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Unit – IV Morphology, structure and reproduction of the following: Anthoceros and Polytrichum. Evolution of sporophyte and gametophyte; ecological aspects. Fossil bryophytes. Economic importance of Bryophytes.
Economic importance of Bryophytes
- Protection from soil erosion: Bryophytes, especially mosses, form dense mats over the soil and prevent soil erosion by running water.
- Soil formation: Mosses are an important link in plant succession on rocky areas. They take part in binding soil in rock crevices formed by lichens. Growth of Sphagnum ultimately fills ponds and lakes with soil.
- Water retention: Sphagnum can retain 18 - 26 times more water than its weight. Hence, used by gardeners to protect desiccation of the seedling during transportation and used as nursery beds.
- Peat: It is a dark spongy fossilized matter of Sphagnum. Peat is dried and cut as cakes for use as fuel. Peat used as good manure. It overcomes soil alkalinity and increases its water retention as well as aeration. On distillation and fermentation yield many chemicals.
- As food: Mosses are good source of animal food in rocky and snow-clad areas.
- Medicinal uses: Decoction of Polytrichum commune is used to remove kidney and gall bladder stones. Decoction prepared by boiling Sphagnum in water for treatment of eye diseases. Marchantia polymorpha has been used to cure pulmonary tuberculosis.
- Other uses: Bryophytes arc used as packing material for fragile goods, glass wares etc. Some bryophytes act as indicator plants. For example, Tortell tortusa grow well on soil rich in lime. Ecological aspects and Economic importance of Bryophytes
- Ecological Importance Bryophytes are of great ecological importance due to following reasons: (a) Pioneer of the land plants. Bryophytes are pioneer of the land plants because they are the first plants to grow and colonize the barren rocks and lands. (b) Soil erosion. Bryophytes prevent soil erosion. They usually grow densely and hence act as soil binders. Mosses grow in dense strands forming mat or carpet like structure.
They prevent soil erosion by: (i) Bearing the impact of falling rain drops (ii) Holding much of the falling water and reducing the amount of run-off water.
- In Experimental Botany The liverworts and mosses play an important role as research tools in various fields of Botany such as genetics. For the first time in a liverwort, Sphaerocarpos, the mechanism of sex determination in plants was discovered.
- As Food Some Bryophytes e.g., mosses are used as food by chicks, birds and Alaskan reindeer etc.
ANTHOCEROS
Most species of Anthoceros, commonly known as ‘horned liverworts’, occur frequently along hillside roads and on very moist clay banks. They differ so greatly from the Liverworts that the present day workers separate them as a distinct Class co-ordinate with the Classes Hepaticae and Musci. Anthoceros is a cosmopolitan genus with about 200 species and is found both in the hills as well as in the plains, in tro•pical and temperate regions. There are about 25 species occurring in India, of which the commonly growing ones are A. himalayansis, A. erectus, A. crispulus, etc. The Gametophyte of Anthoceros The vegetative body of each plant is a small, dorsiventral and very simple, greasy dark- green gametophytic thallus, which is inconspicuously branched or somewhat lobed, and without any internal differen•tiation of tissues. There are numerous smooth-walled rhizo•ids on the under surface of the thallus, the scales being entirely absent. On the ventral side of the thallus there are nu•merous large intercellular spaces, each of which opens externally by a narrow slit. These cavities are usually filled up with mucilage and often contain colonies of an endophytic blue-green alga (e.g. Nostoc). Each cell of the thallus usually contains a single large chloroplast with a conspicuous pyrenoid, which is made up of numerous disc- or spindle-shaped bodies destined to be metamorphozed into small starch grains. Thus, it is evident that sim•plicity is the most prominent feature of the thallus in comparison, with those of Riccia and Marchantia. Reproduction in Anthoceros Anthoceros reproduces both by vegetative and sexual methods. The vegetative reproduction is usually effected by progressive growth and death of the thallus. Under certain conditions of prolonged desiccation the gametophyte often produces tubers, formed due to marginal thickenings. Each tuber is externally protected by a cork layer and under favourable conditions gives rise to a new thallus. Anthoceros is chiefly monoecious, though in some species the antheridia may attain maturity early (protandrous). It is a note•worthy feature that the sex organs are entirely embedded in the dorsal side of the thallus and not borne on special receptacles, as in Marchantia. Antheridia develop in clusters within closed ca•vities (antheridial cham•bers) just beneath the upper surface of the tha•llus. From the floor of each antheridial chamber two to four antheridia develop. The sterile layer, over-roofing each anthe•ridial chamber, may be one or more (usually two) cells in thickness. Each antheridium develops a stalk of several cells in height. Numerous biflagellate antherozoids are produced from each antheridium. When the antheridia attain maturity the sterile cell layers, over-roofing each antheridial chamber, disintegrate and the antherozoids are liberated. Archegonia develop singly and are closely embedded in the thallus. The vegetative cells of the thallus are confluent with a part of the neck and venter of each archegonium, the extreme end of the neck being only protruding. When fully developed, there is a single axial row of cells in each archegonium, consisting of four to six neck canal cells, a ventral canal cell and an egg cell. At matu•rity, the neck canal cells and the ventral canal cell disorganize and fertilization of the egg is brought about by one of the antherozoids passing down the neck into the venter. After fertilization
Each spore, under suitable conditions, germinates and forms a new gametophytic thallus. Evolved Characters
- The sporophytes contain a few chloroplasts by which it can manufacture a portion of its food hence it is partially dependent on the gametophyte for nutrition. It depends on the gametophyte for supply of water only for manufacturing food.
