Sphagnum: Introduction, Structure and Affinities

In this article we will discuss about:- 1. Introduction to Sphagnum 2. Structure of Sphagnum 3. Reproduction 4. Affinities.

Introduction to Sphagnum:

Sphagnum is popularly known as bog moss, peat moss or turf moss because of its ecological importance in the development of peat or bog. The plants are perennial and grow in swamps and moist habitat like rocky slopes where water accumulates or where water drips.

They grow along the bank of lakes and gradu­ally encroach more and more of the water as creeping bogs and in course of time they com­pletely cover up the lake transforming it into a bog. Hence Sphagnum is known as bog moss.

Subsequently, angiosperms encroach the bog. As a result the topography of the bog gets changed and water of bogs becomes very acidic. In this acidic soil the upper portion of the Sphagnum gametophores grows indefinitely, while the basal part dies progressively. The dead plant parts do not decompose easily in acidic soil.

In other way, the acidic medium helps to inhibit the growth of fungi, bacteria and other pathogenic microorganisms, thus slows down the decaying process of the dead materials.

Consequently a large mass of dead remains accumulated year after year followed by compression from plants on top, thus a com­pact, dark coloured substance rich in carbon is formed which is known as peat. Since Sphagnum is the chief constituent of peat, it is often called peat moss.

Structure of Sphagnum:

A. External Features:

The gametophyte phase of Sphagnum is represented by two distinct stages namely, (a) juvenile protonema, and (b) mature leafy or gametophore stage. The mature plants grow in dense clumps and their shoots are of whitish or brownish green in colour.

All species of Sphagnum accumulates water and often grow with bright colour (deep red, rose pink, etc.) due to the presence of water-soluble pigments, anthocyanin. They are perennial showing unlimited growth by means of an apical cell with three cutting faces.

Very young gameto­phytes bear multicellular rhizoids with oblique septa. Mature gametophytes, how­ever, do not bear rhizoids. It is differentiated into an upright branched axis and leaves (Fig. 6.38A-B).

Main Axis and Branches:

The main axis is soft and weak at young stage, but becomes erect and stout at maturity. However, the main axis is much longer in aquatic species, but is relatively short in terrestrial form due to the progressive death of the older basal part.

The axis branches profusely on the lateral sides. Single branch or in tufts of 3 to 8 branches arise from the axils of every fourth leaf of the main axis (Fig. 6.38C). At the apex of the main stem, many small branches of limited growth are densely crowded forming a compact head called coma (Fig. 6.38A & 6.39A).

The coma is formed near the apex due to the condensed growth of apical internodes. As the stem grows in length these short branches elongate and become normal branches.

The submerged species (S. obesum, S. cuspidatum) have all the branches similar in form and structure, but the terrestrial species pro­duce two types of branches viz., (i) pendent branches, and (ii) upwardly divergent branches (Fig. 6.38C).

Pendent Branches:

These are long slender loosely arranged, turn downwards and then grow parallel to the main axis. They are also termed flagelliform or de-current branches.

Divergent Branches:

These are short and stout branches which grow outwards and upwards. They are also termed ex-current bran­ches. Sometimes, one divergent branch in each node develops strongly than others and ultimate­ly gives rise to a new plant when it becomes detached from the mother plant.

Leaves:

The leaves occur both on the main axis as well as on the branches (Fig. 6.38C). On the branches, the leaves are closely set and, there­fore, overlapping and are placed apart on the main axis. The leaves are arranged in spiral phyllotaxy.

Moreover, the leaves on the main axis differ from those on the branches in size, shape and details of cell structure. In general, the leaves are small, sessile, entire, thin and scale­-like with acute apex and without a midrib (Fig. 6.41 A).

B. Internal Structure:

Stem:

Internally, the stem shows distinct differentiation of tissues into three zones viz., outer cortex or the hyalodermis, the middle hadrom (prosenchymatous region) and the central cylinder or medulla (Fig. 6.40A).

(a) Outer Cortex:

The cortex or the hyalo­dermis is the outermost region of the stem (Fig. 6.40A-C). This is bounded externally by a single- layered epidermis. It is composed of large hyaline cells. The genus, Sphagnum has often been divi­ded into two sub-genera based on the nature of hyaline cells.

In the sub-genus Sphagnum or Inophloea, cortical hyaline cells are fibrose and porose, while they are without pores or spiral thickening in the sub-genus Lithoploea (Fig. 6.40B, C). The cortex remains 2- to 4-layered in the main axis (Fig. 6.40A), but it is single layered in lateral branches (Fig. 6.40B). The mature corti­cal cell is devoid of protoplasm.

