Rhizogoniaceae

Rhizogonium novae-hollandiae, photographed by Neils Klazenga.

Belongs within: Bryales.

The Rhizogoniaceae is a family of mosses distinguished by the position of their sporophytes, attached to the gametophytes at the base or basal half of the erect stems. Members of the Rhizogoniaceae are primarily tropical or subtropical, and many species are epiphytic (with a particular liking for tree ferns).

Mosses have a place for reproduction
Published 5 December 2011
Rhizogonium sp., photographed in the Philippines by Leonardo L. Co.

The Rhizogoniaceae are a family of mosses found in tropical and subtropical parts of the world, with a concentration of diversity in the Southern Hemisphere. Many species in the family are epiphytic; in particular, many show a preference for growing on the trunks of tree ferns (O’Brien 2007). The family has been defined by features such as sharply toothed, usually bistratose (i.e. with two cell layers) leaves and sporophytes located in the basal half of the erect stems, but molecular studies have indicated that the Rhizogoniaceae in the broad sense are para- or polyphyletic, and for this post I’ll be using Rhizogoniaceae in a more restricted sense, corresponding to the ‘clade C’ of O’Brien (2007), including genera such as Rhizogonium, Cryptopodium, Calomnium, Goniobryum and Pyrrhobryum. One member of the Rhizogoniaceae, Pyrrhobryum dozyanum, is often used in moss gardens (it appears that there may also be a moss doing the rounds under this name in the European aquarium trade, though I haven’t found anything to confirm whether this species, also being referred to as “Mayaca fern” or “Indonesiae bogoriensis”, is actually P. dozyanum. Many bryophytes and other such plants in the aquarium trade have been misidentified, sometimes dramatically so).

View under microscope of leaf of Pyrrhobryum dozyanum, showing the toothed margins characteristic of Rhizogoniaceae. Image from here.

Most attention on Rhizogoniaceae from an evolutionary point of view has focused on what they might say about the relationship between acrocarpy and pleurocarpy. To explain what these terms mean, we’ll start with the following diagram (from here):

Like other plants, mosses go through an alternation of generations, with both haploid and diploid multicellular stages. The haploid stage of the life cycle, the gametophyte, is the leafy green part of the moss. The gametophyte produces perichaetia, whorls of modified leaves within which the gamete-producing organs are contained. When a female gamete is fertilised, the resulting diploid zygote grows into the sporophyte, the brown thread-like structure you will often see growing out of a moss. The sporophyte produces haploid spores that will be dispersed to grow into new leafy gametophytes.

The diagram above shows an acrocarpous moss, in which the perichaetium is produced at the end of a growing branch of the gametophyte. Other mosses, however, are pleurocarpous, with perichaetia produced on the side of a branch. Whether a moss is acrocarpous or pleurocarpous is one of the first things a botanist will look at when attempting to identify it. However, many Rhizogoniaceae do not easily fall on either side of the acrocarpous/pleurocarpous distinction. They are what is called cladocarpous: the perichaetia are produced at the ends of small side-branches. However, lest any moss enthusiasts accuse me of overly simplifying things, I must point out that a great deal has been written on the exact distinctions between acrocarpous vs cladocarpous vs pleurocarpous. Like so many distinctions in nature, there are examples that blur the distinction between these states. As the perichaetia-bearing side-branches in a cladocarpous moss get progressively shorter, they become less and less distinguishable from pleurocarpy. In light of this, recent authors have suggested that the distinction between cladocarpy vs pleurocarpy should be defined by whether or not the side-branch bearing a perichaetium also bears normal vegetative leaves. If it only bears perichaetial leaves, then it is pleurocarpous: by this definition, some Rhizogoniaceae (including the genus Rhizogonium) are truly pleurocarpous (Bell & Newton 2007).

Goniobryum subbasilare, photographed by David Tng.

