Polytrichum

Bryopsida

Distribution and Habitat:

The moosses are the higher Bryophytes. They are world wide in their distribution.
Occur in almost all situation were life is possible. The only exception is the sea. The
leafy plant body of a moss plant is better adapted for a life on land than the thallus of
liverworts. However, they flourish the most wet and humid region, such as, moist
mountain forest of tropics and sub tropics. A few are aquatic. A few , such as,
Sphagnum grow in bogs. Theya re found in tempetate and arctic tundras. They grow
on soil,fence, rocks tree trunks.

Polytrichum

Division: Bryophyta

Class:      Bryopsida

Sub:Class: Bryidde or Eubrya

Order :    Polytrichales

Family :   Polytrichales

Genus  : Polytrichum

Distribution and Habit:

The genus Polytrichum is represented by about 100 species which are widely distributed all
over the world. They are mainly distributed in cool temperate and tropical regions of the world.

Polytrichum mostly prefer moist and shady places. The plants grow on wet sandy banks of
rivers and ponds, on branches of trees under shade (Epiphytes) and on rocks and cliffs,
on bogs. They often form a green carpet on moist and shaded walls.

In India, the genus is represented by five species found in Himalayan regions –
P. densifolium, P. juniperinum, P. xanthopilum, P. alpinum, and P. formosum.

Gametophyte

External Morphology

Polytrichum gametophyte has two stages 1. Protonema stage 2. Adult gametophyte stage.

Protonema

Spores on germination produce creeping, green, branched and filamentous protonema.
Protonema represents the temporary junvenile phase of the gametophytic generation.
The growth of protonema is apical. Two types of filaments grow on the protonema,
upward and green chlorenematous filaments and colourless rhizonematous filaments.
The rhizonema filaments are meant for attachment. On upright chloronema filaments
buds develops. Each bud develops into an erect leafy gametophore.

Gametophore

The gametophore is a erect leafy shoot. The gametophore is differentiated into two parts :
a horizontal underground rhizome and an erect leafy shoot.

Rhizome

It is underground, horizontal growing portion of the gametophore. The rhizome bears leaves
and fluffy rhizoids.

The leaves are small, scale-like, usually brown or colourless. They are arranged in three
vertical rows (1/3 phyllotaxy).

The rhizoids arise from the base of the rhizome. Rhizoids are long, thread like, multicellular,
branched with oblique septa. Rhizoids are inter woven to form a wick like structure.
Rhizoids absorb water not only through their surface but also hold water externally
by capillary action as in the wick of a lamp. Because of this character Polytrichum can
withstand relatively dry habitats.

Buds and gemmae present on the rhizoids help in vegetative propogation.

Ariel Leafy Shoot

The leafy shoot is an erect axis arising from the horizontal rhizome. It is the most conspicuous
part of the plant. It grows to a height of about 15-20cm.  In P. commune, the leafy shoot grows
upto 45 cm in height

Each leafy shoot consists of a central axis and many lateral expansions called leaves. Typically,
the erect leafy axis is unbranched. The central axis bears two kinds of leaves, the scale leaves
and foliage leaves. The scales leaves are produced on the lower portion of the central axis. The
foliage leaves are large and are arranged spirally on the central axis with a divergence of 3/8.
Each leaf consists of two parts, a broad colourless, membranous sheathing leaf base and a narrow,
brown or green lanceolate limb.

The limb consists of a broad dark green midrib and rudimentary wings. The wings on both sides
of the midrib are thin. The leaf margin may be entire or toothed.

The branches arise from the primordial present below the leaves. The shoot grows by means of an
apical cell with three cutting faces.

Internal structure

Rhizome

The rhizome is roughly triangular in outline with rounded corners. The transverse section shows
three regions an eipidermis, cortex and central cylinder.

Epidermis

It is outermost layer of the rhizome. It is single layered. The epidermal cells are thick-walled due
to deposition of suberin. Several epidermal cells give rise to rhizoids. Stomata are absent.

Cortex

The epidermis is followed by cortex consisting of 3-4 layers of thin walled cells. The cortex is
interrupted by three hypodermal strands, which extend radially from the periphery towards the
centre. They are composed of living prosenchymatous cells, which contain starch grains.

