This circumscription includes the familiy Helodiaceae Ochyra. This family is also accepted in Goffinet, Buck and Shaws systematics propposed in Bryophyte Biology, B. Goffinet and A.J. Shaw (Ed.), second edition, Cambrige University Press 2009
Type is the genus Thuidium Bruch & Schimp.
Order: Hypnales | Subclass: Bryidae | Class: Bryopsida | Subdivision: Bryophytina | Division: Bryophyta
The Thuidiaceae in size range from small to large. The plants usually form flat frondose lose wefts, regularly once to several times pinnate, in a seasonal fashion. The frond is arched, with a basal ascending, a middle parallel and a distal descending part (in regard to the substrate). The distal part is more delicate, with shorter and more distant branches compared to the middle. The apex of the stem is often somewhat stoloniform with a small apical bud. The bud can develop into a new frond in the next season, however lateral growth is also found. Most rhizoids are found at places of contact between the stem and the substrate. Often this is the base and apex of a frond [3]. Branching is often in a single plane parallel to the substrate, with the branches originating laterally, but not in two exact rows. In some species a "pseudotetrastichous" arrangement is found. Primary branches appear to originate in four lines, but actually do along the edges of two lateral sectors[3]. Variations of this architecture include [3]:
While [1] claims that in the stem cross section usually no central strand can be seen, their source [3] says that in most genera a central strand of a few smaller cells with thinner cell walls is present.
Axillary hairs originate from the stem in the axils of leaves. Their number per leaf is variable, but seems to be above two. Usually they are formed by one to two basal and one to seven distal cells. In general the basal cells have brownish to orange cell walls, the distal cells are usually colourless, occasionally yellowish to brownish. The cause of this colour difference could be taxonomic, genetic or because of damage[3].
The hairs are seen as a useful character for organizing the Thuidiaceae into genera. However, they seem to get lost fast during maturation of the leaf. Most reliable they are found in dissection of buds [3].
Both paraphyllia and pseudoparaphyllia are found in all genera of Thuidiaceae but are difficult to differentiate. Branch primoridia are surrounded by foliose pseudoparaphyllia, with a lanceolate to ovate-triangular to ovate outline and denticulate, serrate, spinose, irregular indented or appendiculate margin. Outwards from the primordium the differentiation reduces until the pseudoparaphylllia are indistinguishable from paraphyllia. However, in some species, structures similar to pseudoparaphyllia can be found all over the stem, somtimes quite numerous and noticable by a lack of paraphyllia [3].
Besides a few taxa and specimens with poor development paraphyllia usually form a dense mat on the stem. They consist of one to several smooth, colourless to greenish to occasionally brownish basal cells, intermediate cells and a single apical cell at the tips. In other properties far reaching variation is found [3]:
As already mentionied under "Gametophyte Architecture", rhizoids are often found at contact points between the main stem and the substrate. They are clustered just below the stem leaf insertions. In colour they range from brown to orange-brown to red brown, in branching from simple to strongly ramified, in ornamentation from smooth to papillose. The exact structure seems to vary in a genus and even between members of the same species [3].
The stems branch regularly in a pinnate fashion, with one to three levels of branching. This gives them a more or less frondose appearance. In the cross section of the stem usually no central strand can be seen. Paraphyllia are numerous and dense. At insertion they are uni to pluriserate, they can be branched or single with short to elongated cells and usually show some kind of ornamentation. Pseudoparaphyllia are leaf-like. Axillary hairs are two to eight cells long with one to two basal cells.
Anisophylly is usuall. Leaves on the stem are usually cordate, ovate-triangular or triangular, with a markedly recurved margin. Two types of lamina can be differentiated: thuidioid, with isodiametric to oblong cells and helodioid, in witch at least the upper cells are linear or linear-rhomboid (But see Cytology and molecular biology!). Cells usually are ornamented with one or several papillae (uni- to pluripapillose) either on both or only the dorsal face of the leaf. The alar cells are not or only weakly differentiated. Exception is the Genus Helodium. The costa is single, thick and strong (stout) and often projects as a spine.
Distribution of gametangia is dioicous, cladautoicous or seldom synoicious.
The seta of the sporophyte is usually elongated, smooth or mamillose to spinulose. The capsule is typically curved. The stomata are elongated pores. The peristome is complete, occasionally reduced. The callyptra is cucullate, rarely campanulate, and usually smooth.
Haploid chromosome number n = 10, 11, 12, 20, 22.
Molecular analysis of both platid and nuclear DNA supports this taxon. A separation of heloid and thuidioid taxa is however rejected.
The family is cosmopolitan in distribution, and found on both hemispheres.
It is found on a variety of substrates.
The Thuidiaceae are comprised of 15 genera with approximately 140 species.
Goffinet, Buck and Shaw list Abietinella Müll. Hal., Boulaya Cardot, Cyrto-hypnum (Hampe) Hampe & Lorentz, Fauriella Besch., Pelekium Mitt., Rauiella Reimers, Thuidiopsis, (Broth.) M.Fleisch., Thuidium Bruch & Schimp.