Studies of protonemal morphogenesis in mosses VII. The perennial rhizoids and gemmiferous protonema ofDicranella heteromalla(Hedw.) Schimp

Protonemata of Dicranella heteromalla, grown from spores on nutrient agar, comprise a loose mat of sparsely-branched, initially chloronemal and subsequently caulonemal filaments. Cells of the latter contain spindle-shaped nuclei and elongate or spindle-shaped chloroplasts suspended in longitudinal endoplasmic strands. Some cells are highly polarized with the majority of the organelles crowded at the apical end. Occasional gametophore buds are scattered along the main caulonemal axes whilst many of the chloronemal side branches produce terminal and intercalary filamentous gemmae with well defined tmema (abscission) cells. Detachment of the gemmae involves the swelling of a new, Cellufluor-staining, internal wall in the tmema cells. Polarity reversal, associated with tmema cell formation, is also evident in gemma germination. On nutrient-free agar, protonemata are non-gemmiferous but instead produce filaments of different diameters, down to 4–5 μm, which trace an undulating or spiral course through the medium or form knot-like aggregations on cellophane overlays.The wild rhizoid system of D. heteromalla forms a dense, matted network of filaments of different diameters closely similar to protonemata grown on nutrient-free agar. The nuclei are spindle-shaped and minute chloroplasts and elongate mitochondria, the latter revealed by staining with 3.3'-dihexyloxacarbocyanine iodide, are suspended along longitudinal endoplasmic strands. Walls are brown-pigmented with a delicate outer pellicular layer of non-cellulosic carbohydrates. The older rhizoid and protonemal cells accumulate large numbers of lipid droplets. When transplanted on to nutrient agar, wild rhizoids regenerate normal protonemata.Rhizoids growing through the soil surface, around gametophore colonies of D. heteromalla, produce secondary chloronemata bearing gemmae identical to those in culture Perennial gemmiferous protonemata also form extensive carpets on hare soil in British woodlands. The rhizoid system of D. heterornalla is long-lived and extremely extensive, with cells cytologically closely similar to food-conducting cells in moss gametophores and sporophytes, suggesting that its major role is solute uptake rather than anchorage. The common occurrence of perennial gemmiferous protonemata indicates a need to reconsider the importance of protonemata in plant strategy theory as applied to mosses.