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065 - 078
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Cellular and subcellular changes were observed in Nostoc symbionts which had been removed from lichens and grown in culture. Cultured Nostoc cells had thicker gelatinous sheaths than lichenized cells and in some strains the sheath contained bacteria and vesicles. In culture the photobiont cells were round when they occurred singly but oblong when the cells were part of a chain. Cell size and heterocyst frequency decreased in cultured strains. At the subcellular level, the most apparent difference between lichenized and cultured Nostoc cells was the arrangement and number of thylakoids. Lichenized Nostoc cells of P. canina and P. rufescens had many scattered thylakoids while cultured cells had fewer thylakoids which ran parallel to the cell wall. In P. spuria the lichenized photobiont had only a few thylakoids which were parallel to the cell wall in contrast to the cultured cells which had whorls of thylakoids. Plastoglobuli were present in both lichenized and cultured photobionts. Cultured strains had fewer cyanophycin granules and polvhedral bodies but more polyglucoside granules than lichenized strains. Different types of mycobiont-photobiont interactions were observed. Intrawall haustoria and fungal protuberances were found in all the investigated lichens but intracellular haustoria were absent. Modifications of the fungal hyphae which appeared to compensate for the absence of intracellular haustoria included invagination of the plasmalemma and crenulation of the outer layer of the cell wall.
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