SPHAGNUM
(PEAT MOSS OR BOG MOSS)

CLASSIFICATION OF SPHAGNUM (PEAT MOSS OR BOG MOSS)​

Kingdom :-  Plantae

Division :- Bryophyta

Class :- Bryopsida

Sub-Class :- Sphagnidae

Order :- Sphagnales

Family :- Sphagnaceae

Genus :- Sphagnum

All the 11 species of Sphagnum found in India are restricted to the Himalayas. The plants are aquatic or semi-aquatic forming dense cushions commonly seen in swamps, ponds and lake margins, moorelands and wet hill sides.

EXTERNAL MORPHOLOGY OF SPHAGNUM (PEAT MOSS)

• The plants are aquatic, growing about the margins of small lakes and ponds or growing on dripping rocky banks.
• The pH of water in which Sphagnum grows ranges from 3.7 to 4.9. Since this water accumulates year after year to form peat and hence the name peat moss.
• The size of the plant varies from a few inches to a maximum of 7 inches. 
• The plant is erect, branched and differentiated into stem and the leaves. The colourless rhizoids are formed at the base but soon disappear. Hence, there are no rhizoids on mature gametophores. 
• At the apex of the gametophore there are a number of short branches densely crowded in a cluster, called coma.
• In the posterior part of the stem, the branches arise in tufts in the axil of every fourth leaf and in each tuft there are 3-8 branches.
• These branches are of two types-
  1. diverging branches which are stout, short, growing outward and upward and
  2. drooping or flagelliform branches which hang downward, around and close to main stem. These are absent from submerged forms
• At intervals, one of the branches in the tuft grows and forms an apical cluster of branches like the main stem. This is called an innovation. It helps in vegetative propagation by separating from the main branch.
• When first formed, the leaves are in three vertical rows or three ranked. Later the arrangement changes to 2/5. to. The leaves lack a midrib an exception to mosses.

ANATOMY OF AXIS OF SPHAGNUM​

• The outermost layer is cortex, made of compactly arranged cells. 
• In some species, as S. subsecundum, it remains one cell thick throughout its life but in some others, it becomes 3-5 cells thick. 
• At maturity these cortical cells lack protoplast, become hyaline and dead.
• In some species viz. S. cymbifolium, they develop spiral thickenings and develop even the pores. These cortical cells contain air and water.
• In some species as S. molluscum and S. tenellum, some of the cortical cells become greatly enlarged and become flask shaped. The cells also accumulate water and are known as retort cells. Such type of cortex is also known as hyalodermis. 
• The cells inner to cortex are prosenchymatous. These give mechanical support to the stem. The region is also known as hadrome.
• The innermost region is known as medulla. The cells are colourless. parenchymatous and somewhat vertically elongated.

LEAF : EXTERNAL FEATURES

• The mature leaf is sessile, entire, acute and onecelled thick. 
• The leaf lacks a midrib and the surface view shows two kinds of cells
  1. the narrow, chlorophyll containing assimilatory cells, 
  2. the large, dead, rhomboidal hyaline cells with spiral thickenings and pores.
• The spiral thickenings provide mechanical support and keep the hyaline cells from collapsing when they are empty.
• The pores help in rapid intake of water. S. The green assimilatory cells of the leaf are joined together and form a network with sinuous walls. 
• At the base of very young leaf there are one or more glandular hairs but these disappear as the leaf matures.

LEAF ANATOMY​

• In a cross section, the leaf appears like a beaded structure with hyaline and chlorophyllous cells alternating with each other.
• The chlorophyllous cells are triangular and the base of triangle is towards the upper side e.g. s. acutifolium. In S. tenellum the condition is just reverse i.e. the base of the triangle is towards the lowerside. In S. squarrosum the chlorophyllous cells are not triangular but appear spindle-shaped and are hemmed in, above and below, by the hyaline cells.

