Studies in the genus Riccia (Marchantiales) from southern Africa. 20. R. albovestita and its synonyms, R. duthieae and R. sarcosa

As has been repeatedly emphasized elsewhere, species in section Pilifer Volk are often very difficult to distinguish (Perold 1990b, 1990c). This is nowhere better illustrated than in the case of R duthieae Volk & Perold and R. sarcosa Volk & Perold. both initially described as new species, which are now regarded by me as synonyms of R. albovestita Volk. Comparisons are drawn between the thallus dimensions, dorsal cell pillars, spore ornamention and other characters, in support of the above conclusion. An updated distribution map and a list of the specimens examined are added, because several new collections have been made since the original publications.

R. albovestita R. albovestita was described from Namibia by Volk (1981).It was only the fourth species in the by now quite large, endemic section he was later to name Pilifer' (Volk 1983), in reference to the free-standing dorsal cell pillars.Volk's early collections of this species (Volk 12458 p.p. and 12462) had been identified by Arnell (1957Arnell ( , 1963) ) as R. albomarginata (Volk 1981), a name which had been generally misapplied since Sim (Perold 1990a).Volk (1981) distinguished R. albovestita from R. simii Perold (= albo marginata sensu Sim), by the reticulate (not radiate) ornamentation of the spores.His good spore drawings (Abb.If) are clearly recognizable, but the SEM micro graphs (Abb.2) are perhaps less so, as the areolae on the distal face are mostly complete and lack a central 'knob'.SEM micrographs of Volk (X)484 (Figure 1A Arnell (1963) but as being R. albomarginata spores (Volk 1981).
The dorsal cells in the free-standing pillars of R. albo vestita as depicted by Volk, are of four cells, the top one conical, with the others rounded and isodiametric, the basal cell the same size as the others or smaller, but not wider.An attempt to revive the dorsal cells achieved limited success, yet it was found that some of the pillars were tapering, with distinctly wider (up to 62 /urn) basal cells.

Riccia duthieae
Subsequent gatherings,  trom Aberdeen in the central Cape, were tentatively also named R. albovestita, but after further study, Volk became convinced that this was yet another new species, later to be named R. duthieae, although in litt.[5.5.83 (PRE)]. 1 had informed him of the close similarity between the two species in the spore ornamentation on both faces, as seen on SEM micrographs (Figures 1 & 2).Unfortunately, I had no living material of R. albovestita from Namibia to study and to compare the dorsal cell pillars with those of the new collection.The cells of these pillars collapse when dry (Volk & Perold 1984), and can hardly be recon stituted in herbarium material, in order to examine their shape and size, which are very important taxonomic characters.R. duthieae was described (Volk & Perold 1985) as bearing a close resemblance to R. albovestita and R. parvoareolata, but was thought to differ from them by having 3 (or rarely 4) cells in the dorsal pillars (Figure 3F), as opposed to their 4 (or rarely 5)-celled pillars, and by having a different spore ornamentation.It has since become evident that the number of cells in the pillars and their shape can vary within a species.Generally, however, the pillars in these three species are relatively short and tapering.The proximal face of the spores of R. duthieae  (Figure 2A) bears a strong resemblance to that of R. albovestita ( Volk 00484, 01164) 2B, F) has fewer complete areolae than R. albovestita, but the larger, central ones contain a 'knob' as shown in the micrograph of S.M. Perold 1347 (Figure 2E).Two micrographs of less mature spores

