Polytrichastrum formosum

Polytrichastrum formosum generally inhabits shaded, poor soils and humus in damp coniferous forests and cool temperate rainforests.[3][8] This species is most common in Europe and grows in grasslands, lowland heaths, acidic moorlands, rocky slopes, and old buildings.[5]

Like all moss, the haploid gametophyte is the dominant phase of the lifecycle of P. formosum. The moss gametophyte has photosynthetic leaves, a stem, and root-like rhizoids that anchor them to the substrate.[9]

Polytrichastrum formosum is a medium to large robust acrocarpous moss, growing in uncrowded, unbranching tufts.[2][3] Its colour ranges from green to dark olive green to greenish black. The stems of mature plants are generally 3 to 8 cm tall, however they can be as short as 2 cm or as tall as 20 cm.[3] P. formosum, like all members of Polytrichaceae, is an endohydric moss, meaning water conduction occurs internally.[10] This process is made possible by a central conducting strand in the stem, made up of hydroids, which are cells specialized for water transport.[11][12] The stem also contains leptoids and specialized parenchyma cells that are used for conducting sugars throughout the plant.[12] The hydrome (made of hydroids) and leptome (made of leptoids) are considered analogous in function to xylem and phloem in vascular plants.[10] For structural support there are also thick-walled stereid cells circling the hydrome, which helps the moss grow tall.[10]

Ventral surface of a leaf from P. formosum.

The leaves of Polytrichastrum formosum are linear-lanceolate, and are usually 6 to 8 mm in length, but can be up to 12 mm long.[3] They are erect to spreading when the moss is dry but become broadly recurved when moist.[3] The leaves have a toothed margin and a prominent costa, where guide cells help conduct water throughout the leaf, and stereids on the dorsal side provide protection.[3][13]

Cross-section of P. formosum leaf, revealing photosynthetic lamellae.

Using a microscope, photosynthetic lamellae are visible in cross section, nearly covering the entire ventral surface of the leaf. [2][3] A feature unique to Polytrichaceae, lamellae are often compared to the mesophyll of vascular plants due to their role in photosynthesis.[14] Each lamella stands 3 to 7 cells high and the cells at the top are smooth and oblong.[2][3] Lamellae help increase desiccation tolerance and photosynthetic efficiency by providing more surface area for light absorption and gas exchange.[13][15] The lamellae are covered by a layer of hydrophobic wax that protects the air spaces between neighbouring lamella from being flooded with water, which would impede CO₂ uptake necessary for photosynthesis.[16][14] In this way, P. formosum is able withstand high irradiance better than other mosses.

P. formosum angled sporophyte capsules (calyptra absent).

The diploid sporophyte generation of P. formosum matures in early summer and is short-lived.[5] The solitary sporophyte grows out of the female gametophyte, relying on it for energy and nutrients. It is anchored to the gametophyte by a foot, and has a stalk (seta) elevating a capsule (sporangia) in which spores will develop via meiosis.[17] The seta is yellowish to reddish brown and is 3 to 6 cm tall.[3] Like the stem of the gametophyte, the seta has a hydrome and leptome for conducting water and sugars from the gametophyte.[17] The slender and short-rectangular capsule is ochre to brown and 4 to 7 mm long.[3] The capsule is terete or 4-6 angled, which distinguishes Polytrichastrum species from Polytrichum species.[6] In the early stages of growth the capsule is covered by a hairy calyptra that protects the capsule during development and influences it's shape. At maturity, the calyptra falls off to reveal a beaked operculum.[17] Like most other moss in Polytrichaceae, under the operculum P. formosom has 64 nematodontous peristome teeth that surround an epiphragm.[8]