A Key to the family Rhachitheciaceae in Africa

Extract from: O'Shea, B.J. 1997. British Bryological Society Expedition to Mulanje Mountain, Malawi. 9. Regmatodontaceae, Rhachitheciaceae, Rhacocarpaceae and Rhizogoniaceae (Bryopsida). Journal of Bryology 19(4).

NB: since this was written, Hypnodontopsis mexicana has been found in Uganda (see Hodgetts, N.G. & Goffinet, B. 1998. Hypnodontopsis mexicana (Thér.) H.Rob., a genus and species new to Africa. Journal of Bryology 20: 251-252.

There are only five species of Rhachitheciaceae in Africa, and all, although distinctive, have been collected only comparatively rarely. The family was created by Robinson (1964) to include two genera previously included in the Orthotrichaceae, Rhachithecium Broth. ex Le Jol. and Hypnodontopsis Iwats. & Nog., but also including Rhachitheciopsis P. de la Varde as a subgenus of Rhachithecium. Jonesiobryum Allen & Pursell was added to the family by Allen & Pursell (1991), and Zander (1993) added Tisserantiella P. de la Varde. The family circumscription is still subject to debate, and it may possibly be paraphyletic. Zander (1993) described the family as "an apparent link between the Pottiaceae and the Orthotrichaceae". The distribution is largely tropical with some occurrences in the subtropics.

The family may be recognised by an orthotrichaceous appearance with distinctive 8-ribbed capsules, spathulate stem leaves, sheathing perichaetial leaves, and capsules annulate and with a single peristome consisting of 16 teeth fused into 8 pairs.

The five species are in three genera: Rhachithecium (3 species), and Jonesiobryum and Tisserantiella with one species each. Jonesiobryum sphaerocarpum Allen and Pursell is known in Africa only from one collection in Nigeria, but is also found in South America. Tisserantiella pulchella (Thér. & Hilp.) Zander is known only from single collections in Cameroon and Zaire.

Generic key to African Rhachitheciaceae

1	Capsule immersed	Jonesiobryum
	Capsule exserted	2

2	Peristome absent	Tisserantiella
	Peristome present	Rhachithecium

For further description and illustration of Jonesiobryum (as J. sphaerocarpum Biz. ex Allen & Pursell), see Vital (1983) and Allen & Pursell (1991), and for Tisserantiella (as T. pulchella (Thér. & Hilp.) Zander) see Potier de la Varde (1941) and Zander (1993).

Rhachithecium Broth. ex de Jolis

The genus is characterised by the green, oblong-spathulate stem leaves contrasting with the pale convolute-sheathing perichaetial leaves, and the erect seta with a twisted apex (often curved when moist), the ribbed, annulate capsules apparently with eight triangular teeth, and the small cucullate scabrous calyptrae. Plants have usually been collected and described with the distinctive capsules, but this may be because it has been overlooked in its sterile state.

Key to African species of Rhachithecium

1	Perichaetial leaves not sheathing seta, operculum obtuse, annulus bistratose, calyptra smooth (only known from two collections from Central African Republic, and until recently separated in the genus Rhachitheciopsis)	R. tisserantii
	Perichaetial leaves sheathing seta, operculum bluntly apiculate, annulus single or double, calyptra scabrous	2

2	Annulus single	R. perpusillum
	Annulus double (only known from type specimen from Angola)	R. welwitschii

Rhachithecium perpusillum is described and illustrated in Crum (1956, 1994) and De Sloover (1976). Potier de la Varde (1926) and Iwatsuki (1957) described R. tisserantii, and Iwatsuki also illustrated it. Rhachithecium welwitschii differs from R. perpusillum only on the basis of the annulus.

Rhachithecium perpusillum is widespread in Africa (Guinea, Nigeria, Central African Republic, Zaire, Burundi, Malawi, Tanzania, Zambia, Zimbabwe, South Africa (Transvaal), Madagascar), as well as occurring in USA, Mexico, Brazil and Argentina in America, and in Sri Lanka, India, China and Philippines in Asia. All authors other than Jones (1980) have described its habit as epixylic, particularly as a coloniser of burnt bark in savanna areas, but Jones mentioned its occurrence on rock in Malawi and Madagascar, suggesting that humus covered rock also meets its habitat requirements, and that when growing on tree bark, burning did not always seem to have taken place.

References
Allen B, Pursell RA. 1991. A reconsideration of the systematic position of Jonesiobryum. Bryologist 94:438-442.
Crum H. 1956. Notes on Hypnodon, a genus of Orthotrichaceae new to North America. Bryologist 59: 26-34.
Crum H. 1994. Rhizogoniaceae, Rhachitheciaceae. In: Sharp AJ, Crum H, Eckel PM, eds. The Moss Flora of Mexico. Memoirs of the New York Botanical Garden 69: 588-590.
De Sloover JL. 1976. Note de bryologie africaine. VII. - Pseudephemerum, Bryohumbertia, Eucladium, Streptopogon, Ptychomitrium, Rhachithecium, Antitrichia, Pterogonium, Lindigia, Distichophyllum. Bulletin du Jardin Botanique National de Belgique 46: 427-447.
Iwatsuki Z. 1957. The genus Hypnodon and its allies. Bryologist 60: 299-310.
Jones EW. 1985. Rhachithecium perpusillum (Thwait. & Mitt.) Broth. in Africa. Cryptogamie, Bryologie. Lichénologie 6: 47-50.
Potier de la Varde R. 1926. Rhachitheciopsis P. de la V., genre nouveau d'Orthotrichacées de l'Afrique tropicale. Bulletin de la Société Botanique de France 73: 74-76.
Potier de la Varde R. 1941. Tisserantiella P. de la V. genus novum familiae Pottiacearum. Bulletin de la Société Botanique de France 88: 469-472.
Robinson H. 1964. New taxa and new records of bryophytes from Mexico and Central America. Bryologist 67: 446-458.
Vital DM. 1983. Two new species of Jonesiobryum (Musci) from the Brazilian cerrado regions. Journal of Bryology 12: 383-391.
Zander RH. 1983. Genera of the Pottiaceae: mosses of harsh environments. Bulletin of the Buffalo Society of Natural Sciences 32: 1-378.

TBG48/rhac Issue 1 - 27.02.97