- In the Californian sp. of Anthoceros (A. fusiformis), the gametophyte sometimes disintegrates so the sporophyte becomes independent.
- Like higher plants stomata are present in the epidermis of the capsule.
- The columella acts as a conducting tissue and it seems that in higher plants the columella is replaced by conducting cylinders.
- The amphithecium forms the sporogenous tissue and the jacket layer, while the endothecium forms only the columella—a higher character.
- Presence of sterile filaments known as Pseudoelaters.
- Owing to the presence of the meristematic tissue at the base of the capsule (sporophyte) is indeterminate in its growth.
- Presence of single chloroplasts in the cell of the thallus suggests evolved characters, as in Selaginella, Isoetses, etc.
Type Polytrichum – Occurrence , Structure & reproduction life cycle
Occurrence Polytrichum have worldwide distribution. They are very common in cool temperature and tropical regions. Plants live in cool and shady places. General structure The main plant body is gametophyte. The adult plant consists of two parts: rhizome and upright leafy shoot.
- Rhizome: It is horizontal portion and grows underground. It bears three rows of small brown or colourless leaves. It also bears rhizoids. The cells are rich in protoplasm and oil globules.
- Upright leafy shoot: The leafy shoots are much longer. It is the most conspicuous part of the plant. It arises from rhizome. These branches consist of central axis. These branches bear large leaves arranged spirally.
- Leaves: Leaves have broad bases. Leaves in the upper portion are green. But the lower ones are brown. Each leaf has a broad. colourless sheathing leaf base and narrow distal limb. The mid-rib forms the major part of the leaf. These leaves possess extra photosynthetic tissue in the form of closely set vertical plates of green cells. These are known as lamellae. Green lamellae act as additional photosynthetic tissue. Leaf: Polytrichum have complex internal structure. The mid-rib region is thick. But the margins are only one cell thick. The lower surface is bounded by epidermis. One or two layers of sclerenchymatous tissues are present above the epidermis. The central tissue of leaf is composed of thin-walled parenchymatous tissues. Above this are again sclerenchymatous cells. The upper surface is formed of a layer of large cells from which arise numerous lamellae. This upper portion is the main photosynthetic region of the leaf. Stem: The T.S. of stem shows three regions: medulla, cortex and epidermis. The medulla is again differentiated into two zones: central zone and peripheral zone. The cortex consists of thick-walled cells. The innermost layer of cortex around the conducting strands is known as a mantle. Its cells contain starch grain. Epidermis is present over the cortex. Life cycle Vegetative reproduction I. Protonema: The spores germinate to form protonema. Several buds grow on the protonema. Each bud by of its apical cell develops into gametophyte.
- These are also called vegetative buds. They are formed on the rhizoids.
- Fragmentation: The rhizome gives rise to erect lea& shoots at intervals. Death or breaking of shoots separates the erect branches. These branches behave as independent plants. Sexual reproduction
- The primary stalk cell forms a massive stalk. The archegonial mother cell forms the main body of archegonium. It undergoes three vertical division s to form three peripheral cells surrounding an axial cell.
