Plant Paleontology PDF

Plant Paleontology ‫تطور الكائنات الحيه‬ What is the meaning of Paleontology is the area of geology that examines fossils. Fossils are an important key in identifying and dating past environments and can also be extremely useful for geologic mapping. In Latin the word fossilis means “...

Plant Paleontology ‫تطور الكائنات الحيه‬ What is the meaning of Paleontology is the area of geology that examines fossils. Fossils are an important key in identifying and dating past environments and can also be extremely useful for geologic mapping. In Latin the word fossilis means “dug up.” In the nineteenth century when geology was still a young science, the word fossil was used for virtually anything that was pulled out of the ground- including artifacts and even Egyptian mummies.  fossils are the naturally preserved remains or traces of animals, plants and other organisms that lived in the geologic past. Since Egyptian mummies were preserved by humans and not through natural phenomena, they cannot be considered fossils.  Fossilization Is a natural process produced by nature, rather than by the intent of human beings, by which the animals or plants that existed in some earlier age; were turned to stones called fossils.  Conditions of fossilization: 1- The durability of the organism’s body (hard, internal or external body parts such as teeth, claws, shells, and bones) 2-The environment in which it lives (areas of high deposition, such as marine environments, will often be buried in a period of hours to days )  Types of fossils fossils 1-Body fossils 2-Trace fosssil 3- Chemical fossil 1-Body fossils : include the remains of organisms that were once living such as (Whole body of pleistocene mammoths which can be found at Siberia in ice with their hair, flesh and bloodm), teeth, bones, shell, and leaves 2-Trace fosssil: are the signs organisms were present or the results of their activity (i.e. footprints, tracks, trails, and burrows)  stages of the formation of footprint fossils. 1-organism steps into soft mud (usually silty sediment that is deposited in shallow temporary pools.) 2-the impression is covered with loose sand so that the footprint is filled 3-The sand eventually consolidates into sandstone and 4-the rock splits open along the bedding surface to reveal the original footprint in the shale and its cast in the sandstone.  3- Chemical fossil: it is the organic compounds they produce by biochemical processes, fossil fuels such as oil, gas and coal are chemical fossils. Lecture 2 Modes of organisms’ preservation: 1- Preservation without alteration: In rare instances no alteration can take place and the original parts, including soft body tissue, can be preserved intact. This occurs most often in cold or arid climates where an organism can be frozen in ice or desiccated in dry desert air. Desiccation is the process by which all water is removed from the organism. Examples of these processes include frozen mammoths that have been retrieved from ice in Siberia and early Egyptian remains that have been dried in the dry desert sands. 2-Preservation with alteration: Most fossils have been altered in some way from their original form. Some of the more common alterations that occur in fossils are permineralization, replacement, carbonization, recrystallization, and the production of molds and casts  Fossilization processes 1-Permineralization: is a process of fossilization that occurs when an organism is buried the empty spaces within an organism (spaces filled with liquid or gas during life) become filled with mineral-rich groundwater. Minerals precipitate from the groundwater, occupying the empty spaces. This process can occur in very small spaces, such as within the cell wall of a plant cell. Small scale permineralization can produce very detailed fossils. For permineralization to occur, the organism must become covered by sediment soon after death or soon after the initial decay process. The degree to which the remains are decayed when covered determines the later details of the fossil. Some fossils consist only of skeletal remains or teeth; other fossils contain traces of skin, feathers or even soft tissues. 2-Carbonization: is the complete change through chemical processes of the original plant or animal remains to a thin layer of carbon that outlines the original body shape. In some cases this process allowed the preservation of details in soft body tissue. 3-Recrystallization: is the process by which less stable forms of a mineral structure convert into more stable forms. This is most often seen in the shells of clams and snails. These shells are typically made of a form of calcium carbonate known as aragonite; over time the aragonite structure will slowly turn into the more stable calcite. 4-Freezing: It is a type of preservation in which the organism frozen. Such ideal remains are rare and almost always never very old. Like the hairy mammoth. These remains have preserved bone, skin, muscle, hair and even internal organs. 5-Drying or Dessication Remains of animals that have been found thoroughly dried include camel, ground sloth and even marsupial wolf. These remains were found in caves in arid and semi-arid areas of the Southwestern United States, South America, New Zealand and Australia. The dried dung of cave dwelling giant ground sloths have also been found in caves. 