The hypodermal strands gradually narrow down towards the centre of the rhizome, and are
connected inward with thin walled cells. These thin walled cells together with hypodermal
strands are called radial strands.

Inner to the cortex, there is a layer of radially arranged large cells. The radial and horizontal
walls of these cells have suberized thickening. This layer can be compared with the endodermis
of higher plants. This layer is not continuous but separated by radial strands.
Central Cylinder:
It is the central, compact, 3-lobe mass of tissue forming the core of the rhizome.
Pericycle: In some species, a 2-3 layers of thin walled pericycle is present between the
endoermis and the central cylinder. It is not continuous and is absent in the region of furrows
where the centre of the bay is occupied by the lepoids.
Leptoids:
In the furrows of the interrupted pericycle, is present a group of polygonal proteinaceous
cells. These cells are large, elongated sieve tube-like and termed as leptoids. Collectively the
three groups of leptoids, which more or less resemble the sieve cells of the vascular plants,
constitute the leptom.
These are living cells with oblique end walls and are connected with each other through
plasmodesmata. The leptom is the food conducting system.
Amylom:
The inner most layer of leptoids is seperated from the central cylinder by a single layer
of parenchymatous cells containing starch. This layer is called amylom. The amylom
seperates the leptom from the central trilobed hydrom.

The central cylinder consists of two kinds of elements, the stereids (or sclereids) and hydroids.
The central mass mainly consists of the stereids. These arre thick-walled, elongated cells with
oblique end walls. Collectively the stereids constitute the stereom. It functions as the supporting
tissue.
Interspersed among the stereids are the empty elements in groups of 2 or 3. These are termed as
hydroids. Collectively the hydroids constitute the hydrom. They function as the water conducting
tissue and is equalent to the xylem of the vascular plants.
The stereom and the hydrom together constitute the hydrom cylinder.


T.S of Aerial Stem:
The cross section of the erect stem has an irregular outline due to attachment of leaves.
It is differentiated into epidermis, cortex and central cylinder.
Epidermis:
It is single layered, usually not well defined.
Cortex:
It is differentiated into outer thick walled cortex and inner thin walled cortex. the leaf traces
pass through the cortex and join the central cylinder interrupting the pericycle.
The inner cortex is followed by endodermis. Inner to endodermis is a zone composed of
thin-wlled, sieve tube like cells. It is known as leptom mantle. It performs the function
of phloem.
Inner to leptom mantle is a narrow zone consisting of a single layer, sometimes of two
layers, of cells containing starch. These cells have dark-brown suberized walls. This
zone is called the hydrom sheath or amylom sheath.
Central Cylinder
In the centre of the stem is a compact mass of thick walled cells constituting the
hydrom cylinder. It consists of two kinds of cells, the stereids and hydroids.
Stereids are thick-walled supporting cells constituting the major part of the hydrom
cylinder. Interspersed among the stereids are the thin-walled elongated empty cells
in groups of 2 or 3. These are the hydroids concerned with water conduction.
Surrounding the hydrom cylinder is a zone consisting of two or three layers
of thin-walled cells devoid of contents (empty)

Anatomy of Leaf
The cross section of the leaf shows a broad midrib flanked by a narrow wing or lamina.
The midrib makes up most of the width of the leaf. it is multistratose (several cells thick)
in the centre. Gradually, it thins towards the margins and finally merges into the wing
 or lamina on either sides.
The lower epidermis is single layer consisting of regularly arranged large cells with
their outer walls thickened.
Within the lower epidermis, the midrib shows one or two layers of sclereids cells with
extremely thickened walls and narrow lumen.
There is narrow and interrupted band of similar cells on the upper (adaxial) side.
Between these two bands of sclereids, the midrib consists of thin-walled parenchyma cells.
The midrib lacks the upper epidermis. It is replaced by a layer of large, thin-walled cells.
These cells bear the rows of green or dark green cells. These cells are 4-7 cells high,
standing vertically parallel to each other. These cells are called the lamallae and contains
chloroplast.
The lamallae besides being photosynthetic also hold water due to capillary force. The
terminal cells of the lamallae are colourless and enlarged. In P. commune it is bifid and
in P.junniperinum it is papillose.
The adjoining terminal cells almost touch each other, thus providing a functional equivalent
 of the upper epidermis. It is therefore, termed as pseudo-epidermis.