- Three peripheral cells divide to form 2 - 3 - layered jacket around the venter. The axial cell divides transversely to form inner central cell and outer apical cell.
- Czntral cell forms upper small venter canal cell and lover large egg cell. Apical cell divides to form long neck which consists of 6 vertical rows of cells. The cells cut off from the base foem neck canal cells. Fertilization The sex organs dehisce in the presence of water. The venter canal cell and the neck canal cells dissolve to form mucilage. This mucilage exerts pressure and the neck opens out. The mucilage comes out of the neck. The sperms reached the archegonial heads by rain water. They are attracted towards the archegonia. One of the sperm swims down the open neck and reaches the base. It fuses with the egg to form oospore. Oospore is the first stage of sporophytic generation. Sporophyte Development of Sporogonium
- The hypobasal region forms foot and lower part of seta. The foot region consists of thin-walled cells. It is embedded ir the stalk of the archegonium. The cells of the seta are larger and poor in cytoplasmic contents. 3. The epibasal region forms upper portion of seta and the capsule. Epibasal cell divides to form young embryo. Young embryo is cylindrical and completely surrounded by calyptra. Cells of the embryo divide to form amphithecium and the endothecium regions. 8- amphithecium cells are surrounded by a group of 4 - endothecium cells.
- Endothecium forms central conducting strands of apophysis. It forms columella and spore sac of theca. It also forms membranous tissues of the operculum. The outermost layer of endothecium forms archesporium or spore mother cells. These cells divide meiotically to form haploid spores.
- The amphithecium divides to form seven rings of cells. These cells give rise to spongy tissues and epidermis of apophvsis. They also form outer wall of theca. Structure of Mature Sporogonium (Sporophyte) The mature sporogonium is differentiated into foot, seta and capsule. Foot.: The foot is buried deep in the tissue of gametophyte. It is absorptive in function. It consists of thin-walled narrow cells containing dense cytoplasm. Seta: The seta is several inches long. It carries the capsule high into the air. It also conducts water and food. It consists of epidermis, cortex and central conducting strands. Capsule: The upper part is capsule. It is differentiated into three regions: apophysis, theca and operculum.
- Apophysis: It is the lower part of capsule. It is continuous with the seta. It is in the form of a swollen ring-like protuberance. Its cells are thin-walled, green and loosely arranged. The apophysis is the main photosynthetic region of the capsule.
- Theca: It is the middle part of the capsule. It is four-lobed. Its wall is several layered. The outermost layer is epidermis. Trabecular air spaces are present inside the wall layers. These spaces have filaments of thin-walled elongated cells containing chloroplasts. Outer spore sac wall is present internal to outer trabecular spaces. This is followed by spore-sac proper. Then 2 - layered inner spore- sac wall is present. Then inner trabecular air space is present. The centre is occupied by solid columella. All the sporogenous cells are fertile and form spores after reduction division.
- Operculum: This is the uppermost part of the capsule. It is conical. The operculum is covered by calyptra. The calyptra forms a hairy structure. So Polytrichum is also known as hair moss. A constriction is present between operculum and theta. A rim or diaphragm is present at the base of this constriction. The celumella of the theca is continuous into the operculum. It expands into a fan- shaped epiphragm.Peristome is present in the form of a thick rim. It bears a number of rigid teeth. The epiphragm fills the space inside the ring of peristome teeth and is attached to their tips. Peristome teeth arise from the rim or diaphragm. Dispersal of spores Cells of the epiphragm dry up during dry conditions. It separates the operculum. The calyptra falls. Epiphragm also dries up between the peristome pores. The central tissue of theca region except the spores degenerates. Thus the spores lie free in the centre of the capsule at maturity. Spores come out through pores. They are dispersed by wind. Structure and germination of spores The spores are yellow. Each spore is uninucleate and has two wall laye:s. The outer layer is exosporium (exine). The inner layer is endosporium. The spore germinates under favourable conditions. Exosporium ruptures and endosporium comes out. It forms prntonema. Protonema develops many buds. These buds produce new moss plants. Altrnation of generation Polytrichum shows heteromorphic alternation of generation. 3. Gametophyte: The plant body is gametophytes. Gametophyte is haploid. It develops antheridia and archegonia. Antheridia produce antherozoids and archegonium produces egg. Antherozoids fuse with egg to produce diploid oospore. 4. Sporophyte: The oospore is the first stage of sporophyte generation. It is diploid generation. Sporophyte has three parts: foot, seta and capsule. Haploid spores are produced in the capsule by meiosis. Spore is the first stage of gametophyte. Spores germinate to produce protonema stage. It gives rise to mature gametophyte completing the life cycle
(d) Funaria Sporophyte- In funaria major portion of the Sporophyte remain sterile to form the foot and Seta. The capsule is differentiated into central column of andothecium surrounded by many layered amphithecium. The inner layer of the endothecium forms the sterile columella and the superficial cell form the sporogenous tissue. Thus the archesporium arise from the outermost layer of cells of the endothecium. It is thus extremely reduced and consists of single layer of fertile tissues. The amohithecium become differentiated into the Epidermis , photosynthetic tissue of the capsule wall and the outer spore sac. The Bower theory of sterilization gives a clear explanation of the evolution of sporophye into upward direction. This theory is more conveincing and reliable.
