Group I and group II introns, next to spliceosomal and transfer RNA (tRNA) introns, belong to four main types of introns divided on the basis of splicing mechanism . Although two transesterification reactions are used by group I and group II introns for their splicing, the reaction mechanisms are different. As a result, group I introns are removed in a linear form, and some of them can circularize, whereas group II introns are released as a lariat . Both discussed intron groups are also known as mobile elements. Their mobility is possible thanks to internal encoded enzymes, but the movement mechanism in each of the groups is different. Group I introns can proliferate by a DNA-mediated homing mechanism, where intron-encoded endonucleases play a key role. In contrast, retrohoming (RNA-mediated mechanism) is used by group II introns for propagation . Internal encoded enzymes of this intron class, maturase, reverse transcriptase and endonuclease, enable retrohoming .
In addition to the differences in splicing and mobility mechanisms, group I and group II introns also have a unique structure. Group I introns have a characteristic RNA fold consisting of 10 elements denoted from P1 to P10 , while group II introns typically have a secondary structure consisting of six double-helical domains [2,6].
In plant mitochondrial genomes (chondriomes), group II introns occur more commonly than group I introns . Group II introns are also present in archaebacteria, bacteria and plastids, but are absent in a nuclear genome, unless a sequence with this type of introns was transferred from mitochondrion to nucleus [7,8]. On the other hand, group I introns are common in nuclear ribosomic RNA (rRNA)-encoding genes, frequent in fungal mitochondria and are also present in plant organellar genomes, bacteria and viruses [1,9]. Intron contents of the mitochondrial genome in angiosperms are reported to be rather stable [10,11], as well as in hornworts, mosses and in the earliest land plants: liverworts. On the other hand, intron contents among these four distinct lineages are reported to be significantly different [11,12]. The tendency towards a stable number of introns in the mitogenome among bryophytes is in accordance with the hypothesis that their mitochondrial genomes are slowly evolving .
Retroprocessing is the most frequently reported mechanism for removal of introns [14,15,16,17,18]. According to this model, also known as a reverse transcriptase-mediated model (RT-mediated model), spliced mRNA is reverse-transcribed, and then, the emergent intronless complementary DNA (cDNA) is integrated into the genome by homologous recombination [14,15,16,17]. A loss of introns is thus associated with a loss of editing sites in the genome . Under the standard type of this mechanism, the introns located at 3’ ends of genes are more likely to be deleted, because reverse transcriptase polymerizes the RNA template from 3’ ends to 5’ends of genes and frequently dissociates from the template without completing its rewriting [14,15,19]. Although reports of cDNA production in vivo are very rare [15,20,21], reverse transcription has been widely discussed in gene evolution , and a 3’ bias of intron loss has been reported in some organisms , which may indicate an RT-mediated model of intron loss. Intron losses from quillwort Isoëtes engelmannii , and gymnosperms are the strongest  evidence of retroprocessing in the plant mitochondrial genome.
Other possible mechanisms of intron loss include genomic deletion, exonization, horizontal gene transfer (HGT) and gene conversion. In the case of genomic deletion, introns are excised imprecisely , resulting in the removal of adjacent exonic sequences or retaining small fragments of introns, which are exonized . This exonization may also involve an entire intron, which is no longer cut out of the transcript, but preserved in the mature mRNA and translated . Disorders in intron cutting may be caused by creating alternative splice sites, which are the result of genomic insertions or point mutations in a DNA sequence. In this way, the splicing system includes new sequences (here introns) as exons or elongated existing exons . Intron loss by deletion has been reported only in the case of Petunia , whereas exonization has never been evidenced in plant mitochondrial genome . Intron loss can be also explained by HGT of an intron-less gene and the following gene conversion with an intron-including gene. The above mechanism in the plant mitogenome has only been observed in Magnolia tripetala to date .
The genus Calypogeia Raddi belongs to leafy liverworts (Jungermanniopsida) and consists of about 90 described species . The most characteristic taxonomical feature of this genus is the presence of oil bodies and their color, shape and pattern of distribution in the leaf and underleaf . This genus is the most widespread (but hardly cosmopolitan) genus of the family Calypogeiaceae. Although it includes species with wide Holarctic distribution (e.g., Calypogeia suecica, Calypogeia sphagnicola, Calypogeia neesiana), it also contains neotropical species occurring in Central and South America (e.g., Calypogeia biapiculata, Calypogeia laxa, Calypogeia miquelii) [30,31]. The family Calypogeiaceae is easily recognized, but species identification, due to the presence of environmentally-induced modifications and atypical forms and the frequent absence of sporophyte, can often lead to misidentification [32,33]. Some morphologically similar (cryptic) species are possible to distinguish using molecular markers .
