Three groups of bryophytes are known: mosses, liverworts and hornworts. The gametophyte is the dominant plant with the sporophyte growing attached to it. Fossil bryophytes occur in Carboniferous rocks, the liverworts perhaps occurring earlier. They may have originated independently from tracheophytes.
Vegetative structure of liverworts
Liverworts are highly variable with leafy and thallose forms. Leafy liverworts usually have three ranks of leaves, all one cell thick, one rank usually reduced in size, of many different shapes and arrangements used to distinguish species. Thallose forms have thicker structures which may have cavities and pores.
Vegetative structure of hornworts
Hornworts produce a simple thallus similar to some thallose liverworts but, in most, cells have a single chloroplast.
Vegetative structure of mosses
The first growth from the germinating spore is a filamentous protonema from which the main plant grows. Mosses have spirally inserted tapering leaves, mostly one cell thick, but are highly variable in growth form and in details of leaf structure and cell shape. Many have a midrib andPolytrichum has lamellae. Sphagnum has large dead hyaline cells that retain water.
Most bryophytes rely on surface water and capillary action, often enhanced by leaf arrangement. This limits their size. In Polytrichum and some other large mosses there is a rudimentary conducting system, resembling a simple form of that found in the true vascular plants.
They occur in all habitats, particularly in wet places and deep shade. They dominate some sub-polar regions and bogs where they form peat. They are abundant on the floor of wet woods, by streams and as epiphytes. Some can tolerate desiccation and occur on rocks where no other plants can grow.
Interactions of bryophytes
Pioneer communities may lead to succession. Different growth forms occur and interactions between species may be competitive or beneficial. Only a few specialist invertebrates eat bryophytes; fungi can infect bryophytes and are the main decay organisms. Nitrogen-fixing cyanobacteria can colonize
Bryophytes have been used for bedding and as filling in buildings and have a small role as ornamentals. Sphagnum has been an important wound dressing. Peat, based on Sphagnum, is an important fuel. Scientifically they have been useful as genetic tools and for monitoring pollution. Peat has preserved numerous biological and archaeological remains used for reconstructing history.
The bryophytes, subkingdom Bryophyta, comprise two large groups: the mosses, Bryopsida or Musci, and the liverworts, Marchantiopsida or Hepaticae; and the small group of hornworts, Anthocerotopsida. They are almost entirely land plants, with a few in fresh water. They are complex, multicellular plants, but all are small, often less than 2 cm high, with the largest reaching up to 1 m above the substrate. The three groups of bryophytes share many features but also differ markedly.
Bryophytes show a typical alternation of generations , the main plant body being the haploid gametophyte. The sporophyte consists solely of a basal foot with a stalk and capsule and remains attached to the gametophyte throughout its life. The sporophyte lives for only a few days in most liverworts, and up to a few months in mosses and hornworts, in contrast to the perennial gametophyte. Meiosis in the capsule leads to spore production and dispersal. Vegetative fragmentation is an important mode of propagation in many species and some produce specialized asexual gemmae (rounded groups of cells that disperse to form new plants).
The fossil record of bryophytes is patchy but fossil mosses occur from Carboniferous rocks, while the liverworts may go back to the Silurian and were probably the earliest land plants. They may have originated independently from tracheophytes and are likely to be derived from green algae. The three groups of bryophytes may have arisen separately; they certainly diverged early in their evolution.
Liverwort gametophytes are highly variable in structure, ranging from leafy shoots with leaves one cell thick to flat thalli many cells thick and of indeterminate growth (Fig. 1). The leafy liverworts usually lie prostrate or nearly so in damp places or as epiphytes and normally grow up to a few centimeters inmlength. They consist of a simple stem, leaves and often rhizoids, single cells forming a hair-like projection into the substrate. Most have three lines of simple, toothed or lobed leaves but, in many, one of these is much reduced in size to form ‘underleaves’ on the ventral side, or is lost. All the leaves are a single cell thick. The classification of leafy liverworts is mainly based on the shape and arrangement of the leaves.
Thallose liverworts have a simple thallus (Fig. 1) several cells thick that branches dichotomously. Some consist of little more than this but, in the more complex forms, there is a thickening of the central part into a midrib, pores in the upper surface and rhizoids anchoring them to the substrate. Some species produce gemmae. Most liverworts contain oil bodies in their cells and sometimes these are fragrant when the plant is crushed, e.g.Conocephalum. They also produce antibacterial products but the potential of these for human use has not been examined.
The gametophyte of the hornworts resembles that of a simple thallose liverwort, with no midrib and with no dichotomous branching. They differ in having cavities, often with a symbiotic nitrogen-fixing cyanobacterium in them, and, in most, by having cells containing a single chloroplast, a feature otherwise unknown in land plants but occurring in the algae. They are separated by differences in the sporophyte.
The mosses are the most abundant and important of the three groups and a few grow to 1 m above the ground. After germination, a spore produces filaments of undifferentiated cells known as the protonema, resembling filamentous algae, which spread over the substrate. These normally grow for only a few days in this way before growing into the main gametophyte, though they persist in a few species. Mosses have simple stems with no specialized cells. Most are branched and the shoots may grow upright often forming a tight clump or mat, or trail along the substrate. Rhizoids grow into the substrate, anchoring the plant but with no special adaptations for absorption, and some mosses grow similar hair-like structures along their stems. Mosses normally have spirally arranged leaves, tapered, often to a point, so in general not closely resembling those of leafy liverworts (Fig. 2). One genus, Fissidens, has two ranks of leaves. A few mosses produce gemmae.