6-Petrification Petrification is a geology term denoting the processes by which organic material is converted into stone or a similar substance. It is approximately synonymous with fossilization. Petrified wood is the most well known result of this process. 7-the formation of molds and casts: Many of the fossils that you will observe in the field are actually the mold or cast of the organism. A mold: is formed when the hard body material of the buried organism is removed by dissolution. The print that is left behind will then retain the shape of previously existing body. This is similar to making a handprint in freshly poured concrete. Molds can express the external structure of the object. A cast : is formed when the initial object has dissolved, the mold becomes filled with sediment or mineral deposits, the original shape of the object can be regained in the form of a cast. lecture 3  The importance of fossils: 1-Peeking into the Past Fossil remains can give us informations about how prehistoric plants and animals obtained food, reproduced and even how they behaved. At times fossils can also provide evidence for how or why the fossil organism died.  2-Dating Layers of the Earth Geologists also use fossils for what's called biostratigraphic correlation, which allows researchers to match layers of rock in different locations by age based on how similar the fossils in each rock layer are. This information can be used to help understand when different layers of rock were formed even when large distances separate them.  3-Documenting Changes Fossils help in understanding how the Earth has changed over time. The type of fossil found in a particular location tells us what kind of environment existed when the fossil was formed. For example, if you find fossil marine animals like brachiopods in the sandstone in your backyard, you know that there must have once been an ocean where your house now stands.  4-Fossils and Oil Fossils also have practical and commercial applications. The oil used in our energy and plastics industries tends to collect in specific types of rock layers. studying the fossils that founded when digging oil wells can help workers locate oil and gas reserves. And of course, coal, oil and gas are themselves called "fossil fuels or chemical fossils" because they're formed from the organic remains of prehistoric organisms.  5-Evolution one of the most important functions of fossils is that they help in understanding evolution. Using information from fossil evidence, scientists can reconstruct body types of animals that no longer exist and put together a "Tree of Life" to describe the evolutionary relationships between organisms.  6-studying climate changes Study of fossils also helps to determine how the earth's climate and landscape have changed over time. For instance, presence of a fossil of a particular type of plant in a specific place, and belonging to a specific time period indicates the that the climate was suitable for existence of that type of plant. Plant fossils Plants Plants are an entire kingdom of life forms They are the most essential group of organisms in our world. Without plants, there is no life. Plant fossils, also, were very important to the evolution of the world as we know it. Fossilized plant remains are responsible for the world's vast coal deposits and possibly the huge oil reserves trapped in the Earth's crust. Fossil plants are usually found as individual parts, not complete These parts include leaves, fern fronds, cones, bark sections, spores, flowers and petrified wood.  Domain: Prokaryote Kingdom: Bacteria Bacteria: are microscopic single-celled organisms that can survive in diverse environments bacteria are prokaryotes. The entire organism consists of a single cell with a simple internal structure bacterial DNA floats free, in a twisted thread- like mass called the bacterial chromosome. Morphology of bacterial cells: lecture 4  Bacterial Endospores Microorganisms sense and adapt to changes in their environment. When favored nutrients are exhausted, some bacteria may become motile to seek out nutrients, or they may produce enzymes to exploit alternative resources or form endospores. Endospore: It is a dormant and highly resistant cell which allow the bacteria to preserve the cell's genetic material in times of extreme stress. Endospores can survive in environmental conditions that would normally kill the bacterium. These stresses include high temperature, high UV irradiation, desiccation, chemical damage and enzymatic destruction.  Endospore Structure There is an outer proteinaceous coat surrounding the spore provides much of the chemical and enzymatic resistance. Beneath the coat resides a very thick layer of specialized peptidoglycan called the cortex. Cortex which aids in resistance to high temperature. A germ cell wall resides under the cortex. This layer of peptidoglycan will become the cell wall of the bacterium after the endospore germinates. The inner membrane, under the germ cell wall, prevents the entry of damaging chemicals. The center of the endospore, the core, exists in a very dehydrated state and houses the cell's DNA, ribosomes and large amounts of dipicolinic acid. This endospore-specific chemical can comprise up to 10% of the spore's dry weight and appears to play a role in maintaining spore dormancy. Small acid-soluble proteins (SASPs) are also only found in endospores. These proteins tightly bind and condense the DNA, and are in part responsible for resistance to UV light and DNA-damaging chemicals. Other species-specific structures and chemicals associated with endospores include stalks, toxin crystals, or an additional outer glycoprotein layer called the exosporium.  