LECTURE 4 - BRYOPHYTE DIVERSITY.pdf

Bryophyte Diversity Brief Background Bryophytes are the simplest and most primitive non-vascular land plants The name bryophyte is derived from two words: ‘Bryon’ meaning moss and ‘Phyton’ meaning plant.  Introduction to Bryophytes General Characteristics of Plants form than those...

Bryophyte Diversity Brief Background Bryophytes are the simplest and most primitive non-vascular land plants The name bryophyte is derived from two words: ‘Bryon’ meaning moss and ‘Phyton’ meaning plant.  Introduction to Bryophytes General Characteristics of Plants form than those of other kingdoms. Their tissues are more Members of the Plant Kingdom are more complex and varied in specialized for photosynthesis, conduction, support, anchorage and protection. which suggests that plants descended either from a remote Plants share a number of common characteristics with green algae, common ancestor of the green algae that successfully invaded terrestrial habitats. photosynthetic pigment and chlorophyll b and carotenoids as Like green algae, plants have chlorophyll a as their primary accessory pigments.  Introduction to Bryophytes General Characteristics of Plants Cont’n. Bryophytes have a well-developed alternation of heteromorphic generations (i.e. sporophyte/gametophyte). This type of life cycle is common among the living members of the Plant Kingdom. The sporophyte is the generation which produces spores which germinate into the gametophyte. The gametophyte is the generation which produces male and female sex cells (gametes) which, during sexual reproduction, fuse into a zygote. The latter develops into a sporophyte. All plants are oogamous (i.e. exhibit sexual reproduction) and the zygotes of many plants are nutritionally dependent on the gametophyte generation. Habitat Bryophytes exist in a wide variety of habitats. They can be found growing in different conditions: temperatures (cold arctics and in hot deserts), elevations (sea-level to mountains), and moisture (dry deserts to wet rainforests). Many grow in bogs and marshy areas – (e.g. Peat mosses or Sphagnum moss), some are epiphytic growing on tree trunks (e.g. Porella spp) and epiphyllous growing on the surface of leaves (e.g. Radulla protensa) – all these habitats have typically high moisture conditions On the other hand, some are xerophytic growing in desert conditions – typically dry conditions (e.g. Tortula desortorum). Few Bryophytes are truly aquatic with underwater forms (e.g. Riccia fluitans).  The habitats of bryophytes range in elevation from sea level near ocean beaches to more than 18,000 feet (5,486 meters) above sea level on mountains. They often occur as soft, green mats on damp banks, trees and logs; but some (e.g. Grimmia spp.) are able to withstand long periods of desiccation and are found on bare rocks in the scorching sun. Some bryophyte species are restricted to very specific habitats; e.g. on the antlers and bones of dead reindeer; in the dung of herbivorous animals or dung of carnivorous animals; and also on large insect wing covers. Habitat of Mosses and their relatives Habit The plant body is thallus like and more differentiated than that of algae. The thallus size may measure up to one to two centimeters in height. The thallus may be prostrate or erect, and attached to the substratum by unicellular or multicellular rhizoids. They lack true roots, stem or leaves. They may possess root-like, leaf-like or stem-like structures. Structure and Form of Bryophytes “Bryophyte” is a collective term for three distinct divisions of non-vascular plants, which include: The mosses (Division Bryophyta), The liverworts (Division Hepatophyta), and The hornworts (Division Anthocerotophyta). There are about 23,000 different species of bryophytes, all of which lack true xylem or phloem.  Liverworts Mosses Hornworts Structure and Form of Bryophytes I Mosses (Division Peat moss Bryophyta) There are three distinct classes of mosses commonly called True moss peat mosses (e.g. Sphagnum sp.), true mosses (e.g. Polytrichum sp.), and rock mosses Rock moss  In all three classes, the gametophyte is the dominant plant body. Many mosses have water-conducting cells called hydroids and some have food-conducting cells called leptoids surrounding the hydroids, but these cells do not conduct as efficiently as the vascular tissues in higher plants. Mosses are anchored by filamentous multicellular rhizoids. Their leaves are never lobed or divided nor do they have a petiole. Structure and Form of Bryophytes II Liverworts (Division Hepatophyta) group of bryophytes are called The most common members of this Marchantia thalloid liverworts because of their flattened, lobed, somewhat leaf- like bodies (e.g. Marchantia sp.) They, however, constitute only 20% of the roughly 8,000 species of liverworts. Frullania Liverworts (Division Hepatophyta) cont….. superficially resemble mosses (e.g. Frullania The remaining 80% are “leafy” and sp,). than upright and the plant body is anchored Growth in the liverworts is prostrate rather by unicellular rhizoids. Structure and Form of Bryophytes III Hornworts (Division Anthocerotophyta) hornworts superficially resemble Although the gametophytes of Anthoceros those of thalloid liverworts, there are many features that separate hornworts from other bryophytes: chloroplast in each cell (a few Hornworts usually have only one species have up to eight).  