In some species (S. tenellum, S. molluscum), some outer cortical cells enlarge peculiarly and become bottle or retort-shaped. (Fig. 6.40D, E). The neck of each cell is turned outward away from the axis and has a pore at the distal end. These are called retort cells. They accumulate water and inhabited by small microscopic animals.

(b) Middle Hadrom:

It lies next to the cortex and consists of 4-6 layers of small thick-walled, prosenchymatous cells (Fig. 6.40A). This part is called hadrom which gives mechanical support to the stem.

(c) Central Cylinder or Medulla:

It is the innermost region of the stem (Fig. 6.40A), com­prised of small, vertically elongated, thin-walled parenchymatous cells. It functions as storage region.

Leaf:

In Sphagnum, the cross-section of leaf shows only one cell in thickness and composed of much elongated cells. A young leaf is com­prised of square or rectangular cells of uniform size, while a mature leaf is characterised by two types of cells, the ordinary type hyaline cells and the green chlorophyllose cells or the assimila­tory cells (Fig. 6.41 B).

The hyaline cells are large polygonal and become colourless or hyaline by loosing their protoplasts. Their walls are provi­ded with pores and become spirally thickened (Fig. 6.41 C, D). The hyaline cells have a remar­kable capacity of absorption and retention of water (hence called capillary cells), thus rhizoids are not necessary in the mature plants.

The chlorophyllose cells are small triangular or biconvex living cells with many discoid chloroplasts and have their photosynthetic ability (Fig. 6.41 C,D). The chlorophyllose and the hyaline cells are arranged in an alternate sequence to form a regular reticulate pattern and this leaf- feature alone can be used to identify the genus, Sphagnum.

Reproduction in Sphagnum:

In Sphagnum, reproduction takes place both by vegetative and sexual methods, however, the vegetative propagation is more common:

I. Vegetative Reproduction:

Vegetatively, it reproduces by means of innovation. Sometime, one of the divergent branches grows upwards and becomes as strong as the main stem (Fig. 6.69). Such an apical branch is called innovation.

Due to the progressive death of the lower basal part of the main axis, the inno­vation gets detached from the mother plant and ultimately gives rise to a new plant. This pheno­menon is responsible for the extensive growth of Sphagnum in nature.

II. Sexual Reproduction:

Sphagnum may be monoecious or dioecious, but the antheridia and archegonia are always borne on the special separate antheridial and archegonial branches of the same plant. These branches are much sma­ller than the vegetative branches (Fig. 6.38B & 6.39B). In monoecious plants, the antheridial branches develop first.

(a) Antheridial Branch:

The antheridial bran­ches (Fig. 6.38B) first appear near the apex of the main shoot but eventually carried downwards due to the growth of the apical region. These branches are usually shorter but stouter than the vegetative branches. They are spindle-shaped and densely covered with yellow, red or dark green leaves gene­rally smaller than the foliage leaves (Fig. 6.42A).

Antheridium:

Development and Structure of Antheridium:

The antheridia develop singly and acropetally below the leaves (Fig. 6.42 B^! Each antheridium develops from a superficial intheridial initial of the stem. The antheridial initial develops a small filamentous structure. The terminal cell of this filament grows by two cutting faces to form an apical cell (Fig. 6.42C).

The latter is further differentiated into a 12-15 celled structure, of which 2-5 distal cells by periclinal division form the body of the antheridium and the rest cells form the stalk. Each distal cell gives rise to an outer jacket initial and an inner primary androgonial cell. The primary androgonial cell, by further divi­sions in all possible planes, forms the antheri­dium. A single-layered jacket is formed from jacket initials.

The Mature Antheridium:

(Fig. 6.42 D). It has a long stalk of two to four rows of cells and a globose or ovoid body. The body has a jacket of one layer of cells enclosing a mass of andro­cytes formed from the sperm mother cells. Each androcyte cell metamorphoses into a spirally coiled biflagellate antherozoid or sperm (Fig. 6.42 E).

Dehiscence of the Antheridium:

The apical cells of the jacket of a mature antheridium swell through the absorption of water. As a result of turgor pressure thus generated, the wall of the swollen antheridium breaks into a number of irregular lobes at the apex that eventually turns backwards. The mass of androcytes comes out and the antherozoids are liberated immediately and swim freely in water.

(b) Archegonial Branches:

Archegonia are borne at the apices of the archegonial branches which develop at the apex, or laterally. The archegonial branches are very short and more or less ovoid in shape (Fig. 6.38B and 6.39B). The leaves on these branches are larger than those present on the foliage leaves. The upper leaves of these branches constitute the perichaetium enclosing the archegonia and thus protect arche­gonia from injury.