Endospore Development The process of forming an endospore is complex. It requires several hours to complete. The bacterial cell divides asymmetrically. This results in the creation of two compartments, the larger mother cell and the smaller for spore. The peptidoglycan in the septum is degraded and the spore is engulfed by the mother cell, forming a cell within a cell. This is followed by the final dehydration and maturation of the endospore. Finally, the mother cell is destroyed in a programmed cell death, and the endospore is released into the environment. The endospore will remain dormant until it senses the return of more favorable conditions  Location of endospores: The position of the endospore differs among bacterial species and is useful in identification. 1-Terminal endospores are seen at the poles of cells 2- central endospores are more or less in the middle. 3-Subterminal endospores are those between these two extremes, usually seen far enough towards the poles but close enough to the center so as not to be considered either terminal or central. 4- Lateral endospores are seen occasionally.  Spore germination: The endospore released from the mother cell blown by air reaching a suitable medium germinates into a new bacterium  Domain: Eukaryote Kingdom: Fungi (Mycota)  Fungi: Are a group of eukaryotic organisms they are heterotrophs as they acquire their food by absorbing dissolved molecules, typically by secreting digestive enzymes into their environment. Most fungi grow as hyphae and some of them are unicellular like yeast  Hyphae or mycelium are cylindrical, thread-like structures 2–10 µm in diameter and up to several centimeters in length. Hyphae grow at their tips (apices) Hyphae can be either septate or coenocytic (non septate). Septate hyphae (B) are divided into compartments separated by cross walls (internal cell walls, called septa), with each compartment containing one or more nuclei coenocytic hyphae (A) are not compartmentalized. They may become noticeable and could be differentiated when fruiting. Lecture 5  What are the importance of fungi? 1-Fungi perform an essential role in the decomposition of organic matter, they are the principal decomposers in ecological systems, and have fundamental roles in nutrient cycling and exchange in the environment. 2-They have long been used as a direct source of food, in the form of mushrooms; as a leavening agent for bread; and in the fermentation of various food products, such as wine, beer, and soy sauce. 3-Since the 1940s, fungi have been used for the production of antibiotics, and, more recently, various enzymes produced by fungi are used industrially and in detergents. 4-Fungi are also used as biological pesticides to control weeds, plant diseases and insect pests. Many species produce bioactive compounds called mycotoxins, such as alkaloids and polyketides, that are toxic to animals including humans. Risk of fungi: Fungi can be significant pathogens of humans and other animals. Losses of crops due to fungal diseases (e.g., rice blast disease) or food spoilage can have a large impact on human food supplies and local economies. Fossil fungi: While fungi are not a common fossils, their fossils have not received a great deal of attention compared to other groups of fossils. Their fossils tend to be microscopic; very few large fungal bodies, such as mushrooms, have ever been found as fossils. Fossil fungi are often difficult or impossible to identify. The fungal filaments shown below are found in Cretaceous amber from north France, they resemble living filaments of the common ascomycete Candida. However, since there is little information on how this fossil organism lived or how it reproduced (both important in recognizing modern taxa), its true affinities may never be known. By contrast, the following Miocene fossil has preserved the perithecium, an enclosed reproductive structure. Features of the spores and the perithecium in which they occur suggest that this may be a fossil species of Savoryella. What are the importances of the fossil fungi? Fossil fungi from the Devonian-age have shown that fungi and land plants were forming symbiotic relationships even at that very early stage in terrestrial evolution. Fossil fungi showing that the fungi had successfully invaded the land and begun to diversify before the first vertebrates. The following image is to a section through a silicified stem of Aglaophyton from the Devonian age. The following image is for fungal spores from Lost Chicken Creek. The fungus Candida Classification: Kingdom: Fungi Division: Ascomycota Class: Saccharomycetes Order: Saccharomycetales Family: Saccharomycetaceae Genus: Candida  Candida is a genus of yeasts and is the most common cause of fungal infections worldwide like (candidiasis or thrush) in humans C. albicans is a diploid fungus populating the human body