R. sarcosa
Volk isolated specimens of the white-margined R. sarcosa from a mixed gathering of R. simii Perold (= R. of R. duthieae, .A sterile specimen, J.M. Perold 35, was thought to also belong here.At that time, it was not realized that with prolonged growth, the some what attenuate thallus margins of this species habitually turn white (Figure 3A), nor that we might be dealing with a widespread species (Figure 4) which was bound to show some variation.R. sarcosa was regarded (Volk & Perold 1986) as being distinct, on account of the white margin of the thallus (Figure 3A), its mostly inconspicuous scales that do not project above the thallus margins, the dorsal cell pillars which have inflated basal and smaller terminal cells and the spore ornamentation with deep-set, ringed areolae.As mentioned above, the attenuate thallus margins develop a white colouration with prolonged growth, and this is no longer regarded as a really reliable distinguishing character.In all three species the scales are apically prominent, especially in dry specimens, where the margins are incurved (Figure 3D), but along the body of the thal lus they hardly project beyond the margins (Figure 3C).The dorsal cell pillars are relatively low and tapering, with a wider basal cell (Figure 3F).The R. sarcosa spores that I originally examined and photographed by SEM (the only ones available to me then) were not fully mature and nearly flat, hence the ornamentation and even the size were misleading; nevertheless, comparison between the figures of Volk &Perold (1985: fig. 2.5 and1986: fig. 2.5) show some similarity.Micrographs of more mature spores of R. sarcosa specimens,   5A, C, D, F) and 81-292b (Figure 5B, E), clearly have similar proximal faces and their distal faces, especially of D), when compared with young spores of Smook 4036 (Figure 2C, D), have much in common, and toward the centre, the ornamentation also appears to be in two different levels; my LM photograph of R. sarcosa (Volk & Perold 1986: Figure 2.7) has eight areolae across the diameter on the distal face, as in R. albovestita.

DISCUSSION
It has previously been shown that thallus morphology can vary considerably within a species and that the environment plays an important part in determining thallus colour, width and thickness (Pandê 1924;Abeywickrama 1945).Scale morphology, particularly pigmentation and size are also affected (Berrie 1975;Na-Thalang 1969, 1980), which could account for the possibly somewhat larger, hya line scales in R. albovestita from Namibia with its hot, rather dry climate.
Spore size may vary quite widely under different environmental conditions (Seppelt 1974(Seppelt , 1983)), which could perhaps be another reason for the larger size of R. sarcosa spores, collected from specimens cultured under ideal circumstances.The ornamentation appears to be relatively stable, although Duthie & Garside (1936)   suggested that environmental differences could be correlated with variation in the ornamention in spores of R. crystallina (= R. plana).On the other hand, I could not convincingly prove or disprove the effect of the environment on spore sculpturing (Perold 1989b), as this would require controlled experiments using sophisticated equipment.
The shape and size of the cells in the dorsal pillars can vary from markedly tapering with a small conical top cell and wide basal cell to being almost uniformly wide.R. albovestita, R. duthieae and R. sarcosa [as well as R. alatospora Volk & Perold and R. hantamensis Perold (1989a)l are the only species in section Pilifer with relatively short, generally tapering dorsal pillars.In R. villosa Steph.and R. simii (= R. albomarginata sensu Sim), the dorsal pillars are long (3 0 0 -4 0 0 /xm) and tapering, whereas in all other species in the section they are of ± uniform width.In Table 1, a comparison of R. albovestita, R. duthieae and R. sarcosa is given.
Several more species belonging to section Pilifer have been identified and described in the last few years (Perold 1990b(Perold , 1990c) and hence more experience has been gained with the members of this section.The adoption of a somewhat wider species concept is clearly called for.In cases where no clear breaks occur in the range of characters such as the size, shape and number of cells in the dorsal pillars and the ornamentation of the spores, as shown in the above three species, their continued separa tion cannot be satisfactorily maintained.All three should therefore be treated under R. albovestita and the reduction of R. duthieae and R. sarcosa to synonymy of R. albpvestita is made as follows: ), 01164b (Figure IB, D) and 12462 (Figure 1C, E) taken by me, show low-walled or incomplete areolae, with irregularly raised and thickened or toothed nodes on the proximal face; the distal face of Volk 01164 has incomplete areolae, with only the suggestion of a central 'knob'; the two micrographs of the distal face of Volk 12462 differ in that there are 10 incomplete areolae across the diameter in Figure IE and only six on Figure IF.These spores were also illustrated by