A closer look at Mnium hornum

Above: Mnium hornum, Swan's-neck Thyme-moss a member of an order of mosses called the Bryales. These mosses were growing on a segment of fallen tree log in woodland, where it is often abundant on fallen logs and tree bases. It can also be found on acidic soil and rocks.

Above: a ripening spore capsule of a sporophyte.

Above and below: the margins of the leaves consist of elongated cells with thickened walls and narrow cytoplasm (rich in small chloroplasts). Some cells form marginal spikes.

What is the function of these marginal cells? They likely strengthen the leaf to reduce tearing, but most mosses do not have them. They likely effect the shape of the leaf when it dries (like many other mosses the leaves have the ability to resist a degree of drying and will then absorb applied water remarkably quickly). When dry the leaf margins fold up slightly, forming a gutter that may trap moisture and conduct water by capillary action when rain returns. Perhaps the margins also help to absorb water into their thick apoplasts (apoplasts - the space between cells consisting of cell walls and binding materials in the intercellular spaces) or conduct excess water to the leaf tip where it may drip away, keeping the leaf surface clear for photosynthesis..The function of the spines is not clear, but perhaps they deter tiny herbivores.

Above: a young leaf tip consisting of a projection of marginal cells.

Above: elongated cells form a sheath for the midrib.

Above and below: the leaf midrib consists of elongated cells with chloroplasts. Note the large channels of apoplast (cell wall and intercellular materials) which is porous and permeable to flowing water.

Above: young bright green shoots developing in late winter, alongside more mature shoots with darker leaves, many of which are bearing maturing sporophytes at their tips.

Above: in Mnium hornum the midrib (vein or 'nerve') usually terminates a short distance below the leaf tip, but may extend more-or-less right into it (below):

Above: older leaves may have patches of dead cells.

Above and below: the male 'flowers' of Mnium hornum -modified leaves around the shoot apex surrounding a cluster of orange antheridia (male reproductive organs).