In this article we will discuss about the two subclasses of class bryopsida. The subclasses are: 1. Subclass Sphagnidae 2. Subclass Andreaeidae.
Subclass # 1. Sphagnidae Genus Sphagnum:
Sphagnidae comprise of a single order Sphagnales of a single family Sphagnaceae with a single and very distinct genus Sphagnum. They differ from the other mosses in showing a broad, thallose protonema (Fig. 456) which develops only one normal gametophytic plant.
Leaves on the gametophyte are without midrib and are formed of two different types of cells. The gametophytic body on the whole shows remarkable adaptations for the absorption of water.
Antheridia are formed between leaves on special shoots while archegonia are formed acrogynously. In the sporophyte a gametophytic structure, the pseudopodium takes the place of the seta and the endothecium forms only a sterile columella while the archesporium develops from the inner layers of the amphithecium and arches over the tip of the columella.
Sphagnum presents features which are considered more primitive than in the other Musci. But there is no fossil evidence to show that Sphagnum existed earlier than the Bryidae.
Sphagnum is commonly called bog moss, peat moss or turf moss because of its ecological importance in temperate to subarctic regions in the development of bogs and peat. Sometimes they are known even at sea-level in some tropical countries like Guinea and Trinidad.
The plants are perennial and of aquatic or subaquatic habit growing in extremely moist situations where water accumulates or where water drips along rocky slopes.
When established on the shores of lakes it gradually encroaches more and more of the water as creeping bogs and may completely cover up the lake transforming it into a bog in course of a century. Angiospermic plants then encroach the bog and the topographical feature gets changed. With this development, the water of the bogs becomes acid (the pH value becoming as low as 3.7), a condition when Sphagnum thrives.
In this acid condition there is little decay of the old basal parts of the gametophyte and accumulation of these, year after year, followed by compression from plants on top leads to the development of dark-coloured peat which, when dried, is a very handy fuel in countries like Iceland and, of course of age, may form a type of coal.
Because of its water absorbing capacity, Sphagnum plants are favoured as a packing material for transporting living plants and as a water retaining constituent for preparing seed beds. After proper treatment, Sphagnum has even been used to replace absorbent cotton for dressing wounds, specially during wars. Thus, Sphagnum has some economic importance as well.
342 species of Sphagnum were recognised from all over the world by Warnstorf (1911). Index Muscorum (1967) has brought this number down to 207. They are distributed all over the high, cool and very wet regions of the earth (cosmopolitan). 22 species are known to occur in India.
Of these, only 3 species (S. fimbriatum, teres and squarrosum) occur in the drier Western Himalaya, only 1 species (S. ceylonicum) occurs in warmer South India but as many as 20 species, e.g., S. nemoreum Scop. (=S. acutifolium Schrad.), S. junghuhnianum D. and M., S. teres (Schimper.) Aongstr., S. squarrosum Crom. in Hopp., S. cuspidatulum C.M., S. khasianum Mitt., S. subsecundum Nees., S. palustre L. S. cymbifolium (Ehrh.) Hedw.), S. ovatum Hamp., are from the moist Eastern Himalaya (including Khasia Hills). S. fimbriatum and ceylonicum are not known in the Eastern region.
Basic chromosome number n = 19.
The shoots of the mature gametophytic plant are of a whitish or brownish green colour. These do not bear rhizoids and are of perennial, unlimited growth. Very young gametophytes, however, show delicate rhizoids with oblique cross-walls. It is differentiated into stem and leaves (Fig. 449).
Growth is by means of an apical cell with three cutting faces. Each segmental cell cut off by this cell gives rise to a single leaf and the subtending portion of stem.
At the growing apex leaves are in three rows but, later on, this arrangement is disturbed because of the growth of the stem. At intervals, on the stem, a leaf bears an axillary cluster of lateral branches some of which are pendent and others upwardly divergent (Fig. 449B).
Sometimes, one of the divergent branches becomes as strong as the main stem and ultimately gives rise to a new plant when it becomes separated from the mother plant. At the apex of the main stem the branches are short and densely crowded forming a compact head (Fig. 449A). Although these shoots are weak, in a bog the steins grow erect being supported by one another and may grow considerably long (up to 7 feet).