Hornworts (Division Anthocerotophyta) cont…… They are often rosette-like in form. thalloid liverworts, hornworts have pores and In contrast to the air-filled pores and cavities of cavities filled with mucilage. The most common species belong to the genus Hornworts are also anchored by multicellular rhizoids. Anthoceros. Upright form Rosette form Land adaptations Possession of root-like rhizoids to absorb water from the soil. Surface of epidermis of the thallus is coated with water proof waxy coating (true cuticle is absent) to prevent water loss and protect against desiccation. Possession of stomata – like structures for gaseous exchange (true stomata absent). Possession of multicellular sex organs surrounded by sterile jackets to protect them from drying out. After fertilization, the zygote is left inside the archegonium to provide nutrition for sporophyte development Alternation of Generation All bryophytes require an external source of water (either as rain or dew) in order to reproduce. Bryophytes reproduce both sexually and asexually. Alternation of generations is a characteristic feature in their life cycle, with the gametophyte being the conspicuous and dominant generation. In all bryophytes, the sporophytes are permanently attached to the gametophytes, but vary in their dependence on them. Bryophytes have a well-developed alternation of heteromorphic generations (i.e. sporophyte/gametophyte). This type of life cycle is common among the living members of the Plant Kingdom.  The sporophyte is the generation which produces spores which germinate into the gametophyte. The gametophyte is the generation which produces male and female sex cells (gametes) which, during sexual reproduction, fuse into a zygote. The latter develops into a sporophyte. All plants are oogamous (i.e. exhibit sexual reproduction) and the zygotes of many plants are nutritionally dependent on the gametophyte generation. Gametophytic generation The gametophytic generation is the more conspicuous phase in the life cycle. It is long lived and the dominant phase. It is independent, green, fleshy and autotrophic. In primitive or lower forms, the gametophyte is undifferentiated, prostrate and thalloid e.g. Riccia spp. and Marchantia spp. In the advanced or higher forms, the plant body is erect, differentiated into stem-like axis, lateral appendages (leaf-like structures) and rhizoids (root-like structure) e.g. Mosses. Moss – Advanced Form  Marchantia – Primitive Form Marchantia Thallus - Gametophyte Female Thallus Male Thallus  Rhizoids help in anchorage and absorption and may be unicellular and unbranched or multicellular and branched. Sometimes multicellular scales may be present at the growing regions to protect them. The gametophytic plant body consists of parenchymatous cells only and thick-walled and lignified cells are completely absent in all phases.  Vascular tissues (xylem and phloem) are completely absent. Plant body sometimes internally differentiated into photosynthetic and storage zone. Mosses and liverworts have unisexual gametophytes. The gametophytes of some species of hornworts are bisexual. The gametophytes bear antheridia and archegonia which contain the male and female sex cells respectively. These fuse to form a zygote which grows into the sporophyte. Sporophytic Generation The sporophytic generation is the less conspicuous phase. It is short-lived and completely nutritionally dependent on the gametophyte. The sporophyte in moss is usually differentiated into foot, seta and capsule. The sporophyte develops from the embryo and it is the first stage in the diploid sporophytic generation. The sporophyte (foot, seta and capsule) is therefore a projecting structure from the gametophytic tissue. Life Cycle Reproduction is generally by vegetative and sexual means. Vegetative reproduction 1. fragmentation of thallus 2. production of gemmae in gemmae cups Production of gemmae in gemmae cups Asexual reproduction in liverworts (e.g. Marchantia sp.) is by means of gemmae (singular: gemma), which are produced in gemmae cups scattered all over the upper surface of the liverwort gametophyte. While in the cup, development of the gemma is inhibited by the presence of lunularic acid but, once out of the cap, each gemma is capable