Studies in the Marchantiales (Hepaticae) from southern Africa. 8. The genus Plagiochasma (Aytoniaceae: Aytonioideae) and six local taxa

A taxonomic account is given ot the genus Plagiochasma Lehm. & Lindenh. and its two subgenera. Micropvlum Bischl. and Plagiochasma. The first subgenus is represented in southern Africa by P. rupestre var. rupestre (J.R. & G. Forst.) Steph. and P. rupestre var. volkii Bischl.: the second by P. appendiculatum Lehm. & Lindenb. (newly recorded for the region). P. heccarianum Steph.. P eximium (Schiffn.) Steph. and P. microcephalum (Steph.) Steph. var. microcephalum. Descriptions and illustrations of these taxa together w ith distribution maps and a key to the subgenera and species are provided. (J Steph.

banks. Brunches lingulate and simple to pseudodichotomously or variously furcate, w ith lateral or apical innova tions from keel, sometimes articulated: thickened over midrib, thinning tow ard attenuate, narrow ly purple, scalloped margins, apically notched, with scale appendages recurved over edge. Dorsal epidermis mostly lacking chloroplasts. thick-or thin-walled w ith trigones, roughened or smooth and w ith or w ithout waxy, granular deposit externally. Air pores simple, sometimes ± stellate, minute and very inconspicuous or larger and slightly raised, encircled by 1 ring or by 2(3) concentric rings of (4)5-8 cells in each, radial w alls of cells often forming continuous lines which may be thickened, pores leading below into small, compact, empty air chambers in several irregular layers, bounding walls chlorophyllose. some scattered cells nearly filled with a single oil body, also present in storage tissue, where cells are closely packed: rhizoids ventral, some smooth, others pegged. Scales purplered to violet, in 2 forwardly directed ventral rows, large, extending beyond thallus margins or not. basal portion broad ly ovate, apically with 1 or 2(3) appendages, variable in shape, linear-lanceolate or ovate to orbicular, sometimes con stricted or folded at base, containing a few scattered oil cells: scale margin entire or tix>thed and w ith papillae in subgenus Plagiochasma. but w ithout in subgenus Micropylum.
Monoicous. autoicous or paroicous. Androecia w ith antheridia sunken in tumid, sessile, crescentic to broadly Uor V-shaped dorsal cushions, base encircled by short paleae. Anhegoniopltores dorsal, single to several acropetally arranged along length of leading branch, usually w ith tuft of slender paleae around base and eventually mostly at apex of very short to long unlunrowed stalk, bearing carp(xephalum with ( l-)2-4<-5) capsules, each on a short seta, capsule wall unistratose. cells lacking thickening bands, dehiscing by an irregularly decaying lid and Subgenus Micropylum Bischl.
Micropylum is characterized by very compact, velvety thalli, the dorsal surface covered with a water-repellent, granular deposit and the numerous air pores tiny and in conspicuous; the ventral scales have acuminate ap pendages with undifferentiated margins, lacking teeth and papillae. Plagiochasnui rupestre var. rupestre a n d P rupestre var. volkii are placed here.
Thallus medium-sized to quite large, nearly flat to somewhat concave with the sides slightly raised, oblong to lingulate ( Figure 1A), compact, glaucous to greyish green, dull, surface ± water-repellent, narrow purple edge along margins, pores very inconspicuous, almost imper ceptible as minute dots, subdorsal air chamber walls hard ly visible, when wet; thallus margins incurved or clasped together, exposing deep purple to dull black, transversely striate underside of wings, toward apex sometimes covered with a dull bloom and medianly always by scales, when dry; in crowded, gregarious patches, simple or once, rarely twice furcate or with apical or latero-ventral in novations and then articulated or jointed. Branches 8-25 x (3-)4-6 mm, 375-650 (im thick over midrib, laterally thinning out into attenuate wings ( Figure IF); apex notched, with appendages of 8-10 deep pinkish purple or purple, sometimes partly decolorate scales recurv ed over edge; margins acute, thin, scalloped and undulate: flanks sloping obliquely, purple; ventral face medianly keeled, green, with row of purple-red or purple scales on either side ( Figure IE).