The small leaves (Fig. 450A) are of a typical construction. Each dividing leaf cell gives rise to two asymmetrical cells, one small towards the margin and one large. The large cell again cuts off another small cell towards the apex (Fig. 450B).
Eventually, the small cells mature into elongated green cells and remain living while the large cell enlarges in all directions, becomes spirally thickened, develops pits on the walls and becomes hyaline by losing its protoplast. The hyaline and chlorophyllose cells form a regular reticulate pattern (Fig. 450C).
The leaf remains one-cell thick (Fig. 450D). The hyaline cells perform an important role in the absorption and retention of water—a typical characteristic of Sphagnum. That is why rhizoids are not necessary in the mature plant. The characteristic areolation (cell pattern) of Sphagnum leaf alone is a sufficient diagnostic feature to identify the genus. No other plant has such a leaf.
The stem in t.s. shows an unsheathing cortex of large hyaline cells which are also greatly capable of water absorption.
The genus Sphagnum is divided into two subgenera:
(1) Subgenus Sphagnum (=Inophloea) with cortical hyaline cells fibrose and porose as in the leaves
(2) Subgenus Lithophloea (meaning ‘smooth-skinned’) where these cells are similar to the hyaline cells of the leaf but without the pores or the spiral thickenings.
The cortex remains one or two-layered throughout in some species (e.g., S. ovatum — Fig. 451A), but, more often, it becomes several layered in the mature stem (Fig. 451B). Cortex of the lateral branches always remains one-layered. In a few species, some outer cortical cells enlarge peculiarly appearing like retorts and these are called retort cells (Fig. 451C & D).
Internal to the cortex is the central cylinder formed of smaller, vertically elongated cells. The outer cells of the central cylinder are thick- walled while the inner ones may or may not remain thin-walled. Some cells of the central cylinder may be devoid of cell contents.
Vegetative Propagation of Bryopsida:
As the shoots are perennial, the common mode of propagation is vegetative. Branch shoots get detached from the mother plant by decay of the lower parts and thereby form independent plants.
Reproduction of Bryopsida:
Sphagnum may be monoecious or dioecious but the antheridia and archegonia are always borne on special separate antheridial and archegonial branches.
The antheridial branches (Figs. 449B and 452A) appear first in a monoecious shoot. They arise near the apex of the main shoot but are later carried downwards by the eventual growth of the apex. The leaves on the antheridial branch are closely imbricate and, although resembling foliage leaves, are shorter and often more brightly coloured (yellow, red or dark-green).
The antheridia develop acropetally, apparently in the axils of the leaves (it has now been shown that they are below the leaves and not in the axils) (Fig. 452B), the top leaves usually do not develop antheridia and the apex is free to grow further after the maturity of the antheridia.
Each antheridial initial is a superficial cell of the stem. This first develops a short filament (Fig. 452C), the apical cell growing by two cutting faces. The top cells, later, form the antheridium in a manner resembling the Jungermanniales.
A mature antheridium (Fig. 452D) shows a long stalk of two to four rows of cells and a spherical or ovoid antheridium with a. jacket of one layer of cells enclosing a large number of androcytes formed from the sperm mother cells. The androcytes contain the sperms or antherozoids which are coiled structures with two long cilia (Fig. 452E) of equal lengths each, resembling the Hepatics.
The archegonia develop on the tips of special archegonial branches which develop at the apex or laterally (Fig. 449). The leaves on these branches are larger than the foliage leaves and the hyaline cells on them are less fibrose. These constitute the perichaetium enclosing the archegonia.
The apical cell of this branch forms the primary archegonium. Two to four secondary archegonia develop from 2 to 4 segments cut off by the same initial cell. Usually, only two secondary archegonia are formed so that the mature archegonial branch shows three archegonia at the tip (Fig. 453C).
The initial of the primary archegonium divided either by two cutting faces or transversely, forming a short filament four to six cells high at first. Then, the terminal cell cuts off three jacket initial cells and an axial cell (Fig. 453A).
Further division is similar to the acrogynotis Jungermanniales and the archegonium (Fig. 453B) is developed. A mature archegonium (Fig. 453B & C) shows a stalk, a twisted neck (wall formed by five or six vertical rows of cells with the cover cells not so distinct) with 8 to 9 neck canal cells, a central canal cell and the egg in the venter whose wall has by now become multi-layered.
