Observations concerning morphology, structure, and ontogeny of the thallus of a squamulous species, Endocarpon pusillum Hedw., are presented. The latter is composed of squamules, most of which are interconnected by a well developed network of rhizomorphs. Four types of squamules have been distinguished according to their morphological characters and their location with regard to rhizomorphs. The squamules of E. pusillum are characterized by the lack of lacunae and the presence of numerous rhizomorphic-like hyphae in the medulla, which are in topographical relation with the hyphae of the rhizomorphs. Morphological and structural data allowed us to follow an ontogenic continuum from the smallest squamules, i.e. the least differentiated, to the one bearing fruiting-bodies, which show numerous rhizomorphs.
1987. Le cycle des Nostoc des genres Peltigera et Collema en cultures in vitro et dans le thalle lichtnique. Can. J. Bot. 65 : 1468-1477. L'isolement et la mise en culture des Nostoc de quatre Peltigera et du Collema tenax ont permis de montrer pour la premibre fois l'existence d'un cycle biologique. L'Ctude culturale de ces Nostoc symbiotes comparCe i celles effectuCes par d'autres auteurs sur des espbces libres a permis de prCciser que le photosymbiote du Peltigera canina, du P. horizontalis et du P. spuria serait le Nostoc punctiforme et que celui du Collema tenax serait le Nostoc commune. Dans le thalle lichCnique, ces Nostoc seraient au stade appelC punctiforme. Le cycle est bloquC i ce stade tant que 1'intCgritC du thalle est maintenue. Si une blessure intervient, l'ouverture des gaines polysaccharidiques des trichomes libbre des hormogonies mobiles qui, dans les conditions naturelles, colonisent les isidies squamuliformes du Peltigera praetextata et en culture permettent I'isolement des souches de Nostoc chez les lichens CtudiCs. des Nostoc des genres Peltigera et Collema en cultures in vitro et dans le thalle 1ichCnique. Can. J. Bot. 65: 1468-1477.The isolation and culture of Nostoc symbionts of four Peltigera and of Collema tenax has revealed the presence of a biological cycle. Our observations, taken in conjunction with those of other authors on free-living species, suggest that the photosymbiont of Peltigera canina, P. rufescens, P. horizontalis, and P. spuria is Nostocpunctiforme (Kiitz.) Hariot, that of Collema tenax is Nostoc commune Vauch. In the lichen thallus, Nostoc is at the stage called "punctiforme." This stage stays stable as long as the thallus is undamaged. If wounding occurs, the opening of the polysaccharide sheath of the trichomes causes motile hormogonia to escape. These, in nature, colonize the squamuliform isidia of Peltigera praetextata and, in culture, they allow isolation of Nostoc strains from the lichens studied.
The comparative ontogeny of perithecia of numerous Verrucariaceae indicates that species studied to date show a peculiar, verrucariacean, stage. It is distinguished, at the time of perithecial cavity aperture opening, by short periphysoids and a domelike sporophytic apparatus. The development and structure of the perithecia of Endocarpon pusillum Hedw. are of the ascolocular type. This species is related to the genus Verrucaria, but differs from the genus Dermatocarpon, especially Dermatocarpon miniatum (L.) Mann.
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