The vegetative and sporophytic features of Holomitriopsis that distinguish this genus from Schistomitrium and from Leucobryum are discussed and illustrated using scanning electron microscopy

Newton, Angela E.

Robinson, Harold

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111Tropical Bryology 9: 111-116, 1994The structure of the leaf and peristome of Holomitriopsislaevifolia (Broth.) H. Robins. illustrated with scanning electronmicroscopy.Angela E. Newton & Harold Robinson.Department of Botany, National Museum of Natural History, Smithsonian Institution,Washington D.C. 20560, U.S.A.Abstract.  The vegetative and sporophytic features of Holomitriopsis that distinguish this genusfrom Schistomitrium and from Leucobryum are discussed and illustrated using scanning electronmicroscopy.Leucobryum laevifolium Broth. is an endemic ofthe Guyana Highlands of Venezuela and Guyana.It was described in 1901, but was rather poorlyknown until recently. Sporophytes were stillunknown when Florschütz (1964) commentedon a combination of vegetative characters in thespecies (leucocysts in two layers throughout theleaf and chlorocysts nearer the dorsal surfacefrom the base to a short distance below the apex)that was "distinctive for the Malesian genusSchistomitrium Dozy et Molk." The lack ofsporophytes prevented Florschütz from makingany change of generic placement at that time.The two genera with which Leucobryum laevifo-lium had been associated by Brotherus (1901)and Florschütz (1964) are very distinct in theform of their capsules and calyptrae. Leuco-bryum  normally has capsules that are curved,striate and strumose, cucullate calyptrae thathave no fringe, and peristome teeth that arevertically striate and bifid. (One species, L.incurvifolium C. Mull. has erect capsules). Schi-stomitrium has capsules that are erect, smoothand cylindrical, mitrate calyptrae that have aciliate fringe at the base and peristome teeth thatare papillose and scarcely divided.A sporophyte of Leucobryum laevifolium wasfirst described by Robinson in 1965. The speciesdiffers from both Leucobryum and Schistomi-trium in several features of the sporophyte, andthis formed the basis for description of the newgenus Holomitriopsis Robinson. In addition theperichaetial leaves are very elongate, claspingthe base of the seta. This feature led to thegeneric name, from the similarity to the elongateperichaetial leaves of Holomitrium in the112Dicranaceae, + opsis, meaning to resemble.Robinson (1986) considered that Holomitriopsisis closely related to Schistomitrium.In a recent paper Robinson (1990) surveyed theperistome structure of members of theLeucobryaceae, but Holomitriopsis laevifoliawas omitted since the material then availablewas insufficient for SEM photography. Somefeatures of the vegetative leaves were illustratedin an earlier paper (Robinson 1985.) New materialfrom Guyana (Henkel 68, US) with many maturecapsules ingood condition has now made itpossible to illustrate the distinctive sporophyticfeatures of the genus. Further observations onthe vegetative leaves are made.Materials and methods.Plants from the collection Henkel 68 (US) wereused. Intact mature capsules were soaked inwater until the operculum could be easilydetached. A representative capsule was cut inhalf longitudinally and mounted on the SEMstub so that the outer and inner surfaces of theperistome were exposed. Vegetative leaves weresoaked in water and then sectioned by hand witha razor blade. The stubs were sputter coated andthen photographed using a Hitachi 520 scanningelectron microscope.Leaf structure.The structure of the leaves was described byFlorschütz (1964) as having "the leucocysts intwo layers throughout the leaf and the chloro-cysts nearer the dorsal side of the leaf from baseto a little below the apex, where they becomecentral." This dorsal placement is most apparentin the mid to upper regions of the leaf (Fig. 1,upper leaf section) and near the midline in thelower part of the leaf. However, towards themargin in the lower part of the leaf the chlorocystsapproach the ventral (adaxial) side (Fig. 1, lowerleaf section).A somewhat similar leaf structure is seen inLeucobryum martianum (Hornsch.) Hampe exC. Mull., which is very commonin the Guianasarea. In this species the dorsal and ventralleucocyst layers are also only one cell thick, withthe chlorocyst layer much nearer the dorsalsurface in sections of the mid and upper leaf.However, in sections of the base of the leaf thechlorocysts are clearly closer to the ventral surfaceat the midline of the leaf. The two species alsodiffer in the shape and color of the leaves. Leavesof L. martianum are frequently falcate-secund orsomewhat spreading, with the upper leaf narrowand sub-tubulose. They also have the glossy,glaucous or whitish green color often tinged withpink or purple at the base seen in many speciesof  Leucobryum. Leaves of Holomitriopsislaevifolia are stiffly erect to clasping, with theupper leaf acuminate with a cucullate apex, andare a matt, yellowish or reddish brown color.In Holomitriopsis pores are present in the majorityof the transverse walls between cells (Fig. 1),and are also present in many of the longitudinalcell walls (Figs 2, 3 and 4). The surface cells arenormally intact on the ventral surface of the leaf(Fig 1), but the outer walls of the cells on thedorsal side of the leaf are frequently lost (Fig 2),with older leaves showing almost complete lossof the outer surface except in the region along themargins (Fig 3). In younger leaves few cell wallsmay be lost (Fig. 4). The loss of the outer cellwall appears to result from resorption of the wallmaterial rather than by breakage, since the edgesare smoothly rounded rather than fractured (seecells inFig. 