Two varieties are recognized: P. rupestre var. rupestre and P. rupestre var. volkii. They are distinguished by the longer, acuminate, hyaline appendages of the ventral scales and slightly larger spore dimensions of the latter. la. P. rupestre var. rupestre Obliquely lunate ventral scales rarely longer than 2000 (im and 1050 (im across base; 1 or 2 narrow ly to broadly triangular appendages up to 9(X) (im long. 1.7-2.7 times longer than wide, and apically acuminate with uniseriate 1 or 2(3) terminal cells, quadrate or rectangular in shape and thin-walled: cells in curved margin of body of scale rectangular, sometimes bulging slightly outward where 2 cells join: male paleae around basal part of androecium 550-580 (ini long. 110-180 (im w ide at base, tapering to a narrow tip. with 1-3 cells in series, ± 32 x 20 (im ( The typical variety is subcosmopolitan and w idely dis tributed, especially in xerothermic regions. Frey & Kurschner (1988) regard it as a xerothermic Pangaean taxon. In southern Africa it is quite common and frequent ly collected in rocky crevices, moist ledges, under boulders, or at seepages, on calcareous substrates or on soil overlying cave sandstone or dolomite. It sometimes grows together with Targionia hypophylla, Athalamia  well as from Northern, Western and Eastern Cape ( Figure  3). Its range in Africa extends northward into Zimbabwe from where it was also reported by Best (1990) and by Bischler (1978), who reported it from Kenya, Tanzania, Uganda, Ethiopia. Djibouti, Sudan. Chad. Morocco, Al geria and Angola, as well as from the islands of Madeira, Azores, Ascension, Cape Verde, St Helena and Reunion (Bischler 1990).
Plagiochasma rupestre var. rupestre is easily identified by its dull, velvety and glaucous thalli with simple, very inconspicuous pores and by its reddish pink or purple scales, with ovate-lanceolate or acuminate appendages, the margins of which are entire.
Puiple ventral scales lai^er (up to 3000 x 1350 |im) and more conspicuous than those of typical variety, espe cially 2 or 3 hyaline appendages, which are narrowly tri angular, 1370-1450 |im long and 4 or 5 times longer than wide ( Figure 10, P). Apex rather fragile, 3-5 elongated cells in series, walls thickened; cells at rounded margin of body of scale irregular in shape and size, cross walls often oblique; male paleae ± 600 |im long, 150 fim wide at base, tapering to a narrow tip where up to 4 cells, 50 x 15 jam, are in series ( Figure IQ): female paleae up to 1500 pm long, 1 (X)-150 pm wide at base, tapering to a narrow tip with 3 or 4 cells, 37.54-2.5 x 12.5-17.5 pm. in series ( Figure 1R); spores ( Figure 2F-H) 92.5-105.0 pm in diameter, slightly larger but very similar in ap pearance to those of typical variety.
This variety is quite rarely collected in southern Africa, but fairly frequently both varieties grow together. In the present investigation specimens of P. rupestre var. volkii from Namibia, the Northwest, Gauteng (PWV), Northern Transvaal, as well as KwaZulu-Natal have been examined ( Figure 3). Bischler (1978) had also studied plants from the Western Cape, Orange Free State. Lesotho and Zim babwe, so that it occurs throughout most of southern Africa. Schuster (1992) states that P. rupestre var. volkii also occurs in Peru and Argentina and it is thus not en demic to southern Africa.
In P. rupestre var. volkii the thallus is generally some what narrower than in the typical variety, but otherwise it is very similar in colour, appearance and composition. The very long, decolorate scale appendages are con spicuous, however, and make it easily separable.

Subgenus Plagiochasma
Most species are assigned to subgenus Plagiochasma. which is characterized by less compact green or yellowgreen thalli, dorsally with quite large, raised air pores, surrounded by a hyaline ring and 2 or 3 concentric rings of (5)6-8 cells in each, radial walls generally forming continuous radiating lines that can be somewhat thick ened; air chamber walls faintly visible from above and scale and appendage margins differentiated with smaller cells, teeth or papillae.
