Studies of protonemal morphogenesis in mosses IV. Aloina (Pottiales)

Unipolar spore germination in Aloina gives rise to wide (20-24 mu m diameter) protonemal axes with oblique cross-walls and spindle-shaped chloroplasts. The original protonemal filaments are caulonemal but they produce narrower (9-12 mu m diameter) chloronemal side branches, with ovoid to spherical chroroplasts in shortly cylindrical cells which have transverse cross-walls. The mature protonema comprises only four to six widely spreading major caulonemal axes with occasional gametophores, a pattern of morphogenesis that explains the scattered gametophores of Aloina in nature. After prolonged culture the older chloronemal filaments become;highly attenuated whereas wall thickening occurs in the caulonemal axes and younger chloronemata are transformed into chains of thick-walled brood cells containing large quantities of lipid. This phenomenon, hastened by abscisic acid and drying out of the cultures, is also found in nature. On Parker nutrient medium the filament systems produced from gametophores and isolated leaves are identical to protonemata. In the absence of nutrients the side branches of all the filament systems are very narrow (3-5 mu m diameter) and closely similar to those found in nature. The filaments produced by Aloina never have pigmented walls. The protonemal brood cells are closely similar to leaf lamellar cells; both are naturally occurring thick-walled vegetative diaspores, which germinate into protonemata after long periods of desiccation.