Minggu, 12 April 2009

Vegetative Features Common to Many Bryophytes

Gemmae in Bryophytes
are a means of asexual reproduction found in many bryophytes. Gemmae are 1 to many celled, specially produced clonal plant fragments. Some specific examples of gemmae are illustrated in the table below. The form of the gemmae is often very useful when identifying certain bryophytes. Gemmae are variously dispersed (e.g. by wind, water) and are capable of growing into new plants.

Click thumbnail images to enlarge

Single-celled gemmae arising from leaf margin of Nowellia curvifolia (grown in liquid culture)
Single-celled gemmae arising from leaf margin of Lophozia capitata - a leafy liverwort gemmae.jpg (78915 bytes)
Gemmae from leaf margins of the leafy liverwort Scapania scaundgcm1.jpg (86286 bytes) scnem01cm.jpg (14810 bytes)
Discoid, multicellular gemmae along leaf margins of Radula australis - a leafy liverwort radaus01.jpg (55248 bytes) radausgmcm1.jpg (58085 bytes) radausgmcm2.jpg (29954 bytes)
Polygonal gemmae from leaf margin of the leafy liverwort Lophozia bicrenata lopbicgm2.jpg (38069 bytes)
Gemmae of the moss Syrrhopodon texanus syrrgemc.jpg (50869 bytes)
Miniature leaf-like gemmae clustered at shoot tips of the moss Tortula pagorum torpagcm.jpg (111431 bytes)
Propagule arising from leaf surface in the liverwort Plagiochila plavircm.jpg (94296 bytes)
The moss, Tetraphis pellucida, with gemmae cups tetpelcm.jpg (38387 bytes)
Gemmae cups in the liverwort Marchantia MarchPolygemcups.jpg (59984 bytes)

Rhizoids anchor the gametophyte to the substrate. Rhizoids can be important taxonomic characters. Their presence/absence, color, and papillosity are sometimes used in making identifications.

Rhizoids in Bryophytes

Purple rhizoids of Fossombronia foveolata fossmrxd.jpg (58372 bytes) click image to enlarge
Rhizoids restricted to underleaf base in a liverwort rhizoidUlbasechipol10CM.jpg (48672 bytes) click image to enlarge
Unbranched rhizoids of a liverwort conorzd1.jpg (57779 bytes) click image to enlarge
Pegged and smooth rhizoids in the Marchantiales rhizpegm.jpg (68191 bytes) click image to enlarge
Branched rhizoids of a moss mossrzdm.jpg (61957 bytes) click image to enlarge


Bryidae protonemata mossprotcm1.jpg (57631 bytes) click image to enlarge

Antheridia and Associated Structures

lopcapancm.jpg (32315 bytes) androeciumhepcm.jpg (45000 bytes) poranth.jpg (66560 bytes)

Archegonia and Associated Structures

archegonjunevacm.jpg (71769 bytes) porarch.jpg (68349 bytes)

Kamis, 21 Agustus 2008

Bryology (mosses, liverworts and hornworts)

What are bryophytes?

Bryophytes are the oldest land plants on earth, and have been around for 400 million years or more.
Although small, they can be very conspicuous growing as extensive mats in woodland, as cushions on walls, rocks and tree trunks, and as pioneer colonists of disturbed habitats.
They comprise three main taxonomic groups: mosses (Bryophyta), liverworts (Marchantiophyta) and hornworts (Anthocerotophyta) which have evolved quite separately.
Worldwide there are possibly 10,000 species of mosses, 7000 liverworts and 200 hornworts.
Most bryophytes have erect or creeping stems and tiny leaves, but hornworts and some liverworts have only a flat thallus and no leaves.

How do bryophytes live their lives?

Bryophytes have a two-stage life cycle (alternation of generations). The ‘gametophyte' generation is the green photosynthetic part (the familiar moss or liverwort plant) attached to the substrate by threads (rhizoids), and the ‘sporophyte' generation which consists of a stalk and capsule which are dependent on the gametophyte for support and nutrients. The capsule when ripe releases thousands of tiny spores. Sexual reproduction occurs on the gametophyte generation and requires water for fertilization. Many bryophytes also produce ‘gemmae': tiny buds, discs or leaf fragments which spread the plants vegetatively.

