STUDIES OF PROTONEMAL MORPHOGENESIS IN MOSSES .3. THE PERENNIAL GEMMIFEROUS PROTONEMA OF RHIZOMNIUM-PUNCTATUM (HEDW) KOP

The highly branched perennial protonema of Rhizomnium punctatum, found on damp rocks, rotten wood and woodland rides is identical to the filament system produced from the bases of excised leafy gametophores. In both, main axes, best described as caulonema, 24-32 mu m in diameter with oblique cross-walls, discoid to spindle-shaped chloroplasts and a finely papillose surface, produce three kinds of side branches all with transverse cross-walls and spherical to ovoidal chloroplasts: 1) Upright, regularly branched chloronemata 19-21 mu m in diameter with upwardly curving and tapering ultimate ramifications. 2) Very narrow (6-9 mu m diameter) rhizoidal branches which coil around objects in the substratum and form appressorium-like structures on detached gemmae. 3) 22-30 mu m diameter gemmiferous side branches forming 4-10 celled filamentous gemmae. Secondary gemmae arise by percurrent proliferation. Gemmae germinate freely in distilled water producing filaments which adhere strongly to solid objects. The walls of all parts of the filament system are brown-pigmented though this is most intense in the gemmae and main axes. Unlike the highly vacuolate cells of young caulonema, the mature cells of the main axes in Rhizomnium contain fine longitudinally-orientated cytoplasmic strands in which the spindle-shaped and discoidal chloroplasts are suspended. The possible functional significance of these strands in either nutrient transport or the coordination of protonemal development should now be explored. When allowed to dry out the protonema and gemmae of Rhizomnium retain their viability, as measured by the capacity to regenerate new filaments, much longer than detached leaves or intact gametophores. Rapid desiccation reduces longevity. In that it extends the ecological range of the species and is the principal means of spread, the protonema should be regarded as the dominant phase in the life cycle of Rhizomnium punctatum.