Acromastigum A.Evans
Acromastigum A.Evans, Bull. Torrey Bot. Club 27: 103. 1900.
Type: Acromastigum integrifolium (Austin) A.Evans (≡Mastigobryum integrifolium Austin)
Plants subisophyllous to markedly anisophyllous, ± stiff and wiry, prostrate to suberect, at times with yellowish to reddish brown pigments, small to medium. Branching frequent, often but at times not uniformly pseudo-dichotomous, never pinnate, the branches of Frullania type, often originating from only one side of axis, subequal to main axis in vigor and with the main axis losing its dominance; branch half-leaf undivided, positioned in the fork of each “dichotomy”; Acromastigum -type branches present, stoloniform, the half-underleaf to one side at branch base. Ventral-intercalary branches also present, stoloniform, geotropic. Stems usually rigid and wiry, the cortex in 6–8 rows, in a number of species consistently 7, in some species 14–16, the cells larger than the medullary, with firm to thick walls; medulla in many rows of rather firm- to thick-walled cells (in only 3 rows in A. rigidum R.M.Schust.). Rhizoids from cells of underleaves and leaves of stoloniform branches. Leaves with insertion transverse to distinctly incubous (rarely slightly succubous), inserted to stem midline dorsally, with apices often variably deflexed, undivided or bidentate to bilobed (rarely 3-dentate), the ventral lobe or tooth often longer than the dorsal. Cells with walls slightly to distinctly thickened, with trigones at times nodulose, with a vitta of elongated, differentiated cells present in some species; surface smooth or papillose. Oil-bodies papillose or coarsely botryoidal or homogeneous and then 1(2) per cell. Underleaves with shape and form ± similar to the leaves or more commonly clearly differentiated, undivided or 2–3-dentate, the teeth often reduced to slime papillae. Asexual reproduction lacking, or occasionally by caducous leaves in a few species.
Dioecious. Androecia on short, determinate, tightly spicate ventral-intercalary branches from leading shoots or from geotropic stoloniform axes; bracts strongly ventricose, usually bidentate to bifid; antheridia 1 per bract, the stalk biseriate; bracteoles concave, usually bidentate, sporadically with antheridia. Gynoecia on abbreviated ventral-intercalary branches; bracts of innermost series much larger than leaves, ovate, variously dentate, laciniate or lobate but typically with 2 main segments; bracteole similar in size and form. Perianth ± fusiform, terete below, trigonous above, the distal sector pluriplicate and narrowing to the contracted mouth, the mouth lobate or laciniate-ciliate; perianth unistratose throughout or 2-stratose in basal sector.
Seta with 8–16 rows of outer cells surrounding an inner core of numerous, much smaller cells. Capsule ovoid, the wall 2–4-stratose; outer layer of cells with two-phase development, the alternating longitudinal walls with nodular and often confluent thickenings lending them a ± sinuous appearance; innermost layer of cells with semiannular bands complete or at times incomplete, the bands at times forked, the radial walls also sometimes with nodular thickenings.
Spores minutely papillose or papillose-vermiculate. Elaters bispiral.
Key to Species
Acromastigum is a genus of 35–40 species (Schuster, 2000a). In the New World it occurs only in southern South America, with three species— A. laetevirens (Sande Lac.) A.Evans, A. anisostomum and A. cunninghamii, the latter two of which also occur in our area (see Fulford, 1966; Grolle, 1961d). One species, A. exile (Lindenb.) A.Evans, is present in South Africa (Arnell, 1963). Ten species occur in New Guinea (Grolle & Piippo, 1984). One species, A. integrifolium, the type of the genus, occurs in Hawaii. Eight species occur in New Zealand.
The two genera comprising the Bazzanioideae may be differentiated by several characters. Acromastigum has ventral stoloniform branches that are terminal in origin, replacing one half of an underleaf. This Acromastigum -type branching is found in other genera of hepatics, but does not occur in Bazzania. These branches occur, for example, in a few Lepidozioideae (e.g., Lepidozia spinosissima, L. pumila [Fig. 42: 3]), a few species of Telaranea and some Zoopsidoideae (Paracromastigum). In a number of Acromastigum species the stem cortex consists of a fixed number of 7 rows of cells that are larger than those of the medulla. The number of cortical cells may be as many as 16 (A. cavifolium, A. colensoanum and A. verticale) and the extreme reduction in number is reached in A. rigidum R.M.Schust. (Schuster, 1997b) of New Caledonia, in which the medulla is formed of only 3 cell rows. None of the species of Bazzania has a fixed number of stem cortical rows or has such extreme reduction in medullary cell number. Also, all New Zealand species of Acromastigum have either unlobed or bilobed leaves vs. tridentate or 3-lobulate leaves in all of the New Zealand representatives of Bazzania. In general, Acromastigum species are much smaller and more rigid than species of Bazzania.
References: Engel and Merrill (1994); Hodgson (1954; rev.); Schuster (2000a).