The majority of mosses have leaves one cell thick (Fig. 2) but there is frequently a line of narrow thick-walled cells forming a midrib or nerve and some have another line around the edge. The nerve can extend beyond the end of the lamina as a hair point. Polytrichum and its relatives have lamellae of cells along the leaf (Fig. 2), making the leaves fully opaque and much tougher than
those of other mosses. The leaves of the bog mosses (Sphagnum) have normal photosynthetic cells interspersed with much larger dead cells, known ashyaline cells, with prominent spiral thickening and pores (Fig. 2). The hyaline cells act as a water reservoir giving the mosses their sponge-like character and allowing them to be particularly effective as bog builders. The details of leaf and leaf cell structure and their arrangement are some of the main features distinguishing moss species.
All bryophytes except Polytrichum and its relatives have no internal conducting system and are dependent on surface rain water and capillary action across their surfaces. The leaves frequently form sheaths along the stem enhancing water flow. Many are able to dry out and remain dormant in a place free from competition such as a wall top. The leaves usually distort in the dry state (often twisted) and some can remain dormant for months, but they absorb water and resume growth within minutes of rain starting. The cushion form of many mosses will slow water loss.
Polytrichum and its relatives are the tallest of bryophytes and do possess a rudimentary conducting system in the stems that conducts water and solutes effectively around the plant. The long vertically arranged water-conducting cells lose their contents and connect by pores, like tracheids of xylem tissue, and solute conducting cells have pores, oblique walls and degenerating nuclei and are associated with cells with high metabolic rates, resembling phloem sieve elements and companion cells . Other large mosses may have a similar system in a less well-developed form.
Bryophytes tend to occupy places that other plants cannot grow in and frequently fit in spaces between other plants. They occur throughout the world and in almost every habitat, living mainly in wet places and places with dim light, e.g. under dense woodland, or in places with little or no soil, as epiphytes or growing on rocks. They dominate some plant communities in polar and subpolar regions, where extensive areas are covered with moss carpets, sometimes with very few or no other plants (though lichens, a symbiosis of fungus and alga, frequently occur too). In acid cold conditions, such as peat bogs, bog mosses (Sphagnum) dominate. They are able to store water and secrete acid from their cells inhibiting other plants and creating conditions of limited decay. The result is that few other plants can grow on the Sphagnum cushions. Sphagnum moss is the basis of much of the world’s peat, partially decayed, waterlogged, compressed plant matter that gradually accumulates in boggy ground.
Extensive bryophyte communities occur on the floor of wet woods and by streams. Numerous bryophytes occur as epiphytes throughout the world and they can smother tree branches, particularly on tropical mountains. In temperate regions they are often the only epiphytic plants. In tropical rainforests some species grow on leaves, when they are known as epiphylls. Smaller bryophyte species may be some of the only plants growing on walls and as epiphytes on tree trunks in drier woods; growth is slow and interspersed with frequent dormant periods, but they are often the only visible life form growing there except for the even more resistant lichens. In almost all situations mosses are more common than liverworts, which tend to fit around the mosses. A few mosses have specialized ecology such as species in flowing fresh water and those that grow on mobile sand dunes. A few species have colonized deserts and rely on dew for water, and some live by hot springs.
Bryophytes (mainly mosses) may follow an ecological succession similar to that of flowering plants , e.g. colonizing bare rock, although change is often slow. In cold and waterlogged climates where mosses predominate, moss hummocks persist for decades or even centuries with little change. Pioneer species are usually low-growing but retain some water and allow other species to colonize. There may be competition between species, but frequently the presence of one bryophyte stimulates others to grow and the role of competition andbeneficial interactions are not well understood.
A few invertebrates, mainly fly larvae, are specialist moss feeders but bryophytes are nutrient-poor and are not eaten by animals to a great extent. Many microscopic animals live in moss cushions and birds use them as nesting
material. Fungi can infect mosses, particularly in the high latitudes, and other fungi are responsible for decay. Nitrogen-fixing cyanobacteria can
associate with certain species and enhance their growth.
Mosses have been used for centuries as bedding material owing to their soft quality. They were also used as padding for building, e.g. between timbers, and blocking air vents in chimneys, etc. The wiry stems of Polytrichum have been used for baskets. Sphagnum can be used as an absorbent anti-bacterial wound dressing and to retain water in window boxes or plant nurseries. Mosses have occasionally been used as ornamentals, particularly in Japan. Peat derived from Sphagnum has been an important domestic fuel when dried out, e.g. in Ireland where there are few trees, and several power generating stations use peat. Peat is also used in the horticultural trade as a soil conditioner, but many peat bogs have been destroyed through over-extraction and its use is not favored by conservationists.
In science bryophytes have several uses. They are haploid and have been useful in genetic studies since geneticists can look directly at gene expression (although many are polyploid). Some species are sensitive to pollution, particularly by sulfur dioxide, and their presence, along with that of lichens, has been used to monitor pollution levels. Peat accumulation has led to the preservation of numerous remains of plants, animals and human artifacts. Reconstruction of vegetative and human history over the past tens of thousands of years has been possible through examination of these remains.
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