Rhizanthella R.S.Rogers
J.Roy.Soc.Western Australia 15:1, t. I, II. (1928)

Name Status: Current
Browse to the list of specimens for Rhizanthella R.S.Rogers

Scientific Description
H.R. Coleman, Thursday 8 September 2016

Family Orchidaceae.

Habit and leaf form. Herbs. Plants of very peculiar form; achlorophyllous, completely subterranean. Leaves absent. Plants succulent; possibly epiparasitic, relying on an association with mycorrhizal fungi for nutrients. Perennial; rhizomatous, or tuberous (rhizome-like tuberoids). Helophytic, or mesophytic, or xerophytic.

Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Pollination mechanism conspicuously specialized, or unspecialized (? unknown, as in most cases the floral bracts remain completely below the ground litter, creating a barrier for the pollinating agents).

Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in heads. The terminal inflorescence unit racemose. Inflorescences terminal; the capitulum comprised of rows of inward-facing flowers, surrounded by several fleshy overlapping bracts; with involucral bracts; pseudanthial (the surrounding bracts somewhat petaloid). Flowers small; fragrant; very irregular; zygomorphic; resupinate. The floral asymmetry involving the perianth and involving the androecium. Flowers 3 merous; cyclic; supposedly basically pentacyclic. Perigone tube absent. Perianth of ‘tepals’, or with distinct calyx and corolla; 6, or 5 (by misinterpretation); 2 -whorled; isomerous (but zygomorphic), or isomerous (by misinterpretation); free (in R. slateri), or joined (in R. gardneri, the lateral sepals and petals united basally, the upper parts free but overlapping); dark red to purple. Calyx (if the outer whorl be so designated) 3 (the median member ostensibly posterior); 1 -whorled. Corolla (i.e. the members of the inner whorl) 3, or 2 (by misinterpretation); imbricate. Androecium 3, or 1 (by misinterpretation). Androecial members free of the perianth; united with the gynoecium (fused with the style to form a ‘gynostemium’); coherent (via the gynostemium); 1 - adelphous; theoretically 2 -whorled, or 1 -whorled (by misinterpretation). Androecium including staminodes, or exclusively of fertile stamens (by misinterpretation). Staminodes 2 (these anterior (ostensibly posterior), supposedly the abaxial pair of the inner whorl). Stamens 1 (this across the flower from the labellum, i.e. anterior but ostensibly posterior, supposedly representing the outer whorl); reduced in number relative to the adjacent perianth; alterniperianthial (i.e. with reference to the single stamen, across the flower from the labellum); filantherous, or with sessile anthers. Anthers dorsifixed to basifixed; dehiscing via longitudinal slits; introrse; tetrasporangiate. Pollen shed in aggregates, or shed as single grains; when in aggregates, in tetrads, or in the form of pollinia (the single grains or tetrads held together by strands of sterile sporogenous material ?). Gynoecium 3 carpelled. The pistil 1 celled. Carpels isomerous with the perianth. Gynoecium syncarpous; eu-syncarpous; inferior. Ovary unilocular; 1 locular. The ‘odd’ carpel anterior (away from the labellum). Gynoecium stylate. Styles 1 (inflexed); apical. Stigmas 1; 3 - lobed (the apex of the median lobe forming the ‘rostellum’); wet type; papillate; Group III type. Placentation parietal. Ovules not differentiated; in the single cavity 30–100 (i.e. very numerous ?); non-arillate; anatropous.

Fruit and seed features. Fruit fleshy; indehiscent; a drupe; to 250 seeded. Seeds endospermic (endosperm development arrested very early), or non-endospermic; minute; without starch. Embryo rudimentary at the time of seed release, or weakly differentiated. Seedling. Seedling collar not conspicuous. Primary root ephemeral.

Geography, cytology, number of species. Native of Australia. Endemic to Australia. Australian states and territories: Western Australia, Queensland, and New South Wales.

Additional characters Perianth of 5 similar members and the median inner member modified into the labellum (perianth segments triangular; labellum small, heart-shaped). Column not prominently winged. Labellum not insect-like. Labellum motile. Perianth not glossy.

Taxonomic Literature

  • Hopper, Stephen D.; Brown, Andrew P. (2001). Contributions to Western Australian orchidology. 1, history of early collections, taxonomic concepts and key to genera [electronic resource].
  • Brown, Andrew; Hoffman, Noel (1995). Orchids of south-west Australia. University of W.A. Press. Nedlands, W.A.