Polytrichum: Distribution, Structure, Reproduction

Polytrichum is one of the largest mosses and a genus of the order Polytrichales. It is found in all parts of the world.

Polytrichum is commonly called hairy cap moss due to the shaggy calyptra enclosing the mature capsule.

Distribution of Polytrichum

Polytrichum is a prominent genus in the family Polytrichaceae. It is represented by about 111 species and is distributed all over the world, mainly in cool temperate and tropical regions.

Polytrichum mostly prefers to grow in moist and shady places during the rainy season. They are found on wet sandy banks of rivers and ponds, on branches of trees under shade (as epiphytes), on rocks and cliffs, and on dry forest bogs.

Polytrichum juniperinum occurs in extremely dry sites, while P. strictum grows in peat lands. P. sexangulare is found in late-snow areas.

The species of Polytrichum often form a green carpet of vegetation on moist and shaded walls.

  • Some of the common Polytrichum species are P. densifolium, P. alpinum, P. juniperinum, P. xanthopilum, P. strictum, P. formosum, P. sexangulare, etc.

Gametophyte of Polytrichum

The plant body of Polytrichum is gametophytic and consists of an erect leafy shoot.

Diagram of Polytrichum
Figure: Plant body of Polytrichum bearing sporophyte

External Morphology of Polytrichum

External morphology is the external structure of Polytrichum.

The plant body is gametophytic. Like Funaria, the gametophyte of Ploytrichum also shows two distinct stages of development: protonema and gametophore.

The protonema represents the juvenile stage, while the gametophore is the leafy stage of the adult gametophyte.

Protonema

The protonema is a haploid branched, green filamentous, algal-like structure. It is produced by the germination of the spore.

Later, the protonema metamorphoses into the leafy gametophore.

Gametophore

The gametophore i.e., adult gametophyte in Polytrichum is rhizomatous and is differentiated into two parts: the rhizome and the erect leafy shoot.

Rhizome

The rhizome is the horizontally growing underground portion of the gametophore. It bears scale leaves and numerous fluffy rhizoids.

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

The rhizoids are long, branched, multicellular, thick-walled and characterised by the presence of oblique septa. They are interwoven to form a dense tangled mass.

Rhizoids are colourless and arise from the base of the rhizome.

Rhizoids attach the leafy shoot to the substratum and also help in absorbing water and mineral nutrients from the soil.

Leafy Shoot

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

Externally, the leafy shoot is differentiated into a stem-like central axis. The central axis bears two types of leaves: scale leaves and foliage leaves.

The scale leaves are small and produced on the lower portion or transition zone of the central axis. The foliage leaves are comparatively large, dark green, and are arranged spirally on the upper portion of the central axis with a divergence of 3/8.

Each foliage leaf is 6–10 mm long. It consists of two parts; the proximal sheathing leaf base and the diverging narrow limb or blade.

The sheathing leaf base is well-developed, broad, colourless, membranous, and unistratose (one cell thick). It is traversed by a relatively narrow midrib.

The limb is green or brown, lanceolate in outline and has an accuminate apex. It consists of a broad dark green multistratose midrib (nurse of costa) and rudimentary wings. The wings on both sides of the midrib are thin, narrow, and unistratose. The margin of the leaf may be entire (e.g., P. juniperinum) or coarsely toothed (e.g., P. commune).

The erect leafy axis is usually unbranched. Sometimes it is branched and the branch arises from a primordium below the young leaf. The branch primordia occur at intervals both on the rhizome and the erect axis, roughly at the base of every 12th leaf.

The stem grows by means of an apical cell with three cutting faces. It cuts off segments in such a way that the position of the apical cell is shifted in a spiral manner.

Internal Morphology of Polytrichum

Internal morphology, or the internal structure of the plant body, is the anatomy of Polytrichum.

Internal Structure of Rhizome

In a cross-section, the rhizome is circular in outline in some Polytrichum species (e.g., P. formusum, P. commune) and triangular with rounded corners in many others. It can be differentiated into the following regions:

Epidermis

The epidermis is the outermost layer of the rhizome. It consists of a single layer of thick-walled cells. The rhizoids, which are the direct outgrowth of the epidermal cells, form a dense tangle.