Thallus large, robust, Hat with sides sometimes curved slightly downwards or upwards, broadly lingulate ( Figure  4A, B). bright green, shiny, with narrow purple edge along margins, pores visible, small and slightly raised, when wet: thallus margins incurved or inflexed, exposing shiny, reddish purple to deep purple transversely striate and wrinkled underside of wings, not covered by scales, when dry; in crowded, gregarious patches, simple or once. sometimes twice furcate, rarely jointed with apical or latero-ventral innovations. Branches 10-20 x 5-8 mm. 750-925 pm thick over midrib, laterally thinning out into attenuate wings ( Figure 4D); apex notched, with large orbicular reddish or partly hyaline scale appendages recurved over edge in 2 layers: margins acute, thin, scal loped and slightly undulate: flanks sloping very obliquely, reddish or purple: ventral face medianly keeled, green, with row of deep red or purple red scales on either side ( Figure 4C). Dorsal epidermal cells unistratose, hyaline, ± rectangular to polygonal. 22.5-42.5 x 15.0-27.5 pm. walls thin but thickened at comers, in transverse section 30.0-37.5 |im thick, smooth externally, along margins 2 or more rows of cells, rectangular, up to 25 x 12 pm or shorter than broad. ± 15 x 22 pm ( Figure 4G); air pores not so numerous. 125-225 pm distant from each other, slightly raised ( Figure 4E), simple. 7.5-10.0 |im wide, sur rounded by an innermost ring. ± 2.5 |im wide, of tiny collapsed cells and then by 2 (occasionally partly by 3) concentric rings of larger cells, 5 or 6 inner ones transver sely oval or round. 10-15 x 15-20 pm ( Figure 4F) partly overlying outer row' of 5 or 6 bluntly triangular cells, ± 22.5 x 30.0 pm across widest part, radial walls not thick ened. Assimilation tissue 375-500 pm thick. ± thick ness of thallus, air chambers empty, in several layers, upper ones ± 25 pm wide, lower down wider, ± 62.5 pm wide, cells in bounding walls 30-45(-50) x 22-37 pm. some with a brown oil body. 25-30 pm wide: storage tissue occupying ventral */* of thickness of thallus. cells angular, up to 45 pm wide, with spaces between them, some smaller cells also with oil bodies; rhizoids either smooth. 12.5-25.0 pm wide, or pegged. 12.5 pm wide. Scales deep red. appendages mostly decolorate and some times base as well, arranged in 2 forwardly directed ventral rows, one on either side of midrib, asymmetric, obtusely triangular with flatly arched base, gradually nar rowed above, deeply constricted and folded where joined with large, mostly single, orbicular appendage ( Figure 4H. I), the latter up to 750 pm long. 550 pm across widest part in middle. 300-375 pm w ide at base, at margin 1 or 2 rows of small rectangular cells 10.0-17.5 x 7.5-12.5 pm. alternating with somewhat larger cells, in centre of appendage toward base, cells large, rectangular. ± 75.0 x 37.5 pm. surrounded by several rows of irregularly shaped cells, variable in size ( Figure 4J); body of scale up to 1250 pm long. 11 (X) pm across base, cells rectangular or 5-sided. ± 70 x 25 pm. 6 or 8 smaller, scattered ones w ith remains of oil bodies. ± 27.5 x 20.0 pm: at margins cells small, ± 25.0 x 12.5 pm. walls thin, curved, occasionally with long, projecting papillae.
After P. rupestre, P. appendiculatum was one of the earliest species in the genus to be described. It is generally a large plant and is easily recognized by its mostly single, very rarely double, large, orbicular scale appendages. Bischler (1978) examined a large number of specimens and found quite considerable variation in the size of the thallus and spores as well as in the thickness of the radiat ing cell walls at the air pores. In the present study only three southern African specimens were available for study: a fourth one is from Zimbabwe and was collected at Es sex vale by Borle (also reported by Bischler), but the exact locality could not be traced. Three other specimens seen, were from elsewhere. Plagiochasma appendiculatum has not previously been reported from southern Africa, al though it had been collected by Bottomley in 1930 at Pelindaba, the specimen being held at PRE. Unfortunately it was misidentified as a species of Asterella and was not sent on loan to Bischler, when she was revising the genus. Recently I have collected specimens of it twice (the second time with M. Koekemoer) in Bakker's Pass, east of Thabazimbi in southwestern Northern Transvaal (Fig  ure 6), where it grew between boulders in a shady, damp gulley, together with Fissidens sp. and Philonotis sp. Arnell (1963) refers to P. fischeri, the synonym of P. appen diculatum, in his key to the species of Plagiochasma, but there is no description or illustration of the species.
Plagiochasma appendiculatum is chiefly an Asiatic species and has been reported from Afghanistan, Burma, Celebes, China, Taiwan [Formosa], India, Kashmir, Nepal, Pakistan, Philippines, Sikkim and Vietnam by Bischler (1978). She has also reported it from Yemen and Socotra as well as from Ethiopia, Kenya and Zimbabwe, where it is rare. Bizot et al. (1985) reported it as new from Tan zania and regard it as a palaeotropical species. Frey & Kurschner (1988) also reported it from the Arabian Penin sula and from Socotra; they assume that it is of xerother mic Pangaean origin.
