The growth of six saxicolous lichens transplanted to lime-rich and lime-poor substrates in south Gwynedd, Wales

The objectives of this study were to investigate: (1) whether foliose lichen thalli could be transplanted from one substrate to another and (2) whether such transplants could be used to study the influence of the substrate on growth. Hence, six saxicolous lichens, with contrasting distributions on lime-rich and lime-poor substrates in South Gwynedd, Wales, were transplanted onto slate, granite, asbestos and cement. Fragments of the perimeters of thalli were glued to the different substrates using Bostic adhesive. Parmelia conspersa (Ehrh. Ex Ach.)Ach. and Parmelia saxatilis (L.)Ach., fragments increased in area over 15 months on slate and granite but decreased in area or did not survive on asbestos and cement. Fragments of Xanthoria parietina (L.)Th.Fr. and Physcia tenella (Scop.)DC. em Bitt. did not survive on slate and granite while some fragments survived but grew poorly on asbestos and cement. Parmelia glabratula ssp. fuliginosa (Fr. ex Duby)Laund. fragments decreased in area on all substrates and especially on cement and asbestos while Physcia orbicularis (Neck.)Poetsch fragments increased in area on granite and cement, decreased on asbestos and did not change significantly on slate. The results suggested that the distribution of P. conspersa and P. saxatilis was determined primarily by physico-chemical properties of the substrate. By contrast, P. glabratula ssp. fuliginosa may have responded to the transplant procedure while X. parietina, Ph. tenella and Ph. orbicularis may require nutrient enrichment to grow successfully on a substrate.

Lobe formation and division in the foliose lichen Xanthoparmelia conspersa

New lobe development and lobe division was studied in the foliose lichen Xanthoparmelia conspersa (Ehrh. ex. Ach.) Hale. In thalli with either the centre or margin removed, the inside edge of the perimeter, the outer edge of the reproductive centre, and fragments derived from the thallus perimeter all regenerated growing points (‘lobe primordia’) within a year. Thalli possessing isidia had the greatest ability to regenerate growing points. In reproductive thalli, there was a positive correlation between the density of new growing points and thallus size. When fragments were cut from the perimeters of mature X. conspersa thalli and glued to pieces of slate, the ratio of growing points to mature lobes increased over 54 months. Lobes within a thallus exhibited different degrees of bifurcation. In some bifurcating lobes, the point of origin of the bifurcation advanced at the same rate as the lobe tips over 4 months but in most lobes, the bifurcation point either advanced less rapidly than the lobe tips or retreated from its original location. Removing adjacent lobes had no significant effect on the radial growth of a lobe over 4 months or on the location of the bifurcation point but it increased the number of growing points. These results suggest that for X. conspersa: 1) all portions of of thalli can regenerate growing points, 2) few growing points actually develop into mature lobes, 3) individual lobes within a thallus grow and divide differently, and 4) adjacent lobes inhibit the development of growing points on their neighbours.

The investigation of the sensitivity gradient of the visual field, as a function of stimulus dynamic range, in the normal and abnormal eye

The study utilized the advanced technology provided by automated perimeters to investigate the hypothesis that patients with retinitis pigmentosa behave atypically over the dynamic range and to concurrently determine the influence of extraneous factors on the format of the normal perimetric sensitivity profile. The perimetric processing of some patients with retinitis pigmentosa was considered to be abnormal in either the temporal and/or the spatial domain. The standard size III stimulus saturated the central regions and was thus ineffective in detecting early depressions in sensitivity in these areas. When stimulus size was scaled in inverse proportion to the square root of ganglion cell receptive field density (M-scaled), isosensitive profiles did not result, although cortical representation was theoretically equivalent across the visual field. It was conjectured that this was due to variations in the ganglion cell characteristics with increasing peripheral angle, most notably spatial summation. It was concluded that the development of perimetric routines incorporating stimulus sizes adjusted in proportion to the coverage factor of retinal ganglion cells would enhance the diagnostic capacity of perimetry. Good general and local correspondence was found between perimetric sensitivity and the available retinal cell counts. Intraocular light scatter arising both from simulations and media opacities depressed perimetric sensitivity. Attenuation was greater centrally for the smaller LED stimuli, whereas the reverse was true for the larger projected stimuli. Prior perimetric experience and pupil size also demonstrated eccentricity-dependent effect on sensitivity. Practice improved perimetric sensitivity for projected stimuli at eccentricities greater than or equal to 30o; particularly in the superior region. Increase in pupil size for LED stimuli enhanced sensitivity at eccentricities greater than 10o. Conversely, microfluctuation in the accommodative response during perimetric examination and the correction of peripheral refractive error had no significant influence on perimetric sensitivity.