A critical survey of the evidence for the occurrence of Helminthocladia agardhiana Dixon [H. hudsoni J. Ag. pro parte] in Europe

An examination of cystocarps from the rarely collected southern Australian alga currently known as Lomentaria corynephora (J. Agardh) Kylin has shown it to be a member of the Rhodymeniales but incorrectly placed in Lomentaria. As it is not referable to any of the genera presently ascribed to the order, the new genus Semnocarpa is proposed to accommodate its suite of unique features. Semnocarpa closely resembles Lomentaria in habit and in having basally septate branches, a peripheral network of widely separated medullary filaments around the cell-free (but mucilage-filled) centres of the main and lateral axes, gland cells directed inwardly on scattered medullary cells, and tetrasporangia produced laterally from surface cortical cells that line deep cavities in the branch surfaces. Features of the mature cystocarp, however, strongly differentiate Semnocarpa from Lomentaria. The carposporophyte has a fusion cell in which outlines of the component cells remain discernible, as opposed to having a fully consolidated fusion cell, and is laxly enclosed in a system of filaments derived from surrounding inner cortical cells. The cystocarp is entirely submerged within the bearing branch, there being no protuberant pericarp derived from the outer cortex of the sort previously thought to be a uniform feature of the family Lomentariaceae and virtually all Rhodymeniales. These features suggest that Semnocarpa is likely to be a highly derived member of the Lomentariaceae. A second species is newly described from material collected in Western Australia. Semnocarpa minuta sp. nov. differs from S. corynephora in its exclusively epiphytic habit, two-layered medulla, smaller stature and extensive crustose holdfast.