worldwide, inhabiting 80% of everyone's intestinal tract, colon, and mouth with no problems. It is unusual in that it is polymorphic, meaning it can grow as both a yeast and as filamentous cells. It is a popular cause of oral and vaginal infections ("thrush"), but is easily treated with common anti-fungals in people who are not immunocompromised. Structure, Metabolism, and Life Cycle C. albicans can take on either a unicellular (yeast) or multicellular (hyphae, pseudohyphae) form. The yeast form is 10-12 microns across, and is Gram-positive. While multicellular, the pseudohyphae are formed by yeast buds that attach to one another. Spores form on the pseudohyphae called chlamydospores. The form it takes depends on environmental cues, switching to the hyphae phase based primarily on temperature and pH changes. Additionally, C. albicans can switch between different phenotypes. The change is spontaneous and reversible, though possibly controlled by regulatory gene expression. In one form, the microbe is white, round cells in smooth colonies; the other form is opaque, rod-shaped in flat, gray colonies. This flexibility makes it highly adaptable as environments change. Reproduction The microbe is asexual, and does not perform meiosis. However, the cells of opposite mating types (different from the characteristic male/female form) perform cell fusion to create a tetraploid. This unit then undergoes a split to return to a diploid state. While splitting, chromosomes are randomly lost. Chromosomes are not typically exchanged, though some gene conjugation does occur to provide genetic diversity. For efficient mating, the microbes must switch from their white form (white and rounded cells forming dome-shaped colonies) to the opaque form (opaque, elongated cells forming a flatter colony). The opaque form is more efficient for mating than the white form. lecture 6  Fossil Algae Algae: are a large, diverse group of eukaryotic photosynthetic organisms, most are aquatic and autotrophic and their body called thallus. Algae are very important producers in the aquatic environment's food chain. Through photosynthesis, by using energy from sunlight, algae can convert carbon dioxide and important nutrient elements such as Carbon, Magnesium, Hydrogen, Oxygen, Potassium, Iodine, Nitrogen, Calcium and Iron into carbohydrates and proteins that sustain all animal life. Thallus: is an undifferentiated body which lack many of the distinct cell and tissue types, such as stomata, xylem, and phloem, which are found in land plants. Morphology of algae: Algae Included organisms range from 1- Unicellular algae: as the whole algal body is consist of one cell, such as Chlorella and the diatoms. 2- Multicellular algae: as the whole consist of more than one cell such as the giant kelp, a large brown alga which may grow up to 50 m in length. Multicellular algae may be: 1-Clonial algae: such as Gonium. 2-Filamentous algae: such as Spirogyra. 3-Membranous algae: such as Ulva. Algal pigments Algae contain many pigments such as: *Chlorophylls: a, b, c, and d which are green in color *Carotenoids: which are yellow, orange and red in color and they in the protection of the chlorophylls. *Phycobiliproteins: blue or red in color. Importance of algae: 1- Food for sea animals and fishes: The algae are used as a direct source of food by several sea animals and fishes. 2- Mineral contents: Algae contain high mineral content, which are very important for human and animal physiology. 3- Algae as a food for human: Several sea weeds and fresh water algae have been used as direct source of food to human beings. 4- Algae as a source of vitamins: The marine algae are the richest source of vitamins like vitamins A, B and E which are found abundantly in sea weeds. These vitamins are essentially required for the development of human body. 5- Algae as a source of agar: The best agar is manufactured from Gelidium of Rhodophyceae, which is also called vegetative agar. 6- Manufacture of iodine: The World’s iodine supply is fulfilled from the sea weeds. 7- Alginic acid, algin and mannitol: The alginic acid is manufactured from the cell wall of Phaeophyceae. 8-Medicinal use: algal extracts have many medicinal applications such as antiviral, anticancer, anti-inflammatory, antimicrobial…..etc. 9- Used as fertilizers: Due to the presence of potassium chloride (KC1) in sea weeds, they are used as fertilizers in many countries. 10- Manufacture of paper: It is probably thought that a rough quality of paper may be manufactured from sea weeds; as yet it is not practised. Importance of Algal fossils: Algae are very important fossils for many reasons: 1-They are important in helping geologists and paleontologists to understand the ancient environments of depositions and ecosystems that existed in the geologic past. 2-The kind of algae present in a rock can give the geologist some idea as to the depth of water in which the rock was deposited. 3-Fossil algae indicate that some wavelengths of light penetrate the water column deeper than other wavelengths s different species of algae photosynthesize at different wavelengths of light. For example, red wavelengths of light penetrate deeper than blue wavelengths so a species of algae that used only red wavelengths would suggest it lived in deeper water.  