The vast majority of pleurocarpous mosses belong to a clade called the Hypnanae, which is massively speciose (probably about half of living mosses are hypnanaens). Because the hypnanaen mosses are so successful, there is a lot of interest in their relationships with other mosses. And as it turns out, the Rhizogoniaceae (with their combination of cladocarpous and pleurocarpous members) are closely related to the Hypnanae. Indeed, the Hypnanae are nested within the older, paraphyletic grade referred to the Rhizogoniaceae (O’Brien 2007). The acrocarpous state is the plesiomorphic one for mosses, with cladocarpy evolving in numerous lineages. Pleurocarpous mosses, it seems likely, have then evolved from cladocarpous ancestors, though either a number of times or with a number of reversals.

Systematics of Rhizogoniaceae

Characters (from S. P. Churchill, Bryophyte Flora of North America): Plants small to large, in loose to dense tufts. Stems erect or curved, few branched; central strand well developed; radiculose, often densely tomentose. Leaves spirally arranged or appearing 2-ranked, distant or rather crowded, ovate to narrowly or broadly oblong-lanceolate to linear-lanceolate, apex acute to acuminate, base decurrent or not; margins plane or reflexed to recurved, crenulate, dentate or sharply serrate, teeth single and margins 1-stratose or double and margins 2-stratose; 1-costate, strong, percurrent to short-excurrent, toothed abaxially distally or smooth; laminal cells mostly isodiametric and smooth or bulging mammillose, walls firm and entire; alar region undifferentiated. Specialized asexual reproduction lacking or present, in leaf axils of distal stems and branches, cylindrical. Sexual condition synoicous, autoicous, or dioicous. Perigonia bud-like, usually below perichaetia. Perichaetia lateral, at base or at mid stem, leaves small and differentiated. Seta elongate, smooth. Capsule horizontal to erect, short to long-cylindrical, symmetric to asymmetric, curved or straight; opercula conic, short to long-rostrate, oblique; peristome double or absent, exostome teeth 16, cross-striate proximally, papillose distally or papillose throughout; endostome basal membrane moderately high, segments 16, keeled, perforate, cilia usually present, 2–3. Calyptra cucullate. Spores spherical, lightly papillose.

Rhizogoniaceae
    |--Goniobryum Lindb. 1865SK02
    |    `--G. subbasilare (Hooker) Lindb. 1865 (see below for synonymy)SK02
    |--Hymenodon Hooker & Wilson 1844SK02
    |    |--H. aeruginosusJ87
    |    `--H. pilifer Hooker & Wilson 1844SK02
    |--Mesochaete Lindb. 1870SK02
    |    |--M. taxiforme (Hampe) Watts & Whitel. 1906 [=Rhizogonium taxiforme; incl. M. grandiretis]SK02
    |    `--M. undulata Lindb. 1870 [=Rhizogonium undulatum; incl. R. plumaeforme (n. n.)]SK02
    |--Leptotheca Schwägr. 1824SK02
    |    |--L. gaudichaudii Schwägr. 1824 [=Aulacomnium gaudichaudii, Bryum gaudichaudii]SK02
    |    |    |--L. g. var. gaudichaudii [incl. L. beccarii, Weissia leptocarpa]SK02
    |    |    `--L. g. var. wattsii (Cardot) Churchill & Buck 1982SK02
    |    `--L. spegazziniiD03
    |--Rhizogonium Brid. 1827SK02
    |    |--R. alpestre Müll.Hal. 1897SK02
    |    |--R. distichum (Swartz) Brid. 1827 [=Mnium distichum; incl. R. muelleri]SK02
    |    |--R. graeffeanum (Müll.Hal.) Jaeger 1875 [incl. R. geheebii, Mnium geheebii]SK02
    |    |--R. mediumBW15
    |    |--R. novae-hollandiae (Brid.) Brid. 1827 (see below for synonymy)SK02
    |    |--R. pennatum Hooker & Wilson 1854 [incl. R. pennatum var. aristatum (Hampe) Dixon 1926]SK02
    |    `--R. setosum [=Pyrrhobryum setosum]SK02
    `--Pyrrhobryum Mitt. 1868SK02
         |--P. bifarium (Hooker) Manuel 1980 [=Mnium bifarium, Rhizogonium bifarium]SK02
         |--P. medium (Besch.) Manuel 1980 (see below for synonymy)SK02
         |--P. mnioides (Hooker) Manuel 1980 [=Rhizogonium mnioides]SK02
         |    |--P. m. ssp. mnioidesSK02
         |    `--P. m. ssp. contortum (Wilson) Fife 1995 (see below for synonymy)SK02
         |--P. paramattense (Müll.Hal.) Manuel 1980 [=Mnium paramattense, Rhizogonium paramattense]SK02
         `--P. spiniforme (Hedw.) Mitt. 1868 [=Rhizogonium spiniforme; incl. R. spininerve Brid. ex Bailey 1890 (n. n.)]SK02
              |--P. s. f. spiniformeB57
              `--‘Rhizogonium’ s. f. samoanaB57
Nomina nuda: Rhizogonium spiniforme var. australe Müll.Hal. 1900SK02
             Rhizogonium parramattense var. gracilior Watts & Whitel. 1906SK02
             Rhizogonium spiniforme var. minus Jaeger 1875SK02
             Rhizogonium parramattense var. minus Watts & Whitel. 1906 non R. spiniforme var. minus Jaeger 1875SK02
             Rhizogonium parramattense var. nanum Bailey 1883SK02

