washington Publ. No 248.

BUXBAUM, F. 1950. Morphology of Cacti. Abbey Garden Press. Pasadena Section 1.

BUXBAUM, F. 1953. Morphology of Cacti. Abbey Garden Press. Pasadena Sections 2 and 3.

CRAIG, R. T. 1945. The Mammillaria Handbook. Abbey Garden Press. Pasadena.

DARLINGTON, C. D. and A. P. WYLIE. 1955. Chromosome atlas of flowering plants. George Allen and Unwin Ltd. London.

EWELL, E. E. 1896. The chemistry of the Cactaceae. Jour. Amer. Chem. Soc. 18: 624-643.

KURTZ, E. B. JR. 1948. Pollen grain characters of certain Cactaceae. Bull. Torrey Bot. Club 75: 516-522.

MARSHALL, W. T. 1946. Revision of the genus Ariocarpus. Cact. Succ. Jour. 18: 55-56.

MARSHALL, W. T. and T. M. BOCK. 1941. Cactaceae. Abbey Garden Press, Pasadena.

RETI, L. 1950. Cactus alkaloids and some related compounds. Progress in the chemistry of organic natural products. Springer Wien.

SCHULTES, R. E. 1937a. Peyote and plants used in the peyote ceremony Bot. Mus. Leafl. Harvard Univ. 4: 129-152.

SCHULTES, R. E. 1937b. Peyote (Lophophora williamsii) and plants confused with it Bot. Mus. Leafl. Harvard Univ. 5: 61-68.

1. A more technical article dealing with Roseocactus was published earlier by the author in Amer. Jour. Bot. 47, 582-589, 1960, under the title A revision of Ariocarpus (Cactaceae) I The status of the proposed genus Roseocactus.

2. This study was made possible through a grant to Pomona College by Dr Gordon A Alles of Pasadena, California. The author also wishes to thank Dr Lyman Benson for his advice and criticism.

The quality of some of the illustrations accompanying this article, does not meet with today's more exacting standards. This is a consequence of the reproduction technology of the time,

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