Archegonium:

Development and Structure of Archegonium:

The arche­gonia develop on the apex of the archegonial branches either singly or in groups (Fig. 6.38B & 6.39B). The apical cell of this branch forms the primary archegonium. Two to five secon­dary archegonia develop from derivatives of the apical cell.

Usually, there are three archegonia in a group i.e., one primary archegonium at the apex and two secondary archegonia emerge from the base of primary archegonium (Fig. 6.43C). The development of both the primary and secondary archegonia are similar. The archegonial initial divides transversely to form a four- to six-celled filament.

Then the terminal cell, by three intersecting vertical walls, cuts off three periclinal jacket initial cells and an pri­mary axial cell (Fig. 6.43A).The primary axial cell divides transversely to form an upper cover initial and lower central cell. The central cell divides transversely to form an upper primary neck canal cell and lower primary ventral cell.

The primary neck canal cell, by repeated trans­verse divisions, forms a row of 8-10 neck canal cells, while the primary venter cell, by a single transverse division, forms a ventral canal cell and an egg. The cover initial divides vertically to form a group of eight or more cover cells that form the upper portion of the archegonial jacket.

The jacket initial, by anticlinal and peri­clinal divisions, subsequently forms the neck and the middle and basal portion of the archegonial jacket. The cover cells form the upper portion of the archegonial jacket.

Mature Archegonium:

The mature archego­nium is a relatively large structure. It has a long stalk, a long twisted neck with 8 to 9 neck canal cells, a massive multilayered venter containing a ventral canal cell, and an egg (Fig. 6.43B).

Fertilisation of Archegonium:

The process of fertilisation takes place only in the presence of water. The antherozoids swim freely in water and reach the archegonia. At maturity, the neck canal cells and the ventral canal cell disorganise and form a passage for the antherozoids.

The antherozoids reach near the archegonia attracted chemotactically and pass into the passage to reach the egg. Ultimately, only one antherozoid fuses with the egg and forms a zygote.

The Sporophyte:

Development of the Sporophyte:

The diploid zygote is the first cell of the sporophytic generation. Among the few archegonia only one is developed to form embryo in an archegonial branch.

The zygote enlarges, invests itself with a cell wall and then divides transversely to form an upper epibasal cell and a lower hypobasal cell. Transverse divisions in both the cells continue until a 6- or 7-celled filament is formed (Fig. 6.44 A). The lower half of the filament undergoes irregular divisions forming a parenchymatous bulbous foot (Fig. 6.44B). The foot acts as a haustorium.

The upper cells of the filament divide by two vertical divisions at right angle to each other — a quadrant is formed. The cells of the quad­rant divide periclinally to form an inner endothecium and an outer amphithecium (Fig. 6.44 B). The cells of the endothecium repeatedly divide and form a central sterile part, columella.

The amphithecium divides peri­clinally to differentiate an inner 2-4 layered archesporium and the outer 3-7 layered capsule wall. The archesporium forms a dome-shaped arch over the columella. The cells of the arches­porium later develop into 2-4 layered sporoge­nous tissue.

All sporogenous cells function as spore mother cells that divide meiotically and form haploid spores. The spores are enclosed within a spore sac developed from the surroun­ding sterile tissue.

There is only a short neck like inconspicu­ous seta connecting the upper capsule and the lower bulbous foot.

Structure of the Mature Sporophyte:

The mature sporophyte consists of a bulbous foot, a neck-like inconspicuous seta and an almost spherical black to dark-brown capsule (Fig. 6.44C). The whole sporophyte is covered by the calyptra. The lowest part of the calyptra that covers the foot is called the vaginula. The perichaetial leaves are present below the sporophyte.

The elongated archegonial branch at the base of the sporogonium is called pseudopodium. It increases in length and pushes out the capsule above the perichaetial leaves to facilitate the spreading of spores (Fig. 6.39A, B & 6.44C).

The capsule in longitudinal section shows an outer jacket and middle spore-sac with spores which overarches the dome-shaped inner columella (Fig. 6.44C).

The capsute wall (jacket) is several layers thick. The outermost layer of the jacket is thick which bears several rudimentary non-functional stomata. The circular biconvex disc-shaped lid, called operculum, is present at the top of the jacket. The operculum is delimited from the rest of the jacket by a groove of thin-walled cells, called the annulus (Fig. 6.44C).