Goniobryum subbasilare (Hooker) Lindb. 1865 [=Rhizogonium subbasilare, Trachyloma subbasilare; incl. Photinophyllum pellucidum, Goniobryum pellucidum, Rhizogonium pellucidum, R. reticulatum]SK02

Pyrrhobryum medium (Besch.) Manuel 1980 [incl. Rhizogonium brevifolium, Mnium brevifolium, Pyrrhobryum brevifolium (Broth.) Manuel 1980]SK02

Pyrrhobryum mnioides ssp. contortum (Wilson) Fife 1995 [=Rhizogonium mnioides var. contortum; incl. Polytrichum gulliveri, Pogonatum gulliveri (Hampe) Jaeger 1879, Mnium hookeri, Rhizogonium hookeri, R. mnioides var. lutescens, Mnium mossmanianum, R. mossmanianum]SK02

Rhizogonium novae-hollandiae (Brid.) Brid. 1827 [=Fissidens novae-hollandiae, Conomitrium novae-hollandiae Müll.Hal. 1901 (n. n.), Hypnum novae-hollandiae, Mnium novae-hollandiae, Skitophyllum novae-hollandiae; incl. Bartramia billaridierei (nom. illeg.)]SK02

*Type species of generic name indicated

References

[B57] Bartram, E. B. 1957. Additional Fijian mosses, III. Journal of the Washington Academy of Sciences 46 (12): 392–396.

Bell, N. E., & A. E. Newton. 2007. Pleurocarpy in the rhizogoniaceous grade. In: Newton, A. E., & R. S. Tangney (eds) Pleurocarpous Mosses: systematics and evolution pp. 41–64. CRC Press.

[BW15] Brotherus, V. F., & W. W. Watts. 1915. The mosses of the New Hebrides [part 1]. Journal and Proceedings of the Royal Society of New South Wales 49 (1): 127–144.

[D03] Dusén, P. 1903. Patagonian and Fuegian mosses. In: Scott, W. B. (ed.) Reports of the Princeton University Expeditions to Patagonia, 1896–1899 vol. 8. Botany pp. 63–126. The University: Princeton (New Jersey).

[J87] Judd, W. S. 1987. Floristic study of Morne La Visite and Pic Macaya National Parks, Haiti. Bulletin of the Florida State Museum—Biological Sciences 32 (1): 1–136.

O’Brien, T. J. 2007. The phylogenetic distribution of pleurocarpous mosses: evidence from cpDNA sequences. In: Newton, A. E., & R. S. Tangney (eds) Pleurocarpous Mosses: systematics and evolution pp. 19–40. CRC Press.

[SK02] Streimann, H., & N. Klazenga. 2002. Catalogue of Australian Mosses. Flora of Australia Supplementary Series 17. Australian Biological Resources Study: Canberra.

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