2), and may be the result of a similarprocess to that which forms the pores betweencells.  Loss of the dorsal surface cell walls occursto a certain extent elsewhere in the Leucobryaceae(Schistomitrium species, Leucobryum martia-num) but is nowhere so extensive as in Holomi-triopsis laevifolia, where it is responsible for thematt appearance of the leaves. The remainingdorsal surface cell walls, very young intact lea-ves, and the ventral surface of the leaf show thecharacteristic iridescent highlights which con-tribute to the glaucous appearance of many of theLeucobryaceae.Perichaetial leaves.The upper leaves of the perichaetium are elonga-te, surrounding the base of the seta (Fig. 5). This114feature is unique in the Leucobryaceae. Theprogressive loss of cell walls on the dorsalsurface of the leaves is clearly visible.Capsule and peristome.The capsules are cylindrical, smooth and erect,as are those in Schistomitrium. However, thebase of the capsule is elongate, asymmetricaland slightly twisted, with a tuberculate surface(but lacking stomates), suggesting a relationshipto the curved capsules of Leucobryum.The peristome teeth are variably bifid, withsome teeth showing two distinct distal portions,while others are entire, with a weak median line(Figs 6, 8 and 10). In this respect Holomitriopsisdiffers from both Schistomitrium, with undivi-ded peristome teeth thatlack a vertical medianline (Robinson 1990, Figs 14-15) and fromLeucobryum, which has strongly divided teethwith a well developed median line.The inner and outer surfaces of the teeth arepapillose and show no sign of vertical striations(Figs 7, 9 and 10). In this respect Holomitriopsisresembles Schistomitrium, which has papilloseornamentation on both peristome surfaces (Figs14-15, Robinson 1990). In contrast, Leucobryumhas well developed vertical bars of the Dicrana-ceous type on the outer surface and papillaearranged in rows on the inner surface (Figs 10-13, Robinson 1990).A well developed preperistome is present inHolomitriopsis (Fig. 7), but was not seen ineither Leucobryum or Schistomitrium.Calyptra.The calyptra in Holomitriopsis is cucullate (Fig.11), and lacks the fringe of elongated inflatedunicellular hairs seen on the mitrate calyptra ofSchistomitrium.Dwarf males.Dwarf males, which are common in the Leuco-bryaceae, are also present in Holomitriopsis.The plants seen were from one to four millime-ters tall and showed one to three cycles ofantheridia production, followed by continuedvegetative growth from a sub-apical innovation.The narrow lanceolate leaves were approxima-tely one quarter of the size (1 mm) of those seenon non-dwarf plants. Male plants in Schistomi-trium are similar to the females but slightlysmaller (Eddy 1990.)A further Malesian genus, Cladopodanthus Dozy& Molk., isvery close to Schistomitrium andshares some further similarities with Holomi-triopsis. Both Cladopodanthus and Holomitriop-sis have two layers of leucocysts to the extremeapex of the leaf, while in Schistomitrium theventral side of the leaf at the apex is filled withmulti-stratose layers of cells (Eddy 1990, Magill1993). Cladopodanthus also lacks a ciliate fringeon its calyptra, which is however mitrate ("oblongcampanulate" Magill 1993.)If these character states in Schistomitrium areconsidered derived (Robinson 1990, Magill1993), the similarities between Holomitriopsisand Cladopodanthus must be considered to beplesiomorphic. Such similarity due to absence ofa derived character state does not indicate anyparticular relationship between the two genera.Similar comments might also be made aboutother characters discussed.It seems clear that Holomitriopsis, although itshares some similarities with Leucobryum on theone hand, and with Schistomitrium andCladopodanthus on the other, is quite distinctfrom all three genera.Acknowledgements.We would like to thank the Smithsonian Institution’s BiologicalDiversity of the Guianas Project for funding support.Scanning electron negatives were taken in the SEM Laboratoryat the Smithsonian Institution by Peter Viola.Literature cited.Brotherus, V.F. 1901. Musci in Report on two botanicalcollections made by McConnell and Quelch at Mount116Roraima in British Guiana. Transactions of theLinnaean Society of London, series 2, 6: 88Eddy, A. 1990. A handbook of Malesian mosses, Vol. 2.Leucobryaceae to Buxbaumiaceae. Natural Histo-ry Museum Publications, London.Florschütz, P.A. 1964. The mosses of Suriname, part 1. E.J.Brill, Leiden.Magill, R.E. 1993. A revision of the genus Cladopodanthus(Leucobryaceae, Musci) Bryologist 96: 233-241Robinson, H. 1965. Venezuelan Bryophytes collected by Ju-lian A. Steyermark.  Acta Botanica Venezuelica 1:73-83Robinson, H. 1985. The structure and significance of theLeucobryaceous leaf. Monographs in SystematicBotany of the Missouri Botanic Garden. 11:111-120Robinson, H. 1986. Notes on the Bryogeography of Venezue-la. Bryologist 89:8-12Robinson, H. 1990. A functional evolution of the Leucobrya-ceae. Tropical Bryology 2:223-237.

The structure of the leaf and peristome of Holomitriopsis laevifolia (Broth.) H. Robins : illustrated with scanning electron microscopy

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