ANTHERIDIAL BRANCH AND ANTHERIDIUM​

• The plants are either monoecious or dioecious and both antheridial and archegonial branches occur at the apices of the gametophores. 
• The catkin-like antheridial branches are either red, brown or yellow in colour with club-shaped appearance. 
• The antheridia present in the axil of leaf are arranged acropetally. 
• The mature antheridium has a long stalk, 2 cells in breadth and a globular body. 
• The body of the antheridium consists of singlelayered jacket surrounding number of androgonial cells.
• The androgonial cells ultimately form the biciliate antherozoids.

archegonial branch and archegonium

• The archegonial branches are purple coloured. These bear large leaves known as perichaetial leaves.
• Each archegonial branch has usually 3 archegonia (sometimes 1 or 5 also). Paraphyses are absent.
• The position of the archegonium is acrogynous i.e. the first archegonium is formed by the apical cell and is known as the primary archegonium, whereas the other two archegonia are developed from the last two segments cut off by the apical cell and are known as secondary archegonia.
• The structure of both types of archegonia is similar. A mature archegonium is a stalked structure with a broad venter and a long twisted neck. 
• The venter as well as lower portion of neck are 2-4 cells in thickness. 
• The cover cells are not distinct and in the axial row are found 8-9 neck canal cells, one venter canal cell and one egg cell.

SPOROPHYTE

• It consists of a large bulbous foot and a nearly spherical, black coloured capsule. Seta is absent and occurs as a small constriction between foot and capsule.
• At maturity the apex of the archegonial branch elongates to form stalk-like portion called pseudopodium. 
• The calyptra covers the upper part of the mature capsule. 
• Longitudinal section of the sporophyte shows following characters. 
• Sporophyte consists of a bulbous foot and a spherical capsule connected by a neck-like suppressed seta. 
• The jacket of the capsule is 4-6 layered. The outermost layer is differentiated into an epidermis with rudimentary, non-functional stomata.
• All cells of the capsule wall contain chloroplasts, hence the sporophyte is not completely dependent on the gametophyte. 
• At the top of the capsule there is a convex operculum separated from the remainder of the thallus by a thin-walled transverse ring of cells called the annulus.
• Over the operculum is the calyptra which arises from the venter of the archegonium and completely covers the sporophyte.
• The central region of the capsule is occupied by the columella. It is overarched by domeshaped spore sac.
• The spore sac is filled only with spores, elaters being altogether absent. Each spore has an outer exine and inner intine. 
• A mature saprophyte is situated at the top of an elongated archegonial branch, the pseudopodium.
• The apex of the pseudopodium is enlarged which together with the basal portion of calyptra is known as veginula. 
• Veginula is a sac-like structure into which the foot of the sporophyte is embedded. 
• When the spores mature, the operculum breaks away at the place of annulus thus releasing the spores.

PROTONEMA

• The spore germinates to form a thalloid protonema.
• At the posterior end of the thalloid protonema are borne the rhizoids which are multicellular with oblique septa. 
• Secondary protonema may arise from the primary protonema 
• Later, at the base of the protonema a single bud is differentiated with a 3-sided apical cell. This bud develops into the leafy gametophore. Thus, only one gametophore is developed from one protonema.

IDENTIFICATION

• DIVISION – Bryophyta
  1. True roots absent and instead are present the rhizoids.
  2. No true vascular strands.
• Class :- Bryopsida
  1. Gametophore erect and leafy,
  2. Rhizoids multicellular with oblique septa.
• SUB CLASS :- Sphagnidae
  1. Protonema thallose
  2. Archesporium from amphithecium
  3. Seta is very small but the capsule is elevated by the elongation of the gametophore apex known as pseudopodium
  4. Peristome teeth absent.
• Order – Sphagnales
  1. Character same as sub-class.
• Family – Sphagnaceae
  1. Characters same as those of Sphagnidae
• Genus – Sphagnum
  1. Characters same as Sphagnidae.

REFERENCES

• https://en.wikipedia.org/wiki/Sphagnum
• https://www.shutterstock.com/search/sphagnum