''Polytrichastrum formosum'', commonly known as the bank haircap moss,
is a species of
moss
Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) '' sensu stricto''. Bryophyta (''sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and hor ...
belonging to the family
Polytrichaceae
Polytrichaceae is a common family of mosses. Members of this family tend to be larger than other mosses with a thickened central stem and a rhizome. The leaves have a midrib that bears photosynthetic lamellae on the upper surface. Species in ...
.
It has a
cosmopolitan distribution
In biogeography, cosmopolitan distribution is the term for the range of a taxon that extends across all or most of the world in appropriate habitats. Such a taxon, usually a species, is said to exhibit cosmopolitanism or cosmopolitism. The ext ...
, found mostly in temperate latitudes in the Northern Hemisphere
and especially dominant in Europe and North America. However, it has also been identified in India, China, Nepal, Japan, Algeria, Australia, New Zealand, Russia, Turkey, Syria, and the Atlantic islands (i.e. Iceland).
This species was previously called ''Polytrichum formosum'' but has been reclassified as ''Polytrichastrum formosum'' due to distinct sporangial features.
Recent molecular studies suggest that it should be moved back to its original genus (''Polytrichum''),
however
bryologists have not yet reached a consensus.
Habitat
''Polytrichastrum formosum'' generally inhabits shaded, poor soils and humus in damp coniferous forests and cool temperate rainforests.
This species is most common in Europe and grows in grasslands, lowland heaths, acidic moorlands, rocky slopes, and old buildings.
Gametophyte
Like all moss, the
haploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
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.
''Polytrichastrum formosum'' is a medium to large robust acrocarpous moss, growing in uncrowded, unbranching tufts.
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.
''P. formosum'', like all members of Polytrichaceae, is an endohydric moss, meaning water conduction occurs internally.
This process is made possible by a central conducting strand in the stem, made up of hydroids, which are cells specialized for water transport.
The stem also contains leptoids and specialized parenchyma cells that are used for conducting sugars throughout the plant.
The hydrome (made of hydroids) and leptome (made of leptoids) are considered analogous in function to
xylem and
phloem in vascular plants.
For structural support there are also thick-walled stereid cells circling the hydrome, which helps the moss grow tall.
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.
They are erect to spreading when the moss is dry but become broadly recurved when moist.
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.
Using a microscope, photosynthetic
lamellae are visible in cross section, nearly covering the entire ventral surface of the leaf.
A feature unique to Polytrichaceae, lamellae are often compared to the
mesophyll of vascular plants due to their role in photosynthesis.
Each lamella stands 3 to 7 cells high and the cells at the top are smooth and oblong.
Lamellae help increase desiccation tolerance and photosynthetic efficiency by providing more surface area for light absorption and gas exchange.
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 CO
2 uptake necessary for photosynthesis.
In this way, ''P. formosum'' is able withstand high irradiance better than other mosses.
Sporophyte
The
diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
sporophyte
A sporophyte () is the diploid multicellular stage in the life cycle of a plant or alga which produces asexual spores. This stage alternates with a multicellular haploid gametophyte phase.
Life cycle
The sporophyte develops from the zygote pr ...
generation of ''P. formosum'' matures in early summer and is short-lived.
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.
The seta is yellowish to reddish brown and is 3 to 6 cm tall.
Like the stem of the gametophyte, the seta has a hydrome and leptome for conducting water and sugars from the gametophyte.
The slender and short-rectangular capsule is ochre to brown and 4 to 7 mm long.
The capsule is
terete or 4-6 angled, which distinguishes ''Polytrichastrum'' species from ''Polytrichum'' species.
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.
Like most other moss in Polytrichaceae, under the operculum ''P. formosom'' has 64 nematodontous peristome teeth that surround an epiphragm.
Reproduction
Sexual reproduction
''Polytrichastrum formosum'' is sexually
dioicous
Dioicy () is a sexual system where archegonia and antheridia are produced on separate gametophytes. It is one of the two main sexual systems in bryophytes. Both dioicous () and monoicous gametophytes produce gametes in gametangia by mitosis rathe ...
, meaning the male and female reproductive structures are on separate plants.
''P. formosum,'' like all members of the Polytrichaceae family, is an acrocarpous or cushion moss with reproductive structures borne at the terminus of the gametophyte stem. The perichaetial leaves surrounding the
archegonia
An archegonium (pl: archegonia), from the ancient Greek ''ἀρχή'' ("beginning") and ''γόνος'' ("offspring"), is a multicellular structure or organ of the gametophyte phase of certain plants, producing and containing the ovum or female ga ...
(female reproductive structure) resembles the other stem leaves besides being longer.
The
antheridia (male reproductive structure) are at the apex of the male gametophyte in a cup-like structure formed by perigonial leaves which are modified stem leaves.
Sperm is produced in the antheridia via
mitosis
In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...
, and when mature they are released from the antheridia by raindrops splashing onto the cup. Sperm can be dispersed up to one meter from the male, and if they successfully reach an archegonia, the egg will be fertilized and grow into a diploid sporophyte.
Asexual reproduction
Asexual reproduction is not extensive in ''P. formosum'', however it occurs on a local scale by the vegetative proliferation of
genets.
Classification
Varieties
''Polytrichastrum formosum'' var. ''densifolium'' (Wilson ex Mitt.) Z. Iwats & Nog.(North American)
''Polytrichastrum formosum'' var. ''formosum'' (Hedw.) G.L. Sm.
Taxonomy disagreement
When this species was first described by
Johann Hedwig in 1801, it was classified as ''Polytrichum formosum.''
Upon further analysis it was determined by Gary L. Smith in 1971 that the moss is better described by the ''Polytrichastrum'' genus.
''Polytrichastrum'' can be distinguished from ''Polytrichum'' by sporangial features including multiple-angled capsules (more than 4 sided), elongated peristome teeth surrounding the ridged epiphragm, and larger spores.
According to several sources, ''Polytrichastrum formosum'' remains the most accurate taxonomic classification.
However, more recent molecular data and phylogenetic analysis suggests that ''P. formosum'' should be reverted to its original genus: ''Polytrichum''.
Their study suggested that species in ''Polytrichastrum'' and ''Polytrichum'' are in fact distantly related, but because the ''Polytrichastrum'' genus is
polyphyletic, authors suggested that some species, including ''P. formosum,'' return to the ''Polytrichum'' genus.
Currently, these names remain synonyms.
References
{{Taxonbar, from=Q301377
Polytrichaceae