Genomic work on liverworts mainly concerns simple (metzgeriid) and complex (marchantiid) thalloid liverworts, and there are still only a few sequenced mitogenomes [12,35,36,37] and plastomes [38,39,40,41,42,43].
In this paper, we introduce for the first time the mitochondrial genome structure of the genus Calypogeia. The four presented Calypogeia mitogenomes are the first sequenced mitochondrial genomes of leafy liverworts. The main goal of our research was to examine the stability of the mitochondrial genome structure among liverworts mentioned in the literature. Unexpectedly, we discovered a lack of introns in the genes atp1 and cox1. It is the first case of intron loss within Jungermanniopsida leafy liverworts. We also discuss the possible mechanisms of intron disappearance.
3. Results and Discussion
3.1. Mitogenome Structure
The mitochondrial genome of Calypogeia is 159,061–163,057 bp in length (Table 1, Figure 1) and is slightly shorter than the closest related species with a known mitogenome structure: P. purpurea (168,526 bp) . The length of the mitochondrial genome of Calypogeia is most similar to the mitogenome of A. pinguis (164,989 bp) . The two other known mitochondrial genomes of the liverwort species, T. lacunosa  and M. polymorpha , differ more and are composed of 151,983 and 186,609 bp, respectively. The GC content in the studied genome (45%) is similar to other liverworts (42–45%) [12,36].
Seventy genes have been identified in the Calypogeia mitogenome: 42 protein-coding genes 25 tRNAs and three rRNAs (Table 2). Chondriomes of liverworts, similar to moss and hornwort mitogenomes, are reported to be rather static in gene content and order [12,13,50] and even with respect to pseudogene contents and retroposed pseudogene pieces . The gene order in Calypogeia mitogenome is identical to the four aforementioned liverwort mitogenomes, although the gene content is slightly different between them. Mitogenomes of M. polymorpha, P. purpurea, A. pinguis and Calypogeia are very similar in gene composition. The differences occur mainly in the content of transfer RNA genes. M. polymorpha has only one copy of the trnRucu, but contains two more tRNA genes: trnRucg and trnTggu. One copy of the trnRucu has probably given rise to trnRucg . The trnT gene is a part of the trnA-trnT-nad7 gene cluster, whose different forms were identified by Wahrmund et al.  in liverwort mitogenome evolution. In leafy (jungermanniid) liverworts and in simple thalloid (metzgeriid), trnT is lacking between trnA and nad7, whereas in Blasia pusilla, representing a sister lineage to all other complex thalloid (marchantiid) liverworts, this gene occurs in conserved Chara-like version. The trnT gene is also present in M. polymorpha and other complex thalloid (marchantiid) liverworts, but its sequence is inverted compared to Blasia. Furthermore, in the A. pinguis mitochondrial genome, there is only one copy of the trnYgua in contrast to other liverwort mitogenomes . Another difference concerns the rtl gene. In all aforementioned mitogenomes, this gene is functional with nucleotide sequence similarity > 84%, whereas in P. purpurea, it may be a pseudogene because of the high level of sequence divergence and several indels in the open reading frame . However, a big part of the reading frame in rtl is intact, so this gene in P. purpurea may be still functional. The other dissimilarities occur between mitogenomes of the above four species and T. lacunosa. In the mitochondrial genome of the latter, either some genes of the cytochrome c biogenesis (ccmC, ccmFN) are missing or some of them are pseudogenized (ccmB, ccmFC). Another conspicuous dissimilarity concerns the nad7 gene, which is only functional in T. lacunosa . In most hornworts and liverworts, this gene is missing or occurs as a pseudogene with a degenerated structure [52,53,54], which is reflected in Calypogeia mitogenome and in the other sequenced mitochondrial genomes of liverworts. The only liverwort species with functional nad7 are Treubia and Haplomitrium  belonging to Haplomitriopsida, a sister clade to the rest of the liverworts: Marchantiopsida and Jungermanniopsida  with the inactive nad7 gene.
