Acrostichum speciosum.
The Mangrove or Leather fern, common in Australia is typically seen on muddy inter tidal areas.
Often present in mangrove forests it can tolerate some salty water but is usually seen on the landward side
of the forests where the water is less salty.
Plants, up to 1.5 m high form thick clumps often in very shady positions.
The rhizomes (modified underground stems) typically spread horizontally just under the surface.
They are sometimes seen on the surface or erect.
The rhizomes have dark scales up to 8 or 9 mm long and 3 mm wide.
There are also fleshy roots that act as props in the soft muddy soil.
The erect and arching fronds are 1 to 2 m long.
Up to one third of this is the stalk or stipe which is smooth or has a few scales at the base.
The scales are under 1 cm long.
On new fronds the lamina may be undivided but older ones are all pinnately divided.
The pinnae alternate with up to 8 each side and a single terminal one.
The lower pinnae are on a short stalk and the terminal ones are directly attached with the base sometimes running down the midrib.
The dull green pinnae are of 2 types – sterile at the base of the frond and fertile at the tip.
The lower sterile pinnae are up to 20 cm long and the fertile upper ones 8 to 10 cm.
All are under around 3.5 cm wide and have a pointed tip and a wedge-shaped base.
The small veins form a network of elongated roughly 6-sided areas.
The fertile pinnae have spore producing sporangia on the whole of the lower surface.
During spore production the whole lower surface is a rusty brown apart from the green midrib.
Acrostichum aureum is very similar but less frequently seen here.
It has wider pinnae around 4.5 cm across and larger rhizome scales.
J.F.
