STUDIES ON TREUBIALES, I. ON APOTREUBIA HATT. ET AL. AND A. HORTONAE SCHUST. & KONSTANTINOVA, SP. N.

Abstract

  Apotreubia hortonae Schust. & Konstantinova, known only from Alaska and British Columbia, is described. This taxon, like other species of Apotreubia, shows a highly “unstructured” organization, with scattered ♂ and ♀ gametangia and a simple but fleshy calyptra that lacks “sterile” archegonia on its surface. A. hortonae differs from A. nana, with which it had been confused, in being unisexual and having short but distinct antheridial stalks. Ability to study hundreds of living, fertile plants of a species of a genus which had been known from only sparing gametophytic material, sterile and/or with immature sporophytes, led to a reevaluation of the status of this genus and the phylogenetic position of the Treubiaceae. Discovery of the sporophyte, which bears a spherical capsule, has reinforced the claims of Apotreubia as a distinct genus. It is shown that Apotreubia differs from Treubia not only in the 2-stratose capsule wall but in the more diffuse gametangia, lack of a shoot calyptra, the simpler axial anatomy with a very “simple” vascular strand, and the furcate, dichotomous branching. In all criteria but the first, Apotreubia is regarded as a less advanced genus than Treubia. The copious production of slime papillae by both genera is regarded as a “hold-over” from amphibious ancestral types. It is speculated that unisexual gametophytes, lacking the capacity for asexual reproduction, producing large amounts of mucilaginous matter bathing the shoot apex, represent the most “primitive” of Hepaticae. Apotreubia and Treubia, Verdoornia and Haplomitrium, together with the even more isolated genus Takakia, all share the capacity to form such mucilaginous matter in considerable quantity. In these genera, aside from Treubia, there is at least a residual capacity to form diffusely oriented gametangia. Lack of organization of antheridia and archegonia into defined androecia and gynoecia (and lack of devices that protect gametangia) is regarded as highly plesiomorphic. In this regard, these genera (aside from Treubia) again show striking convergence. Only the Fossombroniaceae in Metzgeriales show similar diffuse and “naked” gametangia; this last group is usually placed at the beginning of the Metzgeriales.