Ninety-four spacers and 24 introns (Table 3) have been found across the entire mitochondrial genome. The different length (1000–1300 bp) of the nad5-nad4 spacer, containing the inverted sequence of the second cob intron in M. polymorpha, was recognized in different liverwort groups. Almost the entire cob intron sequence is inserted in the nad5-nad4
The 3 Most Important Classes of Bryophyta are mentioned below:
For the first time Braun in 1864 gave the name Bryophyta of this group of plants. However, he included algae, fungi, lichens and mosses in his Bryophyta. Now, it is an established fact that algae, fungi and lichens comprise the group Thallophyta and the mosses are included in Bryophyta.
Thereafter Schimper in 1879 gave the name Bryophyta which is used in full sense for the plants included in this group. Eichler in 1883 divided Bryophyta into two groups-Hepaticae and Musci. Engler (1892) divided the class Hepaticae into three orders-Marchantiales, Jungermanniales, and Anthocerotales.
However, Bower (1935), Wettstein (1935), Bessey, Fritsch (1929), Evans (1938, 1939) and other bryologists still follow the same old system. Underwood (1894) and Gayeh (1897) however, withdrawn the order, Anthocerotales from class-Hepaticae.
According to Campbell, Smith, Takhtajan and others, the Bryophyta has been divided into three classes-Hepaticae, Anthocerotae and Musci.
In 1951, Rothmaler changed the class names. He recognized Hepaticae as Hepaticopsida; Anthocerotae as Anthoceropsida and Musci as Bryopsida. The above given classes have been also recognized by the international code of botanical nomenclature, 1956. Proskauer (1957) however changed the name Anthoceropsida to Anthoerotopsida and thus, the Bryophyta may be classified as follows:
Class 1: Hepaticopsida:
There are 4 orders, 9 families, 225 genera and 8,500 species.
1. The gametophytes are dorsiventrally differentiated. They may be thalloid (thallose) or differentiated into leaves and stem (foliose).
2. In foliose types the leaves are arranged in two or three rows on the axis and are always without mid-rib.
3. The sex organs develop from superficial cells on the dorsal side of the thallus, except when they are terminal in position.
4. The sporophyte may be simple, or differentiated into foot and capsule, or into a foot, seta and capsule.
5. The sporogenous cells develop from the endothecium of sporogonium.
6. The sporophyte is completely dependent on gametophytes for its nutritive supply.
7. The wall of sporogonium is one to several layered thick. The stomata are not present on the wall of sporogonium.
8. The dehiscence of sporogonium is irregular.
The class Hepaticopsida is further divided into several orders - (1) Sphaerocarpales; (2) Marchantiales; (3) Metzgeriales; (4) Jungermanniales; (5) Calobryales and (6) Takakiales.
Order-Sphaerocarpales (3 genera)-two families:
1. Family-Sphaerocarpaceae - Sphaerocarpos (seven species) and Geothallus (single species).
2. Family-Riellaceae-Riella (17 species).
The characteristic features of the order are as follows:
1. Vegetative structure of gametophyte is similar to that of order-Metzgeriales, but in which development and structure of sex organs, as well as the structure of sporophyte are similar to those of order-Marchantiales, and because of this the genera are placed in separate order Sphaerocarpales.
2. The main diagnostic feature by which the order is recognized is the presence of globose or a flask-like envelope or involucre around each of the sex organs (i.e., antheridia and archegonia).
Order-Marchantiales (32 genera; 400 species):
The characteristic features are as follows:
1. The ribbon-like, dichotomously branched and dorsiventral thalli grow prostrate upon suitable substrata.
2. Excluding Dumortiera, Monoselenium and Monoclea, the rest of the genera possess internally differentiated air chambers on the dorsal side of the thallus; such chamber opens outside by an air pore of a particular design.
3. The ventral portion of the thallus consists of parenchyma which acts as storage tissue; oil and mucilage cells may be present in this region.
4. The scales and rhizoids are present on the ventral side of the thallus; the rhizoids are of two types (smooth-walled and tuberculate).
5. The antheridia and archegonia may be found directly on the dorsal surface of the thallus or they may be present on the special branches known as antheridiophores and archegoniophores respectively.
