Introduction to Bryophytes:
Bryophytes are true plants. 3 other groups of organisms are member of plant kingdom. These are pteridophytes, gymnosperms and angiosperms. Together they are known as land and embryophytes. All members of above 4 groups of plants produce the multicellular embryo which is nutritionally dependent on material tissue and represents next sporophytic generation. Embryos within seeds of flowering plants are familiar to you. Bryophytes vary from embryophytes by absence of specialized vascular tissues characteristic of pteridophytes, gymnosperms and angiosperms. There more enhanced groups have xylem and phloem. Xylem is made up of dead conductive cells whose cell walls are reinforced by extremely resistant polyphenolic compound, lignin. Every land plants other than bryophytes are also called as vascular plants. Bryophytes are nonvascular land plants. Few members, like cetain moses, of bryophytes to have conductive tissue which transport water but the conductive cells don't have lignified thickenings feature of vascular plants. Few hotanists restrict term 'land plants' to cascular land plants. Though, it is more pleasing to include bryophytes also as land plant, they have recently been stated as photosynthetic organisms customarily living on land and having relations with other plants living on land. Land plants possess many adaptations which allow them to survive on terrestrial habitat. They comprise protective coverings over plant body and pores called as stomata. Land plants should get water from soil. To prevent evaporation and aridity epidermis of land plants is covered with the highly water resistant cuticle. Spores and pollen are small resistant organic chemicals called, sporopollenin. To regulate entry of carbon dioxide and exit of water vapor epidermis is also given with stomata. Stomata apparatus comprises of 2 kidney-shaped cells surrounding the pore. Most land plants comprising several bryphytes possess stomata.
One of the most interesting fields of plant biology is study of origin of land plants. Fossil proof points out that authentic land plants lived approx 400-430 million year ago. During this period known as Silurian there were small, dichotomously branched plants called as Cooksonia. Cooksonia was the vascular land plant. Microfossils of spores, cuticles and conductive tubes have been found from 450-470 million year old sediments recommending that land plants have existed millions of years before arrival of Cooksonia. Bryophytes are not known from such early periods. This may be because delicate thallus of bryophytes may not have been well conserved in fossils.
Scientists now think that land plants might have initiated from some fresh water algal members approx 470 million years ago. They were possibly derived from green alga ancestors of group associated to modern green algae like stoneworts (Chara and Nitella) and Coleochaete. These charophycean members share many structural and biochemical similarities with land plants. Ancestors of land plant are similar to some modern Coleochaete. Earliest land plants would have been nonvascular embryophytes, not unlike some liverworts. It is probable that 2 subsequent lines of evolution have resulted in bryophyte and vascular land plant groups.
There are approx 23,000 species of bryophytes described so far. All these are tiny green plants estimating in centimeters, and devoid of roots. They happen in the variety of moist terrestrial habitats. As is true of other land plants they are multicellular and parenchymatous. Life cycle comprises of the important gametophytic and less common liverworts and hornworts. Bryologists consider the 3 groups to be closely associated and categorize them as 3 classes under division Bryophyta.
Classes: Hepaticopsida (liverworts)
Few bryologists who consider members of 3 classes to be much less associated to each other raise them to divisional level: 1) Hepatophyta 2) Anthocerotophyta, and 3) Bryophyta. In species abundance bryophytes are dominated by mosses (14,000) followed by liverworts. (8,500) and hornworts (350)
Pteridopphytes are vascular land plants. Unlike bryophytes they have typical xylem and phloem tissue characteristic of vascular plants. Like bryophytes they are also embryophytes. Pteridophytes reproduce by spores but never by seeds. Therefore, it is suitable to further categorize vascular plants in non-seed producing pteridophytes (also called as vascular cryptogams) and seed producing gymnosperms and angiosperms. There are approx 1,000 species of fern allies. They are 52 descendants of extremely ancient groups of vascular plants and are thus of great interest to students of plant evolution. About 10,000 species of ferns are comprised in division, Pteridophyta. We have rich collection of fossils which represent several extinct members of pteridophytes comprising major groups which are known only from fossils. In current years scientists have examined the groups in detail and have set up possible evolutionary relationship among early vascular plants. Though you may not study representatives of all the groups it is necessary that we list major divisions of living and extinct pteridophytes to completely understand diversity and significance of early vascular plants:
Extinct pteridophytes know only from fossil record
Pterophyta are second largest division of plant kingdom. There are 20,000 species of ferns compared to approx 250,000 flowering plants living today. Many ferns are herbaceous plants like Polypodium virginianum (right) that grows in Ohio on damp rockfaces.
