The Internal Cephalodia of the Genus Lobaria

Abstract

The development and structure of internal were investigated in the following representatives of Lobaria sect. Lobaria: L. amplissima, L. erosa, L. linita, L. lobulifera, L. oregana, L. pulmonaria, and L. quercizans. Cephalodia originate from Nostoc cells in contact with the lower cortex of the host thallus. The mycobiont actively envelopes the alga and in- corporates it into the thallus either by an intrusive or nonintrusive mechanism. The thickness of the thallus determines whether the developing cephalodium will be of the superior or the inferior type, the former growing upward through the medulla and inhibiting the green algae in the algal layer. Species with large, loosely attached, reticulately veined thalli have well developed superior and/or inferior cephalodia. Species with smaller, more closely attached thalli lack superior and exhibit fewer, less well developed, inferior cephalodia. Cephalodial Nostoc may be of advantage to the host mycobiont by supplying nitrogenous compounds and an alternative carbohydrate source. In the lichen family Stictaceae, Lobaria is the only genus known to possess cephalodia. Forssell (1883) reported the presence of true in L. sect. Lobaria and classified them as cephalodia immersa. Schneider (1897) recognized the as being completely endotrophic and described the gross morphology of the mature of Lobaria (Sticta) oregana. A description of maturation of internal in Lobaria (Ricasoli) herbacea was presented by Moreau (1927), who interpreted cephalodial growth as a response by the lichen thallus to infection. The purpose of this study was to investigate the mode of origin and development of internal in several species of L. sect. Lobaria. Although northern North American Lobariae were the prime object of study, were observed in several exotic species. The presence of within the genus Lobaria parallels their occurrence in genera of two closely related families, Peltigera in the Peltigeraceae (Thomson, 1950) and Nephroma in the Nephromataceae (Wetmore, 1960). All three genera have been similarly subdivided into subgeneric sections on the basis of algal associations. Those species having only a blue-green algal component comprise one subgeneric section, while those having a green alga as the primary phycobiont and a cephalodiate blue-green form another section. In L. sect. Lobarina the thalli have a continuous algal layer composed of the cyanophyte, Nostoc, and lack any association with a secondary phycobiont. However, all members