6. In most of cases the capsules of the sporophytes possess single layered jacket.
7. The capsule may be simple as in Riccia or it may be differentiated into foot, seta and capsule as in Marchantia.
8. The elaters may or may not be present. According to Campbell (1940), there are five families in this order. The characteristic features of two families are given here:
I. Family-Ricciaceae (3 genera; 140 species):
1. The gametophyte consists of a rosette-like dichotomously branched thallus.
2. In the thallus, the dorsal portion consists of chlorophyllous strips which may or may not have air canals among them; the ventral portion of the thallus is parenchymatous and acts as storage tissue.
3. The sex organs (antheridia and archegonia) are found in the longitudinal groove on the dorsal side from the growing apex to backward in basipetal succession.
4. The sporogonium consists of a simple capsule which is not differentiated into foot, seta and capsule.
5. Elaters not present.
6. The archesporium produces only the spores.
The important genera aie-Oxymitra, Ricciocarpus and Riccia.
II. Family-Marchantiaceae (23 genera; 250 species):
1. The thallus is dorsiventral; it has distinct assimilatory and storage regions.
2. The assimilatory region remains divided into several chambers and each chamber contains branched assimilatory filaments.
3. The pores of the thallus may be simple or barrel-shaped.
4. The archegonia are borne upon special erect, stalked, vertical branches, the archego- niophores.
5. The antheridiophores may or may not present; however, in Marchantia, the antheridia are borne upon these erect, stalked antheridiophores.
6. The typical sterile elaters are found in the sporogonium mixed with the spores. The important genera are - Conocephalum, Cryptometrium, Lunularia, Marchantia, etc.
Order-Metzgeriales (23 genera; 550 species):
The characteristic features of this order are as follows:
1. The gametophyte may be thalloid or differentiated into stem and lateral leaves.
2. In most cases the gametophytes are without internal differentiation of tissues but certain genera have a central strand of thick-walled cells.
3. The ventral surface of a gametophyte bears smooth-walled rhizoids.
4. The sex-organs are found to be scattered on dorsal surface of thallus.
5. The archegonia arise from the young segments cut off by the apical cell.
6. The mature sporophytes lie some distance back from the growing apex of a gametophyte.
7. The sex organs (antheridia and archegonia) are produced on any branch of the gametophyte or only on special branches.
Family-Pelliaceae (Three genera- Pellia, Noteroclada and Calycularia):
1. The thallus is prostrate, dorsiventral and very often lobed by irregular incisions.
2. The rhizoids are simple, non-septate, smooth and thin-walled. The scales are absent.
3. The sex organs (antheridia and archegonia) remain scattered on the dorsal surface of the thallus.
4. The archegonial cluster always remains surrounded by an involucre which is an outgrowth of the thallus.
5. The capsule (sporogonium) is globose or oval in shape. It possesses a basal elaterophore.
Family-Riccardiaceae (two genera):
1. The gametophytes are wholly thallose or have thallose terminal branches.
2. The cells of thallus possess finely segmented oil bodies.
3. The sex organs (antheridia and archegonia) are borne on short lateral branches.
4. A well-developed calyptra is present but there is no involucre.
5. A distal elaterophore is present to which some elaters are attached.
6. The capsule is ovoid to cylindrical and dehisces longitudinally into four parts extending to the base.
Family-Fossombroniaceae (4 genera; Fossombronia, Simodon, Petalophyllum and Sewardiella):
1. Thallus is distinctly foliose.
2. The thallus is dorsiventral and prostrate.
3. The stem is branched. Growth of the main axis and of its branches is by means of an apical cell with two cutting faces.
4. The leaf is thin, one cell-thick except the basal portion which is 2 or 3 cells in thickness.
5. Antheridia develop in acropetal succession singly or in small groups.
6. Archegonia occur in small groups.
7. Young sporophyte remains surrounded by a calyptra and which is ensheathed by a cuplike involucre.
8. The mature sporophyte is surrounded and protected by a bell-shaped sheathing perianth.
9. Important genus-Fossombronia.
Order-Jungermanniales (220 genera; 8, 500 species):
The characteristic features of this order are as follows:
1. The gametophyte is differentiated into stem and leaves; the leaves are borne in a regular spiral succession along the stem.
2. The apical cell is pyramid-like with three cutting faces.
3. The stem generally bears three rows of leaves; two rows are lateral and consist of leaves of normal size; the third row consists of the under leaves which are generally smaller than the lateral leaves.