Fertilisation in Bryopsida:
The mature antheridium opens at the top, the androcyte mass comes out and the antherozoids are liberated immediately. The archegonium also opens at the tip the neck canal cells are disorganised and fertilisation takes place as in other Bryophytes. Bryan (1920) found in S. secundum the persistence of the ventral canal cell and the ultimate fusion of the egg with the ventral canal cell to form a zygote.
Usually, the zygote of only one archegonium in an archegonial branch develops an embryo —the other archegonia wither early. The zygote invests itself with cell wall and then divides transversely. Transverse divisions continue until a filament on 6 or 7 cells (Fig. 454A) is formed.
The development of the embryo resembles the Jungermanniales in the earlier stages and the Anthocerotales in the later stages.
The lower half of the filament divides irregularly forming a bulbous foot (Fig. 454B), the lowest cells of which are haustorial. The haustorial part helps absorption of food material from the gametophyte and is obliterated when the sporophyte matures.
The upper cells of the filament divide periclinally. Soon an inner endothecium and an outer amphithecium are differentiated (Fig. 454B).
The endothecium remains sterile and forms the columella with a dome-like top over which the amphithecium overarches. The inner layers of the amphithecium form the 2 to 4 layers thick archesporium while the outer layer forms a jacket 4 to 6 layers thick. The jacket cells remain green till the sporophyte matures. There is only a short neck and no proper seta connecting the upper capsule and the lower foot.
The function of the seta is performed by the tissue of the gametophyte just below the foot which develops into a long stalk raising the sporophyte and is called the pseudopodium. The archesporial cells form the spore mother cells which divide meiotically developing the spores.
The mature sporophyte (Fig. 454C) shows an almost spherical, black to dark-brown capsule and a bulbous foot connected by a very short neck. The base of the foot is seen disintegrating. The whole sporophyte is covered by the calyptra which is the archegonial wall which has developed with the sporophyte. The lowest part of the calyptra surrounding the foot is called the vaginula.
Below the sporophyte is the pseudopodium which is a stalk increasing in length and pushing out the sporophyte as it develops but is not a part of the sporophyte as it originates out of gametophytic tissue. The capsule shows a jacket of 4 to 6 layers, and the spore-sac, full of spores, overarches over the dome-shaped columella (representing the endothecium).
The outermost layer of the jacket becomes thickened, loses chlorophyll and develops some non-functional stomata in which there are guard cells but the apertures are missing.
The convex top of the jacket becomes differentiated into an operculum after being delimited from the rest of it by a ring of cells with thinner walls forming the annulus. The pseudopodium carries up this sporophyte (Fig. 455) whose capsule has now developed a dark colour.
In sunny days the capsules burst by an explosive mechanism. Air enters the spore-sacs below which the space has increased by the drying up of the columella. This air, expanded by heat, makes the capsules burst so that the operculum breaks at the annulus and this, along with most spores, are thrown off inches high.
The New Gametophyte:
The spore (Fig. 456A) falling on a moist substratum germinates, developing a small thalloid protonema (Fig. 456B). This develops further, forming a prostrate green, irregularly lobed, thalioid structure (Fig. 456C). It is one cell in thickness and is fastened to the substratum by multicellular rhizoids.
On this protonems , a single bud is developed from a marginal cell and this develops the new leafy gametophyte. It has been found (Noguchi, 1958) that the protonema may develop a filament from any marginal cell and on this filament another secondary protonema may develop.
Subclass # 2. Andreaeidae:
Like the Sphagnidae, the Andreaeidae is represented by a single order Andreaeales with a single family Andreaeaceae. While previous authors recognised only a single genus Andreaea, Brotherus (1924) recognised two—Andreaea (109 spp.). and Neuroloma 1 sp. from the Sub-Antarctic region).
But, Reimers (1954) favours three genera: Andreaea, Acroschisma (1 sp., also from the Sub-Antarctic region, taken out of Andreaea) and Neuroloma. These plants are distributed all over the earth in very cool climates, n the Arctic and Antarctic regions and in the alpine heights of the high mountains.
The Andreaeaceae has characters intermediate between the Sphagnidae and the Bryidae. In the sporophyte, both the archesporium and the columella develop from the andothecium but the archesporium arches over the columella like a dome. The seta is almost as short as in Sphagnum and the sporophyte is pushed up by the development of a similar pseudopodium.