In the epidermis, stomata are absent.

Structure of rhizome in Polytrichum
Figure: T.S. of rhizome showing the detailed structure

Cortex

The cortex lies between the epidermis and the central strand. It consists of 3–4 layers of thin-walled parenchymatous cells.

The cortex is interrupted by three hypodermal strands, which extend radially from the periphery towards the centre. Each hypodermal strand is composed of living prosenchymatous cells with a pointed end. Starch grains are present in these prosenchymatous cells.

The hypodermal strands gradually narrow down towards the centre of the rhizome and are connected inward by a group of thin-walled cells. These thin-walled cells, together with hypodermal strands, form a wedge-shaped mass of cells, called radial strands.

Inner to the cortex, there is a layer of large radially elongated cells with suberized thickening on the radial and horizontal walls. This layer can be compared with the endodermis of higher plants. The so-called endodermis is not continuous but is separated by larger cells of radial strands.

Pericycle

In some species of Polytrichum, the pericycle is present between the endodermis and the central cylinder. It is composed of 2-3 layers of thin-walled cells.

The pericycle 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 centre of the bay, a group of polygonal proteinaceous cells is present in each furrow of the central cylinder. These cells are large, elongated, sieve tube-like and are termed the leptoids. The end walls of the leptoids are oblique and, in some cases, the walls are connected to each other through the plasmodesmata.

The leptoids, which more or less resemble the sieve cells of vascular plants, collectively form the leptom. The leptom is the food (photoassimilate) conducting system.

Amylom

On the sideway from the radial strand, each group of leptoids is surrounded by a single layer of parenchymatous cells containing starch. This layer is called the amylom or hydrom sheath.

The amylom thus separates the leptom from the central 3-lobed hydrom.

Central Cylinder

The central cylinder is the central, compact, 3-lobed mass of tissue forming the core of the rhizome. It is made up of two kinds of cells: stereids (or sclereids) and hydroids.

The central mass consists mainly of the stereids. These are thick-walled, elongated, living cells with oblique end walls. The stereids are collectively called the stereom.

The stereom functions as the supporting tissue. In addition, it provides an alternative route for water conduction (Zamski and Trachtenberg, 1979).

Scattered among the stereids are the larger empty cells with very oblique cellulose walls. These are called hydroids.

The hydroids collectively constitute the hydrom. The hydrom is considered to function as the water-conducting tissue.

The stereom and the hydrom together form the hydrom cylinder.

Anatomy of Aerial Stem

A cross-section of the erect aerial stem is more or less circular in outline. Sometimes it is irregular due to the attachment of leaves.

Internally, the erect stem is differentiated into the outermost superficial layer (epidermis), followed by the wide cortex, and the central cylinder.

Epidermis

The outermost superficial layer does not form a clearly defined epidermis. It is single-layered.

Cortex

Within the epidermis is the wide cortex. It is differentiated into the outer and inner cortex.

The outer cortex is thick-walled and composed of compact, elongated prosenchymatous cells that gradually merge into the inner cortex. The inner cortex is made up of loosely arranged, thin-walled parenchymatous cells.

Leaf traces are also seen in the cortical region. Each leaf trace consists of a central patch of colourless, thin-walled, water-conducting hydroid cells (Eschrich and Steiner, 1968). It extends across the cortex and joins the central cylinder.

Internal structure of stem in Polytrichum
Figure: T.S. of aerial stem in Polytrichum

Following the cortex is the endodermis (a layer between the cortex and the leptom, Kawai and Ikeda, 1970). Since the functional aspect of this tissue has not yet been examined, Tranhtenberg and Zamski (1978) suggested that it would be more appropriate to term this layer as the central strand sheath instead of the endodermis. As the cells of this layer bordering the central cylinder contain starch, the endodermis has also been termed the starchy sheath.

Within the endodermis is the rudimentary pericycle, which is not clearly differentiated.