4-Fossil algae indicate the photic zone as all algae live in the photic zone which is the range of water depths that sunlight penetrates and photosynthesis takes place. By using carbon dioxide, the algae produces the carbonate (CO3) ion which is one of the the building blocks of calcite (CaCO3), the mineral component of limestone. What are calcareous algae? They are algae that have developed the ability to secrete or deposit calcium carbonate within or around their tissues. Remains of such algae have been found in appreciable numbers in most of the limestones of Cenozoic age that have been studied on Guam. At some localities, algae occur in sufficient abundance to be important contributors to the building of the limestones. Coralline algae: It is a red alga in the order Corallinales. it characterized by a thallus that is hard because of calcareous deposits contained within the cell walls. The colors of these algae are most typically pink, or some other shade of red, but other species can be purple, yellow, blue, white or gray- green. Coralline algae play an important role in the ecology of coral reefs. lecture 7 They are group of plants which characterized by some features such as: 1-The presence of archegonia: (female sex organ) 2- The presence of antheridium: 3-Alternation of generations:  The classification of archegoniatae: Archegoniatae are classified into three divisions: 1-Division: Bryophyta (mosses). 2-Division: Pteridophyta. 3-Division: Gymnosperms  Division: Bryophyta. Bryophytes are small, herbaceous plants that lack any vascular tissue, so they rely on simple diffusion to distribute water to their cells. They require a fairly damp environment to reproduce, as the sperm of one plant must be able to swim to the egg of another through a water layer, namely the mosses, hornworts, and liverworts General characters of Bryophyta: 1-gametophyte is dominant In life cycle.  2. True roots and leaves are absent as their bodies are either leafy or thalloid. 3. Vascular tissue (like xylem and phloem) is absent. 4. Archegonia with long neck having 6 vertical rows of cells. 5. Sporophyte is capsular and totally dependant on gametophyte. 6. The word bryophyte is the collective term for mosses, hornworts and liverworts  7-Bryology is the study of bryophytes. 8-Includes the simplest and most primitive land plants. 9-Terrestrial but need water to complete lifecycle 10-Mostly, grow on shady damp places 11-Gametophytic Plant Body (n) is thalloid and not differentiated into true roots, stem and leaves, it is green and possess chloroplasts to make photosynthesis, the roots are absent, replaced by unicellular or multicellular rhizoids.  12-The sporophyte dependent on the gametophyte it differentiated into foot, seta and capsule, it produce haploid spores. The spores fall on suitable substratum & germinate to produce gametophytic plant body. 13- Reproduction is always oogamous, male gamete is small and motile, female gamete is large and non-motile. 14-Antheridia are differentiated into stalk, and body Lecture 8  Classification of Bryophyta Class (1): Hepaticae. (the liverworts) Class(2): Musci (Mosses)  Class (1): Hepaticae. (the liverworts) Order(1): Marchantiales Family(1): Riccaceae. Ex: Riccia Family(2): Marchantiaceae. Ex: Marchantia. Order(2): Jungermanniales Sub_order:Anacrogynae Ex: Pellia Order(3): Anthocerotales Family: Anthocerotaceae. Ex: Anthoceros  General characters of liverworts 1- liverworts are restricted to moist environments for two principal reasons. – First, they lack a vascular system for efficient transport of water and food. – Second, their sperm cells must swim through water to reach the egg cells.  2- liverworts use chlorophyll-a, chlorophyll-b, and carotenoids as photosynthetic pigments and store their food reserves as starch. As in mosses and higher plants. 3-their cell walls are composed of cellulose. 4- The thalli of most liverworts have dorsiventral morphology. In other words, they have distinct front and back sides. In this respect, liverwort thalli are similar to the leaves of higher plants. 5- The name "liverwort" is centuries old and was given to these plants because their thalli are liver-shaped  6- Liverworts are small, green, terrestrial plants. They do not have true roots, stems, or leaves. Instead, they have an above ground leaf-like structure, known as a thallus, and an underground structure, known as a rhizoid. 7- The capsule is usually ovoid or spherical and does not have a lid; when ripe, it usually splits into 4 ‘valves’ to release the spores. 8- The seta is colourless and semi-transparent; it lengthens after the capsule has reached its full size, and is structurally much weaker than a moss seta. 9- Typically the thallus is attached to a substrate by means of unicellular rhizoids. Order(1): Marchantiales (Chambered Hepatics)  General characters of Marchantiales 1- The order Marchantiales is the largest order of thalloid forms only. 2-Many of them are remarkably physiological draught resistant and have evolved many anatomical devices to check the water loss. 3- This is the only group in the liverworts, which are basically adapted to grow in exposed sites with intense light. 4- The plant body is dorsiventrally flattened and dichotomously branched thallus, which is attached to the substratum by means of simple and tuberculate rhizoids. 