Dehiscence of the Capsule:

The capsule dehisces on a bright sunny day by an explosive mechanism. The capsule wall and columella become dry and shrivel due to heat. This results in the formation of a large air space below the spore-sac.

The spherical capsule gradually becomes cylindrical and, therefore, an over­pressure of 4-6 atmospheres builds up inside the capsule. Under this condition its operculum bursts open through the annulus with an audible sound. The spores are catapulted up to 20 cm and release in the air. The process is known as air-gun mechanism of spore dis­persal.

The New Gametophyte:

Like other bryophytes, the spore is the first cell of the gametophytic generation. Initially, the spores are arranged in tetrahedral tetrads. Each spore has a distinct triradiate ridge (Fig. 6.45A). The wall of the spore is differentiated into an outer smooth granular or papillate exine and an inner thin intine. Spores may germinate within 2-3 days or may remain viable for 4-6 months.

The spore on falling on a moist substratum germinates to develop a small thalloid primary protonema (Fig. 6.45B). Further development of the protonema produces a prostrate, green, irreg­ularly lobed, one-celled thick thalloid structure (Fig. 6.45C) which is attached to the substratum by multicellular rhizoids.

A single bud is devel­oped from the marginal cell of the primary pro­tonema or may give rise to secondary protonema with rhizoids and leafy buds. The bud eventual­ly develops into a new leafy gametophyte (Fig. 6.45D).

Fig. 6.46 shows the detailed diagrammatic representation of the life cycle of Sphagnum.

Affinities of Sphagnum:

Sphagnum is a unique genus of Bryophyta. It has several features similar to Marchantio­phyta, Anthocerotophyta and Bryophyta. In addition, it has some unique characteristics of its own in which it stands apart from all the three groups.

Resemblance with Marchantiophyta (Liverworts):

Among the liverworts, Sphagnum resembles more closely to the members of the class Jungermanniopsida in the following ways:

(a) The flat, disc-like protonema of Sphag­num resembles the juvenile stage of some lungermanniopsida (e.g., Metzge- riopsis pusilla).

(b) Position, structure and dehiscence mechanism of antheridium are-like that of Jungermanniopsida (e.g., Porella).

(c) Position, structure and development of archegonia are like that of lunger­manniopsida.

(d) Presence of rudimentary stomata scat­tered over the capsule wall is similar to that of Jungermanniopsida.

Resemblance with Anthocerotophyta (Hornworts):

The sporophyte of Sphagnum resembles Anthocerotophyta in these characteristics:

(a) Absence of apical growth in the young sporophyte.

(b) Development of archesporium from the amphithecium

(c) Dome-shaped amphithecium over arches the columella

(d) The entire endothecium transforms into columella.

(e) Presence of chlorophyllose cells in the capsule wall

(f) Presence of large bulbous foot and con­striction like seta.

Resemblance with Bryophyta (Mosses):

Sphagnum resembles the members of Bryophyta in the following characteristics:

(a) Presence of erect, leafy and radial gametophyte.

(b) Presence of multicellular rhizoids with oblique septa.

(c) Development of the leaf, stem and antheridium.

(d) Structure of the archegonium.

(e) Absence of elaters.

(f) Dehiscence of capsule by breaking of operculum.

Distinction of Peat Mosses (Sphagnum spp.) from Other Mosses:

As opposed to the other mosses (granite moss, true mosses, etc.), peat mosses show the following distinctive characteristics:

(a) Protonemata are low structure (thallose).

(b) Stems of adult gametophytes devoid of rhizoids.

(c) Some of their gametophyte branches are spreading, some are pendent.

(d) An intact plant is partly alive and partly dead. Its top part is alive and the bottom part is died for lack of light and is already partially decayed.

(e) The leaves are without midrib.

(f) Presence of retort cells in the cortex of side branches.

(g) The cortex of the mature stem is com­posed of dead, empty cells with large oral pores on their walls. These dead cells absorb water.

(h) Their leaves have a characteristic single- layered cell net with alternating large, dead hyaline cells and photosynthetic chlorophylfose cells. Hyaline cells absorb and store water.

(i) The elongated part of the sporophyte i.e, seta is only rudimentary, its functionas a stalk is taken over by a gametophyte-derived pesedopodium.

(j) The columella has the shape of a hemi­sphere.

(k) The capsule lacks peristome teeth.

(I) Cell walls of peat moss bind large amount of organic substance of colloi­dal nature. The cell walls function as ion exchanger which absorb calcium and magnesium ions and release hydrogen ions. Thus, they create and maintain a nu­trient poor acidic environment (pH 3-4) that fosters their own growth, but is intolerable to all but a small variety of highly-specialised other plants.