Reproduction:
Vegetative Reproduction:
The gametophore of Polytrichum is rhizomatous. Death or decay of the intervening rhizome results
in the establishment of seperate free living plants.
Sometimes, vegetative buds or bulbils developed on the rhiziods also helps in vegetative propagation.

Sexual Reproduction:
All species of Polytrichum are dioecious. The antheridia and archegonia are borne on different
gametophores. The sex organs are borne in groups at the tip of the main axis of the gametophore
which is unbranched.

Antheridial Head:.
Antheridia are borne in groups at the apex of the male plant. The leaves surrounding the antheridia are called the perigonial leaves. They differ from the vegetative leaves in form and colour. The perigonial leaf is comparatively shorter, red-brown or dull-red in colour. The leaves consists of a
broad expanded leaf base terminating in a short bristle point.
The perigonial leaves lie close to each other forming a rosette around the antherida  superficially resembling a miniature flower (moss flower). The apical of the male gametophore is not used in the
formation of antheridia. So, the axis may grow out through the antheridial head and produce antherida year after year. Such growth pattern is called as proliferation.
Intermingled with the antheridia in the antheridial head are multicellular hair-like structures called
the paraphyses. They are simple, filamentous structures consisting of a single row of uniform cells.
Some may have broad spathulate tips.
Antheridium
The mature antheridium consists of an elongated, club-shaped body and a short multicellular stalk.
The body of antheridium has a single layered jacket surrounding a central mass of primary androgonial cells.
Each primary androgonial cell divides repeatedly to form several androgonial cells. The last generation of the androgonial cells are called androcyte mother cells. Finally the androcyte mother cells divide producing androcyte. Each andorcyte metamorphoses into antherozoid.
An operculum is present at the tip of the antheridium, consisting of  4-5 distal tiers of small cells with markedly thickened walls.
The antheridium ruptures at the distal end due to absorption of water.The biflagellate antherozoids comes out. Each antherozoid has a coild body witha cytoplasmic vesicle attached at its posteriod end.

Archegonial Head:
The archegonia occur in a cluster at the tip of female gametophore. The leaves surrounding the archegonia are called the pericheatial leaves. The pericheatial leaves overlap at the top of the archegonial cluster to form a bud-like structure called the pericheatum.
Intermingled with the anrchegonia are the paraphyses. Each paryaphysis is a filamentous structure consisting of a row of uniform cells.
The apical cell of the female gametophore itself functions as the archegonial initial. Consequently further growth of the female gametophore is not there.
Structure of Archegonia
The mature archegonium is flask shaped structure consisting of a very long neck, venter and massive stalk.
The wall of venter is two cell thick. The venter cavity contains a single spherical egg and a ventral canal cell just above it. The long, narrow neck consists of six vertical rows of neck cells enclosing neck canal cells (about 13 or more).
The venter canal cell and neck canal cell in the mature archegonium degenerate forming a mucilaginous mass .It absorbs water and swells forming a passage way leading to the egg in the venter.

Fertilisation:
The perigonial bracts surrounding the terminal antheridial cluster lie close together and overlap forming a shallow cup-like structure. It serves as a splash cup. Falling rain drops from a height would result in splashes coated with sperms travelling considerable distance.
Splashing drops containing brings sperms to the archegonial cluster. The sperms are released into archegonia. A single antherozoid fuses with egg to accomplish fertilisation. The fertilised egg secretes a wall around it to become an oospore.

Sporophyte
The diploid zygote is the first cell of the sporophyte.
Located in the venter cavity, it increases in size and divides by a transverse wall into an upper epibasal cell and a lower hypobasal cell. Each cell divides further and forms an apical cell with two cutting faces.
The apical cell formed in the hypobasal cell gives rise to foot, while the apical cell of the epibasal cell gives rise to seta and capsule.
The cells destined to form capsule divide periclinally and form an outer amphithecium and an inner endothecium. The amphithecium gives rise to the jacket of the capsule, whereas the endothecium gives rise to columella and the archesporium.
Along with the development of sporophyte, the wall venter divides forming 4-6 cells covering called the calyptra. It completely encloses the developing embryo. The calyptra is ruptured by the continuous growth of the sporophyte. The upper part of the calyptra is carried upward on the top of the capsule as dry hairy hood. Due to this hairy calyptra covering the mature capsule Polytrichum is commonly known as Hairy Cap Moss.