- REDUCTION THEORY This theory was put forward by kashyap, church, Goebal and Evans. They hold that the evolution of sporophye has been in downward direction. They hold the fact that evolution of Sporophyte is retrogressive evolution. They mainly based their theory on the reduction of different organs which result in the simplification of Structure of Sporophyte. On the basis of this view the simplest type of Sporophyte of riccia is considered as the most advance one. Simplification of dehiscence appratus. Reduction of the green photosynthetic tissue in the capsule wall. Disappearence of stomata and intercellular spaces. Increase in the thickness of capsule wall. The gradual elimination of foot and Seta. All these changes accompanied by the progressive increase in the fertility of sporogenous cells. The changes eliminates the presence of sterile cells and elater in the capsule. Evidence from comparative morphology and experimental genetics support the view that the simple Sporophyte of riccia is an advanced but a reduced structure.
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This plant shows an axis, covered with helically arranged leaves. Sex organs, sporophyte capsules or rhizoids were not associated with the gameto•phytic plant. Several species of Muscites have been reported from the Upper Carboniferous of France and the Triassic of Africa. An extensive moss flora has been identified by Neuberg (1960) from the Permian rocks of Siberia, of which six identified genera (Intia, Salairia, Uskatia, Polyssaiuria, Bajdaieira and Buchtia) were placed under the Bryales and three (Protosphagnum, Vorentannularia and Jungajia) to a new order, the Protosphagnales. The genus Protosphagnum has leaves comparable to the modern genus Sphagnum, except for the pres•ence of a midrib. Ignatov (1990) described a diverse flora of well-preserved gametophytes of mosses from the Upper Permian of the Russian platform which are comparable to the modern forms like Dicranales, Pottiales, Funariales, Leucodontales and Hypnales. The permineralised well-preserved moss, Mercerea augustica, has been described by Smoot and Taylor (1986) from the Permian of Antarctica. The plant has a delicate axis to which are attached helically arranged leaves containing a midrib and rhizoids. Reproductive organs or sporophytes are not found associated with the plants. The external morphology and anatomy of the axes suggest its affinity with the Bryidae. Several compression fossils of true mosses have been described from the Mesozoic, of which Tricostium and Yorekiella from the Jurassic of the Bureja Basin, Russia and Aulacomnium heterostichoides from deep water varved clays (Eocene) of a fresh water lake in British Columbia. The well-preserved Aulacomnium heterosti•choides has been extensively studied which is very closely related to the present day living species, Aulacomnium heterostichum found in eastern North America and eastern Asia.
- Problematic Fossil Bryophytes The Lower Devonian compression fossil Sporogonites is one of the oldest plants that resembles a bryophyte. The plant consists of many parallel-oriented sporangial stalks that ter•minate in elongate capsules, developed from a common thallus. The sporangium is multilayered and possibly contains a central columella. Numerous trilete spores are present in the spo•rangium. Sporogonites has been considered to be an early hornwort or gametophyte-bearing sporophyte of a moss. A Precambrian bryophytic fossil, Longfengshania, has been described from China which shows striking similarity with Sporogonites. This unusual fossil of Precambrian age makes it doubtful about the validity of its systematic position. Tortilicaulis is an early Devonian fossil described from South Wales that shares a few morphological features common with the modern liver-worth Pellia.
