Green Yoke-moss - Zygodon viridissimus

There are four similar Zygodon species – of which Z. viridissimus is the commonest – are difficult to reliably distinguish from each other without checking under a microscope. They all form small (usually up to 1 cm tall), light green tufts and patches. The shortly pointed leaves are held at a wide angle from the stem when moist, giving the plants a characteristically neat appearance. Dry leaves become lightly twisted and appressed to the stem, considerably altering the appearance of shoots. Leaves are typically 1.5 to 2 mm long, nerved, widest near the middle, and narrow rather abruptly at the tip. Gemmae are normally produced amongst the leaf axils, but only become discernible under a microscope. Capsules are infrequent in Zygodon viridissimus, they are pale yellowish-brown, egg-shaped, borne on a seta 6 to 10 mm long, and produced in spring and summer.

Z. stirtonii differs from Z. viridissimus in its less acutely pointed leaves, with the nerve widest in the upper part of the leaf and excurrent in a stout point.  Z. rupestris is typically more yellow-green than Z. viridissimus. Its shoots tend to be straight when dry, whereas those of Z. viridissimus often curve upwards. Z. conoideus is usually more slender than Z. viridissimus, and forms patches of shoots up to 5 mm tall, and differs further in the tip of its leaves being straight (rather than recurved) when moist. It also produces capsules more frequently than other Zygodon species, the seta of which is usually noticeably curved, whereas the setae of Z. viridissimus, and even more so Z. stirtonii, are straighter and slightly more robust.

Unless identified by a recognised expert, a photo is required and the specimen should be examined with a microscope. In the comments box, state the key or ID method used and describe the size and identifying characters.

Z. viridissimus grows on lightly shaded, base-enriched bark, especially Ash, Elder, Field Maple, Sycamore and Willows (Salix sp.). It particularly favours tree trunks, but is also very common on shaded walls, as well as growing occasionally on base-rich rocks and concrete.

All year round.

Widespread and fairly frequent in Britain.

Status in Leicestershire and Rutland not known.