5- The presence of ventral scales is a unique feature of this order.  6- The thallus is internally differentiated into upper assimilatory zone and lower storage zone. 7- The sex organs are always present on a specialized receptacle, which may be stalked or sessile. 8- Archegonial neck has six rows of cells. 9- The sporophyte is determinate in growth having well marked foot, short seta and the capsule. The seta has lost the ability to elongate. In extreme cases foot and seta is absent also (Riccia). The capsule has unistratose capsule wall, number of small to large spores of diverse sporoderm patterns and elongated elaters.  10- The order Marchantiales has been divided into 13 families and 29 genera. The 13 families are : 1. Cleveaceae 2. Aytoniaceae (Rebouliaceae) 3. Lunulariaceae 4. Conocephalaceae 5. Exormothecaceae 6. Marchantiaceae 7. Monoselaniaceae 8. Targioniaceae 9. Cyathodiaceae 10. Carrpaceae (Monocarpaceae) 11. Corsiniaceae 12. Oxymitraceae 13. Ricciaceae. In India the order is represented by 10 families and 21 genera. Family(1): Riccaceae. Genus: Riccia Systematic Position of Riccia: Distribution and Habitat of Riccia: Riccia, the most widely distributed genus of family Ricciaceae, is represented by about 200 species (Reimer, 1954). Widely distributed in both tropical and temperate regions of the world, this genus is represented in India by about 33 species (Puri, 1973). All species are terrestrial and prefer to grow on moist and shady places except Riccia fluitans, which is an aquatic species and occurs floating in water or submerged below the surface of standing water.  Gametophytic Phase The plant body of Riccia is gametophytic and gametophytes are fleshy, prostrate and dichotomously branched. Repeated dichotomy results into a typically rosette like appearance  Dorsal Surface: Ventral Surface: The dorsal surface is light The ventral surface of green or dark green body, thallus bears many scales each branch having a thick and rhizoids. Scales are midrib like a longitudinal violet coloured, groove which ends with an multicellular and one celled apical notch. thick structures. Scales are arranged all Growing point is situated in along the margin in a single the apical notch. The main row. function of the mid-dorsal groove is to retain water required for fertilization.  They are of two types: (i) Smooth-walled rhizoids: In which both the inner and outer wall layers are fully stretched (ii) Tuberculate rhizoids: in which the inner wall layer modifies into peg-like or plate-like in growth which projects into the cell lumen The main function of rhizoids is to anchor the thallus on the substratum and to absorb water and mineral nutrients from the soil.  Anatomy of the Gametophyte: A vertical cross section of the thallus shows two distinct zones, viz., upper photosynthetic zone and lower storage zone.  Upper Photosynthetic Zone: It is green, dorsal on upper region of the thallus. It is made of vertical rows of un- branched photosynthetic filaments. All the cells contain discoid chloroplasts. Photosynthetic filaments are separated from each other by narrow longitudinal vertical canals called air chambers. Each air chamber opens on the dorsal surface by an air pore. Air chambers spaces help in the gaseous exchange.  Lower Storage Zone: This zone represents the ventral tissue of the thallus and lies below the photosynthetic zone. It consists of compactly arranged parenchymatous cells. These cells are devoid of chlorophyll and contain starch as reserve food material. The lowermost cell layer of this zone forms the lower epidermis. Some cells of the lower epidermis extend to form the scales and both types of rhizoids.  Reproduction in Riccia: Riccia reproduces by: vegetative reproduction sexual reproduction  Vegetative Reproduction in Riccia: 1. Death and decay of the older portion of the thallus: The thallus in Riccia is dichotomously branched and the growing point is situated in its apical notch. The basal or part of the thallus starts rotting or disintegrating due to ageing or drought. When this process of decay reaches up to the place of dichotomy, the lobes of the thallus get separated. Thus detached lobes develop into independent plants by apical growth. It is the most common method of vegetative reproduction in Riccia. 2. By adventitious branches: The adventitious branches develop from the ventral surface of the thallus, these branches develop into new thalli 3. By persistent apices: Due to prolonged dry summer or towards the end of growing season the whole thallus in some species (e.g., Riccia discolor) dries and gets destroyed except the growing point. The growing point grows deep into the soil and becomes thick. Under favourable conditions it develops into a new thallus. It is more a method of perennation rather than multiplication. 4. By tubers: Towards the end of the growing season the apices of the thallus lobes get thickened and form the perennating tubers. These are capable to pass on the unfavorable conditions. On resumption of favourable conditions tubers produce new thalli. 5. By rhizoids: The apical part of the young rhizoids divides and re-divides to form a gemma like mass of cells in some species (e.g., Riccia glauca). These cells contain chloroplast and are capable of developing into new thallus.