Structure of the Mature Sporophyte
The mature sporophyte or sporogonium of Polytrichum is differentiated into foot, seta and capsule.
Foot:
It is dagger-shaped structure buried deep in the tip of the female gametophore. It is composed of thin-walled parenchymatous cells. It functions as an anchoring and absorbing organ.
Seta:
The seta is a long, slender, stalk-like structure present between the foot and the capsule. Structurally it consists of an outer layer of thick walled cells constituting the epidermis. Epidermis is followed by a sclerenchymatous hypodermis. Next to hypodermis is the cortex consisting of green, thin-walled parenchymatous cells with intercellular spaces. Forming the core of the seta is a well developed central strand.
The main function of the seta is support and conduction of the water and mineral salts, raise the capsule to the required height.

Apophysis:
At the base of capsule, the seta is considerably swollen to form a bulbous, sterile structure called thee apophysis. It is demarcated from the Capsule  by a disntict groove. Structurally it consists of a single layer of epidermis continuous with the epidermis of seta below. It has stomata. Next to epidermis is the chlorenchyma consisting of a mass of loosely arranged thin-walled parenchymatous cells containing chloroplasts.
The hypodermis is absent in the apophysis region. In the centre is the conducting strand continuous with the columella above.

Capsule:
The capsule is long, erect and angular in shape. It is differentiated into two regions, (i) the lower, large spore bearing portion forming the body of the capsule called the theca and (ii) a sterile, conical portion forming the lid or operculum.

Theca:
The fertile part of the capsule extending from the apophysis to operculum is called as theca.
The theca consists of several cell layers called the capsule wall. The outermost layer of the wall is epidermis with compactly arranged cells. The epidermis of the capsule region lacks stomata.
Next to epidermis is the chlorophyll tissue typically 2-celled in thickness. It consists of thin-walled parenchymatous cells containing chloroplasts.
Inner to the chlorenchymatous tissue is the outer air space or lacunae. It is traversed by short filaments of green cells called the trabeculae. The trabeculae connect the theca wall layer with the
outer wall of the spore sac.
The outer lacunae is followed by spore sac. The spore sac contains the fertile archesporial tissue. The archesporium originates as a single layer from the outer layer of the endothecium. Later it divides to form 4-6 layers of sporogenous tissue. All cells of the sporogenous tissue are fertile. They divide and form spore mother cells. The spore mother cells undergoes meiosis to form 4 haploid spores or meiospores.
Internal to the spore sac is inner air space or lacunae. It is also tranversed by trabeculae which connect the two cell layer thick inner wall of the spore sac on the outer side  to the central columella on the inner side. Thus, the spore sac is bounded by air space or lacunae  both at its outer and inner face.
The central part of the capsule is occupied by a solid core of sterile tissue forming the Columella. It is made up of parenchymatous cells and is continuous with the central strand of seta below and extends upto the epiphragm of the operculum above.

Operculum or Lid:
It is the terminal, conical portion of the capsule, which forms a cap or lid like structure at the apex of the theca.
The operculum is delimited from the theca region by constriction made of 2-3 layers of  elongated cells forming the rim or diaphragm. The annulus is absent.
Closing the mouth of the theca is a thin, shield-shaped pale membranous structure called the epiphragm. It stretches like the tymphanum of a drum across the mouth of the theca.
Arising from the periphery of the diaphragm is a ring of 64 short, stout pyramidal, teeth like solid structures called the peristome. Each peristome tooth is composed of curved crescent-shaped, fibre-like cells. At their tips, the teeth are joined to the margin of the epiphragm. They do not exhibit hygroscopic movements.

Dispersal of Spores
In the mature capsule, the central, non-sporogenous tissue degenerates. The spores come to lie in the hollow capsule. The calyptra falls off. The exposed mature capsule begins to dry. Minute holes appear between the successive peristome teeth in the margin of the epiphragm by the drying up of cellls between them.
The minute spores are dispersed, by censer mechanism as the capsule sways in the wind.
Spore:
The small yellow spores have a smooth surface. The spore wall is differentiated into outer exospore and the inner endospore. The reserve food is in the form of globules in the spore cytoplasm.

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