Polytrichum is a genus of moss

Moss

Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...

es — commonly called haircap moss or hair moss — which contains approximately 70 species that cover a cosmopolitan distribution

Cosmopolitan distribution

In biogeography, a taxon is said to have a cosmopolitan distribution if its range extends across all or most of the world in appropriate habitats. For instance, the killer whale has a cosmopolitan distribution, extending over most of the world's oceans. Other examples include humans, the lichen...

. (Less common vernacular names include bird wheat and pigeon wheat.)

The Polythrichum genus has a number of closely related sporophytic

Sporophyte

All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...

 characters. The scientific name is derived from the Ancient Greek

Ancient Greek

Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 words polys, meaning "many", and thrix, meaning "hair". This name was used in ancient times to refer to plants with fine, hairlike parts, including mosses, but this application specifically refers to the hairy calyptra

Calyptra

Calyptra is a scientific term used in botany. It describes a feature in plant morphology.-Bryophytes:In bryophytes, the calyptra is an enlarged archegonial venter that protects the capsule containing the embryonic sporophyte . The calyptra is usually lost before the spores are released from the...

s found on young sporophytes. There are two major sections of Polytrichum species. The first — section Polytrichum — has narrow, toothed, and relatively erect leaf margins. The other — section Juniperifolia — has broad, entire, and sharply inflexed leaf margins that enclose the lamellae on the upper leaf surface.

Physiology

Mosses in the genus Polytrichum are endohydric, meaning water must be conducted from the base of the plant. While mosses are considered non-vascular plant

Non-vascular plant

Non-vascular plants is a general term for those plants without a vascular system . Although non-vascular plants lack these particular tissues, a number of non-vascular plants possess tissues specialized for internal transport of water....

s, those of Polytrichum show 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

Analogy (biology)

An analogy is a trait or an organ that appears similar in two unrelated organisms. The cladistic term for the same phenomenon is homoplasy, from Greek for same form. Biological anologies are often the result of convergent evolution....

 to xylem

Xylem

Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...

 in higher plants. The other tissue is called leptom, which surrounds the hadrom and contains smaller cells. This tissue is, on the other hand, analogous to phloem

Phloem

In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...

.

Another characteristic feature of the genus is its parallel photosynthetic

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 lamellae on the upper surfaces of the leaves. Most mosses simply have a single plate of cells on the leaf surface, but those of Polytrichum have 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

Microenvironment

Microenvironment carries different meanings depending on the context.* Medical: a small or relatively small usually distinctly specialized and effectively isolated habitat or environment...

 between the lamellae can host a number of microscopic organisms such as parasitic fungi and rotifer

Rotifer

The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...

s. 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.

Classification

The genus Polytrichastrum was separated from Polytrichum in 1971 based on the structure of the peristome

Peristome

The word peristome is derived from the Greek peri, meaning 'around' or 'about', and stoma, 'mouth'. It is a term used to describe various anatomical features that surround an opening to an organ or structure. The term is used in plants and invertebrate animals, such as in describing the shells of...

 (which controls spore release). However, molecular and morphological data from 2010 support moving some species back into Polytrichum.

Species