The Griffithsieae group of the Ceramiaceae (Rhodophyta) and its southern Australian representatives

Abstract

The genus Griffithsia C. Agardh, with its type species G. corallinoides (L.) Batters, is characterized by subdichotomous filaments of large multinucleate cells; by a fertile axis of three small discoid cells of which the subapical cell produces 1(-2) procarp(s), whilst the lowermost cell (termed the hypogenous) produces abaxially an involucre of 2-celled branches; by a large fusion cell; and by the production of tetrasporangia and spermatangia on whorls of fascicles which often bear involucral cells, or are surrounded by an involucre from the vegetative cell beneath the fascicles, or are naked. Some southern Australian species previously placed in Monosporus Solier or Neomonospora Setchell & Gardner, but which produce tetrasporangia and reproduce sexually, are related to G. tenuis C. Agardh and G. barbata C. Agardh. They are characterized by subdichotomous filaments of multinucleate elongate cells; by a 3-celled fertile axis of which the subapical cell produces a procarp, and the hypogenous cell enlarges during procarp development and produces a whorl of 1-celled involucral branches; by a large fusion cell; and by tetrasporangia and spermatangial heads produced singly either from the basal cell of a trichoblast, or on a separate clavate pedicel. These species are referred to the genus Anotrichium Naegeli, differing from Griffithsia mainly in the spermatangial heads and tetrasporangia. The genus Monosporus Solier, represented with certainty in southern Australia only by M. australis (Harvey) J . Agardh, is retained as a form genus for those species which produce monosporangia but for which sexual reproduction is unknown. Study of Halurus equisetifolius (Lightfoot) Kuetzing, the type species of Halurus Kuetzing, from England, shows that this genus is characterized by irregularly branched axes with dichotomous whorl-branchlets; by the successive production of several fertile axes each of 3 small cells and equivalent to a condensed, dichotomous, branch system; and by a subapical procarp and large fusion cell. The involucre consists of vegetative whorl-branchlets and also a whorl of 1-celled branches from the enlarged hypogenous cell of the fertile axis. Halurus thus differs from Griffithsia mainly in cystocarpic features and the presence of whorl-branchlets. Griffithsia setacea (Ellis) C. Agardh should probably be referred to Halurus. An evolutionary trend towards reduction in the Griffithsieae is suggested: whorl-branchlets have been reduced to trichoblasts and fertile whorl-branchlets to whorls of fascicles. The number of procarps in a branch system and the female, spermatangial and tetrasporangial involucres show a reduction sequence.