A critical investigation of the vegetative anatomy, growth and taxonomic affinities ofAdenocystis, Scytothamnus, andSplachnidium(Phaeophyta)

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Abstract
Light, scanning and transmission electron microscopy have been used to study the apical meristematic regions, growth and the vegetative structure of the thallus of Adenocystis utricularis, Scytothamnus australis, Sc. fasciculatus and Splachnidium rugosum. According to established criteria, the sporophytes of Adenocystis are shown to be halplostichous (filamentous) and those of Scytothamnus spp. and Splachnidium, polystichous (parenchymatous). The problematical classification of the three genera is discussed in relation to the structure of the sporophyte and also the gametophyte generations. It is concluded that Adenocystis, Scytothamnus and Splachnidium are all more readily classified in the order Ectocarpales sensu lato than in either the Chordariales or Dictyosiphonales. Transmission electron microscope studies on Sp. rugosum demonstrate conclusively the presence of an apical cell and conceptacle initials. The ultrastructure of these cells is phaeophycean in character and the conceptacle initials are connected to adjoining cells by plasmodesmata. They are not, as previously thought, an endophytic green alga.