Monoicous. Androecia in small sessile cushions, rounded or V-shaped. ± 1 mm wide, medianly along length of thallus and alternating with archegoniophores, occasionally paired at pseudodichotomy of branches, antheridia opening above via prominent papillae, basally cushions encircled by shallow groove in thallus and sur rounded bv inconspicuous purple paleae that taper slightly toward apex ( Figure 7M), ± 4(X) x 200 pm. cells 32.5-37.5 x 15.0-20.0 pm. several mucilage papillae projecting along margins and from apex. Archegoniophores acropetally arranged along length of thallus and alternating w ith androecia, enclosed by arching, hyaline or purple paleae. ± 500 x 150 pm. tapering toward apex ( Figure 7N). cells rectangular. 35.0-45.0 x 17.5-22.5 pm. sometimes with a few mucilage papillae at margins. Carpocephala small. 1.0 x 2.0 mm. when 2 lobes present ( Figure 7C). raised on stalk, up to 15 mm long and variously twisted, striate and dark red. ± 310 pm in diameter, in transverse section ( Figure 7D) cortical cells in 1 row. thick-walled externally and rounded. 12.5-17.5 x 12.5-15.0 pm. medullary cells thin-walled, angular. 12.5-17.5 pm wide. Spores 70-75 pm in diameter, triangular-globular, polar, light brown, translucent, wing ± 10 pm wide, pore at comers occa sionally present, margins undulate, finely crenulate. or namentation similar on both faces: distal face ( Figure 8A. B) with 4 areolae across. 15-20 pm wide, walls narrow, only ± 2.5 pm w ide, almost snuxuh to granular: proximal face w ith distinct triradiate mark, amis thin. 5.0-7.5 pm high. 4 areolae on each of 3 facets, walls almost smooth ( Figure 8C-E) to granular. Elaters light brown. 212-3(K) pm long. 7.5 pm wide in middle, tapering toward ends. ± 5.0 pm wide, bi-or trispiral ( Figure 8F). Chromosome number, n = 9 (Bischler 1978). Bischler (1978) recognized two varieties of P. micro cephalum. namely var. microcephalum and var. tunesicum Bischl. The latter appears to be restricted to Tunisia, whereas she reported the former from the following African countries: Ethiopia. Uganda. Angola. Namibia and South Africa. It is also known from Madagascar. Yemen and southwest India. In the present investigation, a few more specimens from Northern Transvaal. Gauteng (PWV). the Northwest and Northern Cape have been iden tified ( Figure 6). It can be distinguished by the green colour of the fresh thallus and by the generally large, broadly ovate or triangular scale appendages which have one or two rows of smaller cells along the margin. Bischler (1978) reported that plants growing in drv habitats have scales with a single, broadly ovate ap pendage which is constricted at the base, whereas those from more humid sites usually ha\e one or two triangular This species is equally well adapted to dry or to damp habitats. It grows in dry river beds between limestone boulders or on granite hills or on damp stream banks, but has not been frequently collected. Plants with mature female receptacles are rare. The carpocephala are small (hence the specific epithet) and are raised on a slender, short or long, dark red stalk. Frey & Kurschner (1988) regard it as a palaeotropical taxon with a tropical African distribution. Sim (1926) stated that P. dinteri [placed in synonymy under P. microcephalum by Bischler (1978)] was not known to him and he quoted a few particulars from Stephani (1901b). Amell's (1963) description of P. dinteri contains several inexactitudes as already observed by Bischler (1978): thal lus dimensions '15 cm long and 16 mm wide; peduncle apical, spores 40-50 |im in diameter, without a wing or with a rudimentary one, occasionally with a wing up to 4 (im wide; surface irregularly areolate\ One of the specimens seen by him [Essendale (sic)] is the Borle col lection from Essexdale and belongs to P. appendiculatum. The collection from Hennops River, of which he illustrates the spores, has not been traced. The Holst specimen (the type of P. microcephalum) was seen by Amell and under P. tenue he states that it belongs to another species, which is certainly correct, since P. tenue is a synonym of P. rupestre, but he failed to notice the similarity between P microcephalum and P. dinteri.