The capsule opens by means of a number of longitudinal slits as in some Jungermanniales, there being ,no operculum or peristome. The protonema is thallose.
Genus and Reaea:
As opposed to Sphagnum, Andreaea grows on dry and exposed arctic situations on siliceous rocks, specially granite (hence the name ‘granite moss’). The 109 species are distributed only in the very cool alpine and subalpine regions, i.e., the subarctic regions and the lofty peaks throughout the world.
They grow in dark-brown to blackish dense tufts which become very brittle when dry. Four species: Andreaea indica, A. densi- folia, A. commutata and A. rigida were discovered by J. D. Hooker from above 15,000 ft. in Sikkim. A fifth species (cosmopolitan A. rupestris) was later discovered in Sikkim and has now also been found at 13,000 ft. in Garhwal and also in Western Nepal, i.e., from the Western Himalaya.
Basic chromosome number n = 10.
The general appearance of the gametophyte is like that of a common moss (Bryidae). The stem is prostrate on the rock surface and branching is dichotomous sympodial with one branch growing more strongly than the other (Fig. 458A). There are many rhizoids on the lower part of the stem but these are different from those in other Bryophytes.
Some of them are cylindrical while others are flattened, plate-like—the latter making the stem stick to the rock surface. The stem grows by an, apical cell with three cutting faces, leaves developing in three rows. Leaves are small, one layer of cells in thickness and .without any midrib in some species, while in some other species the median longitudinal axis becomes more than one layer thick.
The stem anatomy shows a uniform mass of parenchyma cells, there being no differentiation into a cortex and a central cylinder. But, the superficial cells are often somewhat thick-walled (Fig. 458B) and darker in colour. Large oil drops are present in the cells.
The zygote divides by a transverse wall. The lower (hypobasal) cell divides irregularly and forms a haustorial tissue which later organises into the foot. The upper (epibasal) cell is destined to form the capsule. It first functions as an apical cell with two cutting faces and soon, after periclinal divisions, develops an inner endothecium and an outer amphithecium. The amphithecium forms the jacket, 3 to 8 cells in thickness.
The inner layers of the endothecium become the sterile columella while its outer cells become the archesporium which develops a dome-like sporogenous tissue, two layers thick, and arches over the top of the columella. Large chloroplasts are present in the sterile cells showing that the sporophyte is not wholly parasitic.
The mature sporophyte (Fig. 460A) shows a swollen foot which is haustorial in function and an elliptical capsule. The two are connected by a short neck which is the seta. The jacket of the capsule is 3 to 8 layers in thickness, the superficial cells of which are thick-walled except along four vertical lines (reaching neither the tip nor the base of the capsule) where the cells remain small and thin-walled.
The club-shaped columella is in the centre and the dome-shaped spore-sac is arching over it.
The whole is raised by a stalk which is the pseudopodium, a growth of gametophytic tissue as in Sphagnum (Fig. 460B & G). The pseudopodium pushes up the main sporophyte on its tip. Just below the tip, the pseudopodium becomes swollen (the foot of the sporophyte being embedded within it).
Lower down, some of the lateral unfertilised archegonia may also be seen on the pseudopodium (Fig. 460 B). The calyptra, remnant of the archegonial wall, is visible on the sporophyte as in other Bryophytes.
At maturity, the capsule splits along the four vertical lines of weakness so that, usually, there are four valves although in a few species there may be more. But, the valves are not completely separate as the slits do not reach the apex or the base (Fig. 460C). These valves are hygroscopic, and the spores are scattered out with their movements.
The New Gametophyte:
The spore (Fig. 461 A) has two coats as usual and contains chloroplasts and oil globules. As soon as the spore is liberated, it begins to divide while still within the exospore and forms a globular mass of cells (Fig. 46IB). The exospore, then, bursts and the cell mass begins to develop one or more filaments from different points (Fig. 461G).
The ultimate appearance of the gametophytic protonema varies. It may be a branched ribbon-like structure (Fig. 461D) or it may be a thalloid leaf-like structure resembling that of Sphagnum.
Unlike most other protonema, the Andreaea protonema may even lie in a dormant stage if the environment becomes too rigorous. Ultimately, buds develop on any part of the protonema and these develop into new leafy gametophytic plants.