4. The archegonia are always restricted to the apices of the axis and its branches.
5. The sporophytes are always terminal in position.
6. The antheridia are borne singly or in groups in the axis of leaves.
Family-Porellaceae (single gemis-Porella):
1. The leaves are arranged in three rows on the stem; ventral leaves are well developed and usually decurrent at the base; dorsal leaves are incubuous; postical lobe distinct.
2. The rhizoids are scarce and arise from the lower side of the stem in tufts generally near the base of underleaves (ventral leaves).
3. The archegonia are borne in terminal cluster on small lateral branches; the archegonia remain surrounded by a large inflated perianth.
4. The spherical capsule dehisces by four valves which split only to half way down.
Family-Frullaniaceae (three genera; important genus Frullania):
1. The thallus is pinnately branched and differentiated into stem and leaves.
2. The leaves are arranged in three rows two laterals unequally lobed and a ventral lobule.
3. The ventral leaves are bifid and trumpet-shaped.
4. The archegonia develop in a group.
Order-Calobryales (2 genera-Calobryum (8 spp.) and Haplomitrium (single spp.):
The characteristic features are as follows:
1. They possess erect leafy gametophytes with leaves in three vertical rows.
2. The leaves are dorsiventrally flattened.
3. They have a pale, substerranean, sparingly branched rhizome from which arise erect leafy branches.
4. Erect branches bearing sex organs have the uppermost leaves close together and in more than three rows.
5. They are devoid of rhizoids.
6. The antheridia are ovoid, stalked, and borne at the apex of the stem.
7. The jacket of the neck of archegonium has only four vertical rows of cells.
8. The sporophyte bears an elongate capsule whose jacket layer is only one cell in thickness except at the apex.
9. The number of chromosomes is-n=9.
Since there is single family Calobryaceae the characters are similar to that of the order. Two genera-Calobryum and Haplomitrium.
Order-Takakiales (1 genus; 2 species):
The characteristic features are as follows:
1. They possess cylindrical, rhizomatic and erect gametophores.
2. They are devoid of rhizoids.
3. They possess copious beaked or non-beaked mucilage hairs on them.
4. They possess terete bifid-trifid-quadrifid leaves or phyllids.
5. The gametophores are about 1 to 1.5 cm. in height.
6. The leafless branches facing downward known as 'flagella' or 'stolons' may be present.
7. Asexual reproduction is not known.
8. Only female (archegonial) shoots are known. They bear conspicuous pedestalled archegonia.
9. The male (antheridial) shoots and the sporophytes are not known.
10. They have lowest chromosome number (i.e., n=4).
11. They are supposed to be most primitive and sometimes known as living fossil. There is one family Takakiaceae, and one genus Takakia.
Class II: Anthocerotopsida:
There are 1 order, 1 or 2 families, 6 genera and 301 species.
1. The gametophyte is thalloid and dorsiventral, bearing simple and smooth-walled rhizoids; tuberculate rhizoids and ventral scales are altogether absent.
2. The tissues of the thallus are undifferentiated; air chambers and air pores are absent; each cell bears a large chloroplast and a conspicusous pyrenoid within it.
3. The sex organs are found to be embedded in the gametophytic tissue.
4. The antheridia arise from the hypodermal cells of the thallus on the dorsal side of it; they develop within the antheridial chambers, singly or in groups on the dorsal side of the thallus.
5. The archegonia are found in sunken conditions on the dorsal side of the thallus, they develop from superficial cells.
6. The elongated and cylindrical sporogonium arises from the dorsal side of the thallus.
7. The sporogonium consists of foot, meristematic region and capsule; the meristem is intercalary and continues its growth thoughout the growing season.
8. The wall of sporogonium contains chlorophyll.
9. The central sterile portion of sporogonium is columella, which remains surrounded by sporogenous tissue and spores; the elaters are also present.
10. The sporogenous mass develos from amphithecium and arches over the columella. The class Anthocerotopsida includes a single order, the Anthocerotales with the same characters. There are two families 1. Anthocerotaceae and 2. Notothylaceae.
Family-Anthocerotaceae (4 or 5 genera; important genus Anthoceros):
1. The capsule is linear and vertical.
2. The stomata are present on the wall of capsule.
3. The archesporium develops from amphithecium.
4. The eleaters are four-celled, smooth or thick-walled; thickening band may or may not be present.
Family-Notothylaceae (single gemis-Notothylas):
1. The capsule is cylindrical and horizontal.
2. The stomata are not found on the wall of capsule.
3. Archesporium arises from endothecium and amphithecium.
4. Elaters are short and stumpy; they have irregular thickening bands.
Class III: Bryopsida:
There are 3 orders, 28 families, 660 genera and 15,504 species:
1. The gametophyte consists of prostrate, thalloid, branched protonema and erect leafy gametophore.
2. The gametophytic plant body consists of the stem, spirally arranged leaves and the sex organs (antheridia and archegonia) at its apical portion.