Plagiochasma eximium has previously been reported from southern Africa by Bischler (1978), who had ex amined a specimen, Sim CH 1163, from Mont-aux-Sources which had earlier been identified as P. rupestre. Two other specimens, Sim CH 1200 (Pietermaritzburg) and Sim CH 1186 (Victoria Falls), have now been identified as P. eximium, as well as some later collections from the  Figure 6). Amell (1963) gave no descrip tion or illustration of P. eximium, merely listing it in his key to the species of Plagiochasma. This species is widespread in Africa and has been reported by Bischler (1978) from Sierra Leone. Guinea. Cameroons, Zaire, Djibouti, Ethiopia, Uganda, Kenya. Tanzania. Malawi and the following islands: Cape Verde. Reunion and Madagas car (Bischler 1990). Its distribution extends to the Arabian Peninsula: Saudi Arabia, Yemen. Oman and to Socotra. Frey & Kiirschner (1988) consider P. eximium to be a palaeotropical taxon with tropical African distribution.
Many of the collections are from high altitudes, such as the Drakensberg in southern Africa, where the plants grow in shady kloofs on muddy rock faces or on soil covering rocks or under boulders. As far as could be as certained. the specimen, Sim CH 11H6, from Victoria Falls seems to be the first record of this species from Zim babwe.
Plagiochasma eximium can be distinguished by its robust, yellow-green thalli with large, deep pink to winered ventral scales narrowed above into (1 )2 or 3 tapering appendages which are only a little constricted or pleated at the base. Ventrally the flanks are wrinkled and deep red, not dark purple. Elaters from the few spore-bearing plants studied are bispiral, but Bischler (1978) also found them to be 3-or 4-spiral and generally distinct along only a small part of their length. Bischler (1978) states that herbarium specimens of P. eximium can look quite different from each other. Those from damp, shady places (e.g. a cave) have thin, olivegreen thalli and decolorate margins, large epidermal cells with thin walls, and smaller, pale scales. Others have thicker, yellowish green thalli with pigmented margins, slightly smaller epidermal cells with well-developed trigones and large, dark red scales. Bischler (1978) con siders them to be ecological variants of the same species. Other characters that are quite variable are the width of the thalli, the thickness of the radial walls at the air pores, the presence or absence of teeth at the base of the scale appendages, the spore ornamentation and the presence or absence of spiral thickenings in the elaters.
Thallus medium-sized to rather large, flat to very slightly concave medianly and gently arching downward toward margins, ligulate to lingulate or broadly ovate (Fig  ure 11 A, B), bright green, shiny, along narrow edge of margins purple; finely and irregularly areolate. pores dis tinctly visible, raised, when wet; thallus margins frequent ly tightly inflexed or sometimes incurved, exposing shiny, ink-black or very dark purple, transversely striate under side of wings, partly covered by purple scales, when dry; in crowded, gregarious patches, once or twice pseudodichotomously furcate, sometimes with apical innovations and then articulated. Branches 8.0-20.0 x 3.5-5.(K-6.0) mm. 600-850 Jim thick over midrib, laterally thinning out into attenuate wings ( Figure HE); apex notched, with several purple ovate-lanceolate scale appendages recurved over edge; margins acute, thin, sparingly scalloped and slightly undulate; flanks sloping obliquely, black or purple, ventral face medianly keeled and green, partly obscured by 2 rows of purple scales, one on either side ( Figure 11C).
Plagiochasma heccarianum is regarded as quite a heterogeneous species, but so closely resembling P ex imium that the two species are often difficult to distinguish when sterile (Bischler 1978). In the few southern African specimens of P. heccarianum available for study, it ap pears that the fresh thalli are a clear green dorsally, not yellow-green, and the underside of the wings as well as the scales are deep purple and not dark red as in P ex imium. The two or three scale appendages of P. beccarianum are ± acuminate or narrowly triangular and not constricted at the base; their margins are irregularly toothed.
This species is rare and has been reported from rela tively few places in Africa; except for Namibia (Volk 00950), it is mostly found along the eastern part of the continent, namely Ethiopia, Tanzania and Zambia (Bischler 1978). It has also been reported from the Arabian Peninsula (Frey & Kurschner 1988) and from Socotra (Bischler 1978;Frey & Kurschner 1988). Some specimens from the Northern and Eastern Transvaal have recently been collected (Figure 6), where they grew in a shady kloof under boulders or in a rocky crevice in Blyde River Canyon respectively. Frey & KUrschner (1988) regard it as a palaeotropical taxon with a tropical African distribution. Bischler (1978) states that P. heccarianum belongs to a complex represented by several species in Asia and America. She thinks that this group probably diversified more rapidly on these two continents than in Africa and that the heterogeneity of P. heccarianum could be due to a slower African evolution of the complex which has not as yet achieved the separation of distinct taxonomic units.