Among ferns we see first main elaboration of leaf morphology, accounting for the large part of the interest for us. Petiole and lamina are distinguished as they are in higher plants and ferns illustrate all likely variations on compound leaf structure. Fossil leaves from Carboniferous rocks are instantly identifiable as the ferns.
Herbaceous ferns have rhizome (with adventitious roots). From this arise leaves that typically start rolled up as croziers or "fiddleheads". Fiddle heads of ostrich fern
Features of ferns:
Ferns are the very varied group, and there is no single trait which states them. The following characteristics are present in many:
Leaves, known as fronds, are megaphylls. Many are compound with the rachis and several pinnae (or compound once again with pinnules). Approximately all have circinate vernation-they are coiled (circinate) strongly in shepherd's crook or crozier fashion over growing tips. These open as they mature (development from base to tip like this is defined acropetal). Croziers are known as fiddleheads and are eaten by few people, though several species are toxic.
Stems, for most part, are rhizomes which develop at, or just under, ground surface. They have only main tissues. "Tree" ferns have upright, thick trunks, bulk coming from roots grouped around small true stem. More primitive species have the protostele, many have siphonosteles, and few have complex dictyosteles.
Roots are plain, uncomplicated and happen adventitiously along rhizomes near base of fronds.
Sporangia are situated, for most part, on undersides of ordinary leaves in groups known as sori (singular, sorus). In early ferns, and few living ones, sori happen on specialized, remarkably unleaf-like leaves. In several ferns small leaf outgrowth known as indusium covers each sorus. Two kinds of sporangia exist:
Eusporangia: The sporangia are thick-walled and open by dividing transversely. They create thousands of spores.
Leptosporangia: Such thin-walled, fragile sporangia are only one or some layers thick. They include area, annulus, where cell walls are thickened. When annulus cells dry off at maturity, sporangium splits and, similar to catapult, throw out spores.
Life cycle of fern:
Large, leafy fern sporophyte alternates with the small (3-4 mm), flat green gametophyte-known as prothallus-in typical life cycle. Sporophytes of ferns are independent, separated in leaves, stems (rhizomes), and roots, and include vascular tissues while gametophytes are small, photosynthetic thalli which live joined to ground with rhizoids. Several are heart-shaped and only one-cell layer thick. Gametangia are sunken or protrude from underside of gametophyte. Fern sperm have many flagella (hundreds in some species). When sperm are released through pore at tip of antheridium, they swim in film of exterior water to opening at top of archegonium and down neck to egg where fertilization takes place. Zygote splits within the some hours after fertilization and supplied at first with nutrients (and may be hormones) through absorbing foot joined to gametophyte. The tiny sporophyte having rhizome, adventitious roots along the surface and juvenile green leaf soon from under prothallus, establishes independence, and prothallus whithers and dies.
Ferns, among vascular plants, are second in number to flowering plants and have modified to all manner of habitats. Few are aquatic; few live in deserts or on dry rock cliffs, few persist in cold arctic desert, but generally modern ferns live in tropics. Here ferns get some of glory of the past. Few tree fern trunks go over 70 feet in height, and 15-feet-long leaves share canopy space having angiosperm tree crowns. Tropical ferns produce strongly as epiphytes on and over everything in understory. Smallest ferns are aquatics which float on subtropical and tropical ponds with altered leaves a centimeter or less in size.
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