Physiology
Polytrichum commune is an endohydric moss, meaning water must be conducted from the base of the plant. While mosses are considered non-vascular plants, Polytrichum commune shows clear differentiation of water conducting tissue. One of these water conducting tissues is termed the hadrom, which makes up the central cylinder of stem tissue. It consists of cells with a relatively wide diameter called hydroids, which conduct water. This tissue is analogous to xylem in higher plants. The other tissue is called leptom, which surround the hadrom and contains smaller cells. This tissue is, on the other hand, analogous to phloem. When these two tissue types are taken into account along with the species' exceptional height, it becomes clear that common haircap moss is quite a unique moss considering that the majority of species show little differentiation of conducting tissue and are restricted to much smaller stem lengths.
Another characteristic feature of the species (and the genus) is its parallel photosynthetic lamellae on the upper surfaces of the leaves. Most mosses simply have a single plate of cells on the leaf surface, but common haircap moss has more highly differentiated photosynthetic tissue. This is an example of a xeromorphic adaption, an adaptation for dry conditions. Moist air is trapped in between the rows of lamellae, while the larger terminal cells act to contain moisture and protect the photosynthetic cells. 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. The microenvironment between the lamellae can host a number of microscopic organisms such as parasitic fungi and rotifers. Additionally, the leaves will curve and then twist around the stem when conditions become too dry, this being another xeromorphic adaptation. 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.
Read more about this topic: Polytrichum Commune
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