Learn more about bryophytes:

Importance of bryophytes

Mosses by their very nature are soft and absorbent and in many cultures have been traditionally used as bedding, padding and packing materials. Linnaeus gave Fontinalis antipyretica (a common north European aquatic moss) its Latin name because he observed Laplanders using it to caulk chimneys. Bog moss (Sphagnum) in particular has many uses: it is the major component of peat used as fuel and compost, as a horticultural substrate, and in the past as nappies and surgical dressings because of its absorbent and antiseptic properties. In World War I, the public around Edinburgh were encouraged to collect Sphagnum for shipment to the battle fronts for medical use. Commercial harvesting of mosses (e.g. for making wreaths and decorations) is now a serious conservation issue.
More important perhaps are the vital ecological roles of bryophytes. On bare and disturbed ground they are primary pioneers helping other plants to gain a foothold. In bogs and forest (especially in the montane tropics) they absorb huge quantities of water, thereby acting as a sponge and maintaining humidity over dry periods and preventing rapid run-off and flooding (excessive flooding in India is thought to be partly due to loss of bryophyte cover in Himalayan forests). They act as a home to many plants and animals, particularly invertebrates, and provide a moist foothold for many other plants such as ferns and orchids in mossy or ‘Elfin' forest, as in the oceanic woods of western Scotland or the mossy forests of the Andes and Himalayas.

Bryophytes show a wealth of adaptive features to all kinds of climates, substrates and habitats. Many are precise indicators: of rock type such as Tortella tortuosa on limestone, Andreaea and Racomitrium on acidic or granitic rocks, of acid bogs, eg. Sphagnum species, of rich fens, eg. Tomentypnum nitens, of metalliferous rocks and soil, eg. Ditrichum plumbicola and Grimmia atrata, of pollution levels such as Dicranoweisia cirrata which thrives on tree bark in polluted areas and many pollution-sensitive species on trees in unpolluted areas, eg. Antitrichia curtipendula. In dry climates many are drought-resistant ‘xerophytes' in contrast to delicate ‘mesophytes' growing in wet habitats.

Bryophytes in Scotland

Scotland has a globally important bryophyte flora of about 920 species, because of its geographical position, its range of climatic types, and diverse geology. The western seaboard is rich in oceanic mosses and liverworts, such as the tropical Cyclodictyon laetevirens and Myurium hochstetteri. In the south-east the sunny sea-banks have a Mediterranean flora. Perhaps the richest habitats are in the mountains: the Ben Lawers range, well-known for its alpine flora, is even richer in rare bryophytes. The high Cairngorm plateau has several rare arctic and alpine species, particularly in snow-beds, such as Marsupella arctica and Andreaea blyttii, whilst the mountains of the North-west Highlands have a unique bryophyte-dominated community the ‘Northern Hepatic Mat' dominated by a range of rare liverworts, some of which (e.g. Pleurozia purpurea, Anastrophyllum alpinum) show astonishing disjunctions to the Himalayas and the mountains of British Columbia, thousands of miles apart. This juxtaposition of arctic habitats and an oceanic climate combine to make Scotland's bryoflora unique in the world. Tropical and arctic species can literally be found within ten or twenty miles of each other.

Scotland is important historically in bryology in that many European species were first discovered here: eg. Anastrepta orcadensis, Marsupella nevicensis, Pohlia scotica, Bryoerythrophyllum caledonicum, Plagiochila atlantica. Pioneers such as Sir William Hooker, Archibald Menzies and James Dickson first explored Scotland for bryophytes at the end of the eighteenth century. Tetrodontium brownianum, named after the celebrated Scottish botanist Robert Brown, was first discovered at Roslin near Edinburgh by Brown in his student days, and still thrives there.

Bryophytes in Scotland's National Botanic Gardens

Mosses and liverworts are common throughout the Royal Botanic Garden Edinburgh at Inverleith but especially on the Peat Walls, in the Rock Garden and in the Cryptogamic Garden. These are all native species. In the glasshouses a number of exotic species thrive, especially in the Fern House and the Peat and Rock Houses, such as the spectacular Hypopterygium tamarisci (formerly called H. atrotheca). These have probably been introduced accidentally with other plants. Many native species grow vigorously, especially the thalloid liverwort Conocephalum conicum which covers large areas of the Fern House in shiny mats. Few are cultivated, though many plant pots are colonised by the weedy greenhouse liverwort Marchantia polymorpha.

At Dawyck Botanic Garden there is a wealth of native bryophytes typical of southern Scotland. The clean air ensures luxuriant growth of epiphytic bryophytes including the locally rare Leucodon sciuroides and Neckera pumila. The Dawyck Cryptogamic Sanctuary now provides a secure haven for a good range of common species typical of upland woods.

In the Benmore Botanic Garden are many of the more oceanic species typical of 'western' native Scottish woodlands. In places bryophytes form the dominant ground flora and along with the ferns form an attractive natural ground-cover under the Rhododendrons and conifers. At Benmore three southern hemisphere liverworts, Lophocolea semiteres, L. bispinosa and Telaranea tetradactyla have become naturalised in the garden. Close to Benmore are some remnants of ancient Scottish oakwood, with very rich communities of oceanic bryophytes including Plagiochila atlantica. A project is now under way to experimentally re-create an area of native woodland with its component bryophyte flora within the garden itself.

