Looking down, haircap moss has a star-shaped appearance due to its pointed leaves arranged spirally at right angles around a stiff stem .

While mosses are considered non-vascular plants, those of Polytrichum show clear differentiation of water conducting tissue.

The leaves of most mosses are simply a single plate of cells, but those of Polytrichum have more highly differentiated photosynthetic tissue.

This minimises water loss as relatively little tissue is directly exposed to the environment, but allows for enough gas exchange for photosynthesis to take place.

It is speculated that the teeth along the leaf's edge may aid in this process, or perhaps also that they help discourage small invertebrates from attacking the leaves.

[citation needed] The genus Polytrichastrum was separated from Polytrichum in 1971 based on the structure of the peristome (which controls spore release).