3. The rhizoids are multicellular, branched and obliquely septate.
4. The sex organs (antheridia and archegonia) develop from the superficial cells of the gametophore.
5. The sporophyte is differentiated into foot, seta and capsule.
6. The capsular wall remains interrupted by stomata at several places.
7. The archesporium or sporogenous mass develops from outer layer of endothecium which in addition forms columella.
8. The elaters are not present in the sporogonium.
The class Bryopsida (Musci) has been divided into three sub-classes (1) Sphagnobrya (Sphagnidae); (2) Andreaeobrya (Andreaeidae) and (3) Eubrya (Bryidae).
I. Sub-class. Sphagnobrya:
The sub-class has a single order, the Sphagnales and a single family, the Sphagnaceae. (Single genus Sphagnum with 326 species). The characteristic features are as follows:
1. They are called 'bog mosses' or 'peat mosses'.
2. The protonema is broad and thallose; It produces one gametophore; the leaves or gametophores lack mid-rib and usually composed of two types of cells-(i) the narrow living green cells and (ii) large hyaline dead cells.
3. The branches arise in lateral clusters in the axis of the leaves borne on the stem.
4. The antheridia are borne in the axis of leaves on the antheridial branch.
5. The archegonia are terminal and formed acrogynously.
6. The sporogenous tissue of a sporophyte develops from the amphithecium.
7. The sporogonium remains elevated above the gametophyte due to elongation of a stalk of gametophytic tissue, the pseudopodium.
II. Sub-class. Andreaeobrya:
This sub-class has a single order, the Andreaeales, and a singly family, the Andreaceceae. The important genus is Andreaea.
The characteristic features are as follows:
1. The gametophores are brittle, and can easily be broken.
2. There is practically no tissue differentiation in plant body.
3. The leaves are generally large, erect and convolute.
4. The archesporium and colonmella develop from the endothecium.
III. Sub-class Eubrya (650 genera; 14,000 species):
This sub-class has been further divided into three cohorts and fifteen orders. The true mosses are included in this sub-class. The characteristic features are as follows:
1. The leaves of the gametophores are more than one cell in thickness and possess midtrib on them.
2. The protonema are filamentous.
3. The sporophyte bears a well differentiated, elongated seta which pushes out the capsule from the gametophore.
4. The sporogenous tissue is derived from the endothecium.
5. The archesporium does not overarch the columella; the columella continues upto the apex of the capsule; both columella and archesporium have been derived from the endothecium.
6. In between spore sac and columella, the partitioned air spaces are present.
7. The mature capsule possesses the complex structure made of many tissues.
8. The capsule opens at its apex by an operculum; the spore dispersal is regulated by a teeth like apparatus, the persistome.
Order-Funariales (26 genera; 356 species):
1. The plants are terrestrial; they are small in size and may be annual or biennial.
2. The leaves possess distinct mid-ribs and arranged in rosettes at the apex of the gametophyte.
3. The capsule is wide and provided with an unbeaked operculum.
4. The peristome of the capsule is double and consists of inner and outer peristomes called endostome and exostome respectively.
5. There are five families in this order, of which Funariaceae is most important.
Family-Funariaceae (9 genera; 200 species):
1. The leaves are one cell in thickness except at the mid-rib region.
2. The small mosses form the velvety appearance on the surface of the sustratum.
3. The calyptra are soon detached from the opercula of the capsules; the calyptra are provided with long beaks.
4. The capsules are pyriform and situated on the long, elongated setae.
1. The gametophyte is perennial and tall.
2. The leaves are narrow and possess longitudinal lamellae on the upper surface of the midrib.
3. The capsule is terminal.
4. The single annular series of cells gives rise to a peristome in the inner zone of the amphithecium.
5. There are 32 to 64 pyramidal teeth in peristome; the tips of the peristome teeth remain joined above to a thin membrance, the epiphragm covers the mouth of the capsule.
6. There is a single family, the Polytrichaceae in this order; the important genera of this family are — Polytrichum and Pogonatum.