Selasa, 17 Juni 2008


Plant scientists recognize two kinds of land plants, namely, bryophytes, or nonvascular land plants and tracheophytes,or vascular land plants. Bryophytes are small, herbaceous plants that grow closely packed together in mats or cushions on rocks, soil, or as epiphytes on the trunks and leaves of forest trees. Bryophytes are distinguished from tracheophytes by two important characters. First, in all bryophytes the ecologically persistent, photosynthetic phase of the life cycle is the haploid, gametophyte generation rather than the diploid sporophyte; bryophyte sporophytes are very short-lived, are attached to and nutritionally dependent on their gametophytes and consist of only an unbranched stalk, or seta, and a single, terminal sporangium. Second, bryophytes never form xylem tissue, the special lignin- containing, water-conducting tissue that is found in the sporophytes of all vascular plants. At one time, bryophytes were placed in a single phylum, intermediate in position between algae and vascular plants. Modern studies of cell ultrastructure and molecular biology, however,confirm that bryophytes comprise three separate evolutionary lineages, which are today recognized as mosses (phylum Bryophyta), liverworts (phylum Marchantiophyta) and hornworts (phylum Anthocerotophyta). Following a detailed analysis of land plant relationships, Kenrick and Crane (1998) proposed that the three groups of bryophytes represent a grade or structural level in plant evolution, identified by their "monosporangiate" life cycle. Within this the geologically oldest group, sharing a fossil record with the oldest vascular plants in the Devonian era.

Of the three phyla of bryophytes, greatest species diversity is found in the mosses, with up to 15,000 species recognized. A moss begins its life cycle when haploid spores, which are produced in the sporophyte capsule, land on a moist substrate and begin to germinate. From the one-celled spore, a highly branched system of filaments, called the protonema, develops. Cell specialization occurs within the protonema to form a horizontal system of reddish-brown, anchoring filaments, called caulonemal filaments and upright, green filaments, called chloronemal filaments. Each protonema, which superficially resembles a filamentous alga, can spread over several centimeters to form a fuzzy green film over its substrate. As the protonema grows, some cells of the caulonemal filaments specialize to form leafy buds that will ultimately form the adult gametophyte shoots. Numerous shoots typically develop from each protonema so that, in fact, a single spore can give rise to a whole clump of moss plants. Each leafy shoot continues to grow apically, producing leaves in spiral arrangement on an elongating stem. In many mosses the stem is differentiated into a central strand of thin-walled water-conducting cells, called hydroids, surrounded by a parenchymatous cortex and a thick-walled epidermis. The leaves taper from a broad base to a pointed apex and have lamina that are only one-cell layer thick. A hydroid-containing midvein often extends from the stem into the leaf. Near the base of the shoot, reddish-brown, multicellular rhizoids emerge from the stem to anchor the moss to its substrate. Water and mineral nutrients required for the moss to grow are absorbed, not by the rhizoids,but rather by the thin leaves of the plant as rain water washes through the moss cushion.

As is typical of bryophytes, mosses produce large, multicellular sex organs for reproduction. Many bryophytes are unisexual, or sexually dioicous. In mosses male sex organs, called antheridia, are produced in clusters at the tips of shoots or branches on the male plants and female sex organs, the archegonia, are produced in similar fashion on female plants. Numerous motile sperm are produced by mitosis inside the brightly colored, club-shaped antheridia while a single egg develops in the base of each vase-shaped archegonium. As the sperm mature, the antheridium swells and bursts open. Drops of rain water falling into the cluster of open antheridia splash the sperm to near-by females. Beating their two whiplash flagellae, the sperm are able to move short distances in the water film that covers the plants to the open necks of the archegonia. Slimey mucilage secretions in the archegonial neck help pull the sperm downward to the egg. The closely packed arrangement of the individual moss plants greatly facilitates fertilization. Rain forest bryophytes that hang in long festoons from the trees rely on torrential winds with the rain to transport their sperm from tree to tree, while the small pygmy mosses of exposed, ephemeral habitats depend on the drops of morning dew to move their sperm.Regardless of where they grow, all bryophytes require water for sperm dispersal and subsequent fertilization.

Embryonic growth of the sporophyte begins within the archegonium soon after fertilization. At its base, or foot, the growing embryo forms a nutrient transfer zone, or placenta, with the gametophyte. Both organic nutrients and water move from the gametophyte into the sporophyte as it continues to grow. In mosses the sporophyte stalk, or seta, tears the archegonial enclosure early in development, leaving only the foot and the very base of the seta embedded in the gametophyte. The upper part of the archegonium remains over the tip of the sporophyte as a cap-like calyptra. Sporophyte growth ends with the formation of a sporangium or capsule at the tip of the seta. Within the capsule, water-resistant spores are formed by meiosis. As the mature capsule swells, the calyptra falls away. This allows the capsule to dry and break open at its tip. Special membranous structures, called peristome teeth, that are folded down into the spore mass,now bend outward, flinging the spores into the drying winds. Moss spores can travel great distances on the winds, even moving between continents on the jet streams. Their walls are highly protective, allowing some spores to remain viable for up to 40 years. Of course, if the spore lands in a suitable, moist habitat, germination will begin the cycle all over again.

