Diderma albo-columella ( Myxomycetes ) , a new species in the Brazilian Atlantic Forest

A new species of Diderma (Physarales, Didymiaceae) was recorded in the Mata Estrela Private Nature Reserve, Rio Grande do Norte state. It is characterized by sessile sporangia, white globose columella, capillitium irradiating from the peridium, forming three layers, the middle part strongly calcareous, the outer layer cartilaginous and the inner layer membranous. A key to species of sessile sporangia Diderma with triple peridium is presented.


Introduction
The genus Diderma was proposed in 1794 by Persoon, who emphasized the presence of a double peridium wall.The type species is D. globosum and in this genus there are 74 currently known species (Hernández-Crespo & Lado 2005) distributed in different neotropic countries.Few species have triple peridium, such as D. lohogadense S.D. Patil, R.L. Mishra

Material and Methods
Sporocarps were collected on an unidentified dead tree in the Mata Estrela Reserve, situated in Baia Formosa municipality, Rio Grande do Norte state (06º22'10" -06º22'43" ).Traditional herbarium methods were used in collecting and dehydrating specimens (Farr 1971).The material was observed through the light microscope (LM) under oil immersion objective and with the scanning electron microscope (SEM).For ultramicroscopic studies, the material was Sporocarps prepared for scanning electron microscopy, were placed on double adhesive cellophane tape mounted on a specimen stub and sputtering with gold-palladium.The nomenclature followed in this paper is that of Martin & Alexopoulos (1969) and Martin et al. (1983).Sporocarps sessile, densely aggregated, bright yellowish-brown, hemispheric-depressed to discoid, 0.4-0.8× 0.3-0.5 mm diam., 0.3-0.4mm alt., slightly umbilicated above.Hypothallus membranous, bright and continuous, dehiscence irregular.Peridium triple, the outer layer yellow, shining, cartilaginous, dehiscence irregular, the middle layer calcareous, white, thick, dehiscing simultaneously with the outer layer to which it closely adheres, the inner layer yellow, membranous.Collumela subglobose, 1/3-1/4 of the sporotheca, white, calcareous.Capillitium abundant, with threads radiating from the inner layer of the peridium, violet brown, with hyaline extremities and short dilatations.Spore-mass dark brown.Spore dark to yellowish brown by transmitted light, globose to subglobose, 10.5-13.0µm diam.

Diderma albo-columella
The specific epithet is in reference to collumela color.
The new species differs from Diderma lohogadense.for not having circumscissile dehiscence and spinulose spores (Mishra & Ranade 1979); from D. nigrum for not having brownish to black sporotheca and frequently lacking columella; from D. subasteroides, D. petalloides and D. stellulum for not having a stalk and the characteristic dehiscence, which gives the name of the species (Farr 1971;Buyck 1983); from D. peyerimhoffii for not having a clavate columella from which the dark threads of the capillitium irradiate, globose to fusiform expansions and spinulate spores; from D. maculatum (species described for Venezuela) for not having whitish peridium, with faint brown spots and the outer layer membranous, very fragile (Buyck 1984); and from D. yucatanensis, described for Ecuador (Lado et al. 2003), for not having stalked sporangia, capillitium threads attached to beige columella and spores less than 10 mm diam.

Figure 2 -
Figure 2 -Diderma albo-columella A.C.C. Bezerra & L.H. Cavalcanti -a.sporangia open, exposing the columella; b. arrow shows the thick wall of the peridium and threads radiating from the inner layer; c. details of the triple wall: limestone; the outer layer (pe) and the middle layer calcareous (pm), thick, with the outer layer to which it closely adheres (Bezerra 45132).