of growing into a new thallus. Fragmentation Asexual reproduction in hornworts is primarily by fragmentation. A piece that breaks off a gametophyte and then lands in a suitable habitat will grow into a new gametophyte. The breakage may be accidental, such as animal trampling or erosion leading to fragmentation of an existing bryophyte colony. Sexual reproduction Sexual reproduction is oogamous type. Sex organs are multicellular. They are located on both ventral side and dorsal side of the gametophyte. Structure and form of male sex organ The male sex organ is called an antheridium. It is stalked and globose, with one cell thick sterile jacket around it for protection. This sterile jacket surrounds fertile cells called androcytes/antherocytes which metamorphoses into antherozoids. These antherozoids are biflagellate and thus motile.  Female Gametophyte Male Gametophyte Structure and form of female sex organ The female sex organ is called archegonium. It is a flask shaped structure, having a basal swollen area called the venter and an elongated upper part called the neck. The venter and the neck are also surrounded by one cell thick sterile jacket for protection. Four to six neck canal cells and one venter canal cell are present Mode of fertilization Water is necessary for fertilization. When antheridia mature, their sterile jacket disintegrates and liberate the motile antherozoids to the surrounding thin film of water. When the archegonia mature, the neck canal cells and venter canal cells disintegrate and forms a mucilage mass. The antherozoids are attracted by chemicals present in the mucilage and move towards it by chemotaxis through the thin film of water. The antherozoids enter the archegonium through the neck and venter.  The antherozoids fuse with egg to form a diploid zygote. The zygote is the first stage in the diploid sporophytic generation. The zygote does not have any resting stage and divides mitotically immediately after fertilization. The first division is always transverse to produce two cells (outer cell and inner cell) the outer cell gives rise to the Embryo. The embryo develops within the ventre of archegonium. The embryo develops to form the young sporophyte inside the archegonium and differentiate into foot, seta and capsule.  Within the capsule of the sporophyte, the sporogenous cells divide meiotically to produce haploid spores. The spores are homosporous because they are similar in shape and size. Within the capsule are hygroscopic structures called Elaters that help to disperse the spores out of the capsule. The spores are non-motile and once out of capsule they are dispersed exclusively by wind. Under favorable conditions, the spores germinate to form the gametophyte In lower forms, the germination is by formation of a germ tube, globose forming one bud protonema which later divides to give rise to the younger gametophyte (Riccia, Marchantia). In the advanced forms (mosses), the spores germinate to form a filamentous forming many buds into branched protonema. From this protonema, many gametophytic plants arise.  ECONOMIC IMPORTANCE OF BRYOPHYTES In Ecological Restoration Some bryophytes and lichens are the first to colonize bare rocks after volcanic eruptions or other geological upheavals. They are, therefore, pioneers of succession and, by their gradual accumulation of minerals and organic matter, create conditions that favour the growth of other organisms. As Biological Indicators Some mosses grow only in calcium-rich soils whereas the presence of others indicates higher than usual soil acidity or salinity. Some mosses, when found growing in a dry area, are a good indication that the area receives running water during some part of the year.  As Packaging Material Some mosses have been used for cushioning in the packing of dishes and other breakable objects, while some have been used for stuffing furniture. For soil conditioning 1 kg of dry peat moss (e.g. Sphagnum sp.) is capable of absorbing up to 25 kg of water. Peat mosses are, therefore, very useful soil conditioners and have been used in nurseries and as components of potting mixtures. As Preservative Peat mosses have a natural acidity which inhibits fungal and bacterial growth. They have, therefore, been used in shipping live shellfish and other organisms.  For Soil Conditioning For Packaging  In Medicine The antiseptic and absorptive capacities of peat moss have made it a useful poultice material for application to wounds. This property of peat mosses was identified during World War I when a species of Sphagnum was used as a substitute for cotton bandages. It was observed that fewer infections developed in the wounds bandaged with Sphagnum. For Plant Propagation Sphagnum and other mosses have also been used in vegetative plant propagation, particularly for air layering. Air Layering