Liverworts and hornworts are like mosses in the fundamental features of their life cycle, but differ greatly in organization of their mature gametophytes and sporophytes. Liverwort gametophytes can be either leafy shoots or flattened thalli. In the leafy forms, the leaves are arranged on the stem in one ventral and two lateral rows or ranks, rather than in spirals like the mosses. The leaves are one cell layer thick throughout, never have a midvein and are usually divided into two or more parts called lobes. The ventral leaves, which actually lie against the substrate, are usually much smaller than the lateral leaves and are hidden by the stem. Anchoring rhizoids, which arise near the ventral leaves, are colorless and unicellular. The flattened ribbon-like to leaf-like thallus of the thallose liverworts can be either simple or structurally differentiated into a system of dorsal air chambers and ventral storage tissues. In the latter type, the dorsal epidermis of the thallus is punctuated with scattered pores that open into the air chambers. Liverworts synthesize a vast array of volatile oils, which they store in unique organelles called oil bodies. These compounds impart an often spicy aroma to the plants and seem to discourage animals from feeding on them. Many of these compounds have potential as antimicrobial or anticancer pharmecuticals.

Liverwort sporophytes develop completely enclosed within gametophyte tissues until their capsules are ready to open. The seta, which is initially very short,consists of small, thin-walled, hyaline cells. Just prior to capsule opening, the seta cells lengthen, thereby increasing the length of the seta upto 20 times its original dimensions. This rapid elongation pushes the darkly pigmented capsule and upper part of the whitish seta out of the gametophytic tissues. With drying, the capsule opens by splitting into four segments, or valves. The spores are dispersed into the winds by the twisting motions of numerous intermixed sterile cells, called elaters. In contrast to mosses, which disperse their spores over several days, liverworts disperse the entire spore mass of a single capsule in just a few minutes.

Hornworts resemble some liverworts in having simple, unspecialized thalloid gametophytes, but they differ in many other characters. For example, colonies of the symbiotic cyanobacterium Nostoc fill small cavities that are scattered throughout the ventral part of the hornwort thallus. When the thallus is viewed from above, these colonies appear as scattered blue-green dots. The cyanobacterium converts nitrogen gas from the air into ammonium, which the hornwort requires in its metabolism and the hornwort secretes carbohydrate- containing mucilage which supports the growth of the cyanobacterium.Hornworts also differ from all other land plants in having only one large, algal-like chloroplast in each thallus cell. Hornworts get their name from their long, horn-shaped sporophytes. As in other bryophytes, the sporophyte is anchored in the gametophyte by a foot through which nutrient transfer from gametophyte to sporophyte occurs. The rest of the sporophyte, however, is actually an elongate sporangium in which meiosis and spore development take place. At the base of the sporangium, just above the foot, is a mitotically active meristem,which adds new cells to the spore-producing zone throughout the life span of the sporophyte. In fact, the sporangium can be releasing spores at its apex, at the same time that new spores are being produced by meiosis at its base. Spore release in hornworts takes place gradually over a long period of time, and the spores are mostly dispersed by water movements rather than by wind

Mosses, liverworts and hornworts are found throughout the world in a variety of habitats. They flourish particularly well in moist, humid forests like the fog forests of the Pacific northwest or the montane rain forests of the southern hemisphere. Their ecological roles are many.They provide seed beds for the larger plants of the community, they capture and recycle nutrients that are washed with rainwater from the canopy and they bind the soil to keep it from eroding. In the northern hemisphere peatlands, wetlands often dominated by the moss Sphagnum, are particularly important bryophyte communities. This moss has exceptional water-holding capacity, and when dried and compressed, forms a coal-like fuel. Throughout northern Europe, Asia and North America, peat has been harvested for centuries for both fuel consumption and horticultural uses and today peatlands are managed as a sustainable resource.

Bryophytes - Mosses, Liverworts & Hornworts

"Bryophytes" is a resource devoted to Bryology, the branch of plant science concerned with the study of mosses, liverworts and hornworts. It provides information on the classification, structural features, natural history, ecology and evolutionary relationships of these plants. Although small in stature, bryophytes play significant roles in diverse terrestrial ecosystems. They are found growing on soil, rocks and/or trees throughout the world, from coastal Antarctica to the peat bogs of the Northern hemisphere, from the deserts of Australia to the rain forests of the Amazon. They are an essential part of this planet's biodiversity. Click on any of the topics listed to learn more about these fascinating organisms.

source from http://bryophytes.plant.siu.edu/