On a recent hike along the lower Stein River, I came across a small thallus liverwort growing in a crevice on a rock face. It is named Asterella gracilis. Instead of stems and leaves like those of mosses, the plant consists of a thallus, a flat, tongue-shaped layer of photosynthetic tissue.
The thallus is quite complex for such a primitive plant and the upper surface of the thallus consists of numerous pores that somewhat resemble the stomata of vascular plants that allow gas exchange to occur within air chambers embedded in the thallus where photosynthetic tissue is located. Unlike stomata, these pores cannot open and close to control water loss from the plant.
The upper surface of the thallus showing the air pores that allow gas exchange to occur inside the thallus tissue.
The most recognizable feature of thin starwort are the stalked receptacle that originate near the tip of the upper surface of the thallus. It contains the developing sporangium on the underside of the umbrella-like receptacle. The sporangium produces the spores that will eventually darken in colour and be released into passing air currents. Each sporangium is surrounded by a white, lacerated skirt – a distinctive feature of Asterella.
Thin starwort often occurs on shaded crevices on rock outcrops. It has an odd distribution in that it is common at low elevations near the coast or at high elevations in the alpine.
