A recent field study indicated that thalli of the beard lichen Ramalina usnea undergo diffuse (“intercalary”) growth throughout their length. We examined thallus sections with TEM to better understand how the highly thickened cell walls of the prosoplectenchymatous cortex behave under conditions of continued expansion. Cell protoplasts were surrounded by massive accumulations of structured electron-dense wall layers interspersed with amorphous, electron-transparent substances, visible as concentric rings in transverse section. Nearest the protoplast, electron-dense wall layers were distinct and more or less alternated with irregular deposits of electron-transparent material. With increasing distance from the protoplast, the electron-dense wall layers were increasingly disrupted and intermixed among the electron-transparent materials. New cell branches grew through the accumulated wall materials, interrupting the layers they penetrated while producing their own concentric wall layers. The differing amounts of cell wall material accumulated was further indication of the different relative ages of such neighbouring cells. These observations suggest that cell walls are disrupted by diffuse tissue expansion and continually replaced by new walls and wall materials deposited to their interior at the interface with the protoplast. This pattern of development, documented previously in R. menziesii and U. longissima, suggests that component cells of lichen prosoplectenchyma behave quite differently from those of diffusely expanding filaments studied in non-lichen-forming fungi, where a single, discrete cell wall is maintained throughout growth.

Imazamethabenz, an acetohydroxyacid synthase (AHAS)-inhibiting herbicide, was applied at a sublethal rate (200 g ai ha–1) to foliage of greenhouse-grown wild oat plants at the 2-leaf stage without tillers. Main shoot tissues were sampled 1 to 2 wk after imazamethabenz treatment and prepared for light microscopic examination. Histological observations showed that imazamethabenz treatment did not produce drastic changes on leaf structures but did affect chloroplast integrity of fully expanded 2nd leaf and inhibited differentiation of the young 3rd leaf 1 wk after application. The internode length and the number of cells in internodes of the herbicide-treated main stem were greatly reduced, presumably due to the inhibition by imazamethabenz of intercalary meristem cell division in early stages of internode elongation. One week after imazamethabenz application, histochemical studies revealed an increased accumulation of starch granules in chloroplasts of the 2nd leaf but reduced starch levels in the main stem. This confirmed the hypothesis that AHAS inhibitors affected phloem transport of photosynthates.

Very little is known about how growth is distributed within thalli of fruticose lichens. To determine whether mature, non-apical regions of the beard lichen Ramalina usnea elongate, segments of the broadest axes were marked with thread and measured; thalli were then returned to the field to resume growth for about one year. Growth was evident in all segments measured and increments were roughly proportional to length, suggesting that elongation occurs throughout the thallus. New thallus axes (branches) were organized from buds that emerged from mature tissue, usually along pseudocyphellae and surfaces exposed by breakage. Axes were very fine at their extremities and possessed an inrolled apex from the time of bud emergence. No evidence of apical branching was observed. The thallus cortex consisted of very thick-walled fungal cells oriented longitudinally. In transverse section, cells gave the appearance of being grouped within fascicles, indicating that branch cells grow within the older cell wall material of their neighbours, as occurs in the cortex of Ramalina menziesii and the medullary cord of Usnea longissima. A continued accumulation of cell wall layers may be related to the mature tissue's capacity to sustain elongation growth.