Inner to pericycle is a broad irregular zone composed of thin-walled, sieve tube-like cells. This zone is known as the leptom mantle. It is regarded as equivalent to the phloem of vascular plants.

Internal to the leptom mantle is the hydrom sheath or amylom layer. It is a narrow zone consisting of a single layer, sometimes two layers, of cells containing prominent starch. These cells have dark-brown suberized walls.

Central Cylinder

At the centre of the erect stem is a compact mass of thick-walled cells forming the hydrom cylinder. The hydrom cylinder is composed of two kinds of cells: stereids and hydroids.

The stereids are thick-walled supporting cells constituting the major part of the hydrom cylinder. Interspersed among the stereids are the thin-walled larger empty cells in groups of 2 or 3. These are the hydroids. The hydroids are considered to function as water conduction.

Surrounding the hydrom cylinder is a zone consisting of two or three layers of thin-walled empty (without content) cells. This layer is called the hydrom mantle.

Anatomy of Leaf

Structurally, Polytrichum leaf is the most complex of all the mosses.

In a cross section, the leaf shows a broad midrib flanked by a narrow wing or lamina. The lamina is unistratose (one cell thick) and composed of hyaline cells.

The wide midrib makes up most of the width of the leaf. It is several cells thick (multistratose) in the centre. Gradually, the midrib merges into the wings.

Internal structure of leaf showing its different parts
Figure: T.S. of leaf

The lower epidermis (epidermis on the abaxial surface) is well developed and consists of a single layer of regularly arranged large cells. These cells have strongly thickened outer walls. Within the lower epidermis, the midrib generally shows one to two, rarely more, layers of a thick band of small sclereid cells with extremely thickened walls and narrow lumina. On the upper side, a narrow and interrupted band of similar slereid cells is present.

Between these two bands of sclereids, the midrib consists of thin-walled parenchyma cells differentiated into a band of large thin-walled parenchyma cells called the deuters and the much narrow empty central parenchyma cells, the hydroids (Hebant, 1971). The deuters lie just beneath the upper band of sclereids. The hydroids help in conducting water, and the deuters are involved in the translocation of assimilates out of the leaf.

On the upper epidermis, there is a layer of large, thin-walled cells from which arise many parallel plates called lamellae. In a cross section, the lamellae appear as one-cell thick (uniseriate) rows of green or dark green cells, standing vertically parallel to each other. Each lamella is made up of 4–7 cells containing chloroplasts.

The leaf lamallae, besides functioning as photosynthetic tissue, also hold water due to capillary force. According to Smith (1971), the lamellae function as pseudomesophyll.

The terminal or distal cells of the lamellae, often referred to as the marginal cells, are slightly larger and colourless. They are variously shaped and are ornamented. In P. juniperinum, the terminal cells are much wider or papillose, but they are bifid in P. commune.

The terminal cells of the adjacent lamellae almost touch each other, thus providing a functional equivalent of the upper epidermis. It is, therefore, termed the upper pseudo-epidermis.

Reproduction in Polytrichum

Polytrichum reproduces by both vegetative and sexual methods.

Vegetative Reproduction

Vegetative reproduction in Polytrichum takes place by the following methods:

Bulbils or Vegetative Buds

Sometimes, vegetative buds or bulbils develop on the rhiziods. Under favourable conditions, bulbils may grow into new plants.

Primary Protonema

The spore, on germination, develops a branched, filamentous structure called the primary protonema. It develops multicellular buds which grow into leafy gametophores.

Secondary Protonema

The protonema that develops from any part of the gametophyte is called the secondary protonema. Each secondary protonema forms buds which grow into gametophores.

Fragmentation of Rhizome

The gametophore in Polytrichum is rhizomatous. The horizontally growing rhizome gives rise to erect, leafy stems at intervals.

Death or breakage of the intervening rhizome results in the development of separate, independent Polytrichum plants.

Apospory

Development of the gametophyte from the sporophyte without the formation of spores is called apospory.

The diploid gametophytic protonema may develop from the unspecialised cells of the various parts of the sporophyte.

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