BIO 102 ZAMSUT LECTURE 2024.pdf

DEPARTMENT OF BIOLOGY FACULTY OF SCIENCE ZAMFARA STATE UNIVERSITY TALATA AMAFARA (ZAMSUT) BIO 102 (GENERAL BIOLOGY II) FIRST PART LECTURE NOTE SECOND SEMESTER 2023/2024 ACADEMIC SESSION COMPILED BY: MAL. SUFIYANU SAMINU...

DEPARTMENT OF BIOLOGY FACULTY OF SCIENCE ZAMFARA STATE UNIVERSITY TALATA AMAFARA (ZAMSUT) BIO 102 (GENERAL BIOLOGY II) FIRST PART LECTURE NOTE SECOND SEMESTER 2023/2024 ACADEMIC SESSION COMPILED BY: MAL. SUFIYANU SAMINU Page 1 of 36 PLANT KINGDOM (PLANTAE) Kingdom Plantae includes all the plants. They are eukaryotic, multicellular and autotrophic organisms. The plant cell contains a rigid cell wall. Plants have chloroplast and chlorophyll pigment, which is required for photosynthesis. Characteristics of Kingdom Plantae The plant kingdom has the following characteristic features: 1. They are non-motile. 2. They make their own food and hence are called autotrophs. 3. They reproduce asexually by vegetative propagation or sexually. 4. These are multicellular eukaryotes. The plant cell contains the outer cell wall and a large central vacuole. 5. Plants contain photosynthetic pigments called chlorophyll present in the plastids. 6. They have different organelles for anchorage, reproduction, support and photosynthesis. Types of Plants The following are types of plants which include the following: Non-vascular Plants The first classification of plants is the non-vascular plants; as their name implies, non-vascular plants lack vascular tissues that can help them transport water and nutrients. Non-vascular plants are considered to be the earliest living plants on the planet. Example Algae, Bryophytes. Vascular Plants Vascular plants have been allowed by evolution to possess vascular tissues (xylem and phloem) that aid them to transport water and minerals. Plants, like the members of the Pteridophyta, Gymnosperms, and Angiosperms, are classified as vascular plants. Page 2 of 36 Classification of Kingdom Plantae Classification of plants is based on the following criteria: 1. Plant body: Presence or absence of a well-differentiated plant body. E.g. Root, Stem and Leaves. 2. Vascular system: Presence or absence of a vascular system for the transportation of water and other substances. E.g. Phloem and Xylem. 3. Seed formation: Presence or absence of flowers and seeds and if the seeds are naked or enclosed in a fruit. The following are classification of plant kingdom, these include: Thallophytes, bryophytes, Pteridophytes, gymnosperms and angiosperms. THALLOPHYTES Thallophytes are also referred to as thallophyta or thallobionta. Thallophytes are identified and classified based on the presence of identical or similar characteristics. The body of thallophytes does not contain a vascular system and hence conducting tissues are absent. The types of living organisms falling under thallophytes include algae, slime molds, fungus, and lichens. Thallophyta is a division of the plant kingdom including primitive forms of plant life showing a simple plant body. Including unicellular to large algae, fungi, lichens. Thallophytes are a polyphyletic group of non-motile organisms traditionally described as ―thalloid plants‖, ―relatively simple plants‖ or ―lower plants‖. Characteristics of Thallophytes 1. They are mostly autotrophic plants because most members of this group prepare their own food spontaneously. But some members such as fungi are dependent on other sources of food. 2. After the process of photosynthesis, glucose is developed and consumed almost instantly, the remaining glucose is transformed into complex compounds known as starch. The plant reserved food in the form of starch. 3. Their sexual reproduction occurs through the fusion of two gametes. There may or may not be any changes of generation present or available. The life cycle may be categorized into diplohaplontic, diplontic, or diploid. Page 3 of 36 4. The sex organs of thallophytic are simple and single-celled, there is no embryo formation after fertilization. 1. ALGAE Algae are photosynthetic organisms that possess photosynthetic pigments such as chlorophyll. However, they lack true roots, stems, and leaves characteristic of vascular plants. Some of them are unicellular whereas others are multicellular. They may also form colonies. Most algae are aquatic. Others are terrestrial and may be found on moist soil, trees, and rocks. Some algal species form symbiosis with other organisms. For example, a lichen is a symbiotic association between fungi and green algae. Algae belong to a polyphyletic group. This means that the organisms in this group are not necessarily closely related and do not have a common ancestor. However, they share a common feature; they are eukaryotes capable of photosynthesis with chlorophyll as their primary pigment but they lack other morpho-anatomical features common among vascular plants. The scientific study of algae is called phycology. Characteristics of Algae 1. Algae are either photosynthetic organisms or unicellular or multicellular organisms 2. Algae has not a well-differentiated body structure such as roots, stems, or leaves. 3. Algae are placed in adequate moisture areas. 4. Algae reproduction can take place in both modes, asexual and sexual. Sporulation is the method used for asexual reproduction. 5. Algae can make up symbiotic relationships with other organisms, but most are independent living things. Classification of Algae 1. Green algae (Chlorophyta) Green algae refer to any of the photosynthetic algae characterized by containing the pigments, chlorophyll a and chlorophyll b. They store food as starch within plastids. They have diverse forms: unicellular (e.g. Micrasterias sp.), multicellular, or colonial. Multicellular forms are those that appear filamentous or form leaf-like thallus (e.g. Ulva sp.). Some of them form colonies, such as Volvox sp. Green algae include charophytes (mostly in freshwater habitats) and chlorophytes (mostly marine). There are also green algae that live in terrestrial habitats (e.g. soil, Page 4 of 36 rocks, and trees). Certain green algal species have been found to form symbiosis on land. For instance, Chlorella sp. forms a symbiosis with Hydra sp. Red algae (Rhodophyta) Red algae are those belonging to the phylum Rhodophyta. They are characterized by their reddish color due to the presence of accessory pigments, such as phycoerythrin, phycocyanin, and allophycocyanins in phycobilisomes, aside from the chlorophyll. Examples of red algae are Rhodella, Compsopogon, Stylonema, Bangia, Porphyra, Hildenbrandia, Nemalion, Corallin a officinalis, Ahnfeltia, Gelidium, etc. Brown algae (Phaeophyta) Brown algae include those of the phylum Phaeophyta. They are characterized by their brown or greenish-brown color due to the presence of brown pigments, such as fucoxanthin, in addition to chlorophyll. Apart from Phaeophyta, other phyla that have predominant brown pigment apart from chlorophyll are Dinoflagellata (dinoflagellates) and olive-brown Bacillariophyta (diatoms). Golden algae (Chrysophyta) Golden algae are those belonging to the phylum Chrysophyceae. They are distinguished mainly by the presence of two specialized flagella wherein one has mastigonemes and the other is smooth. Prymnesium parvum is one of the well-known golden algae due to its association with fish kills. Blue-green algae (Cyanophyta) The blue-green algae include the members of the Cyanophyta. They are not considered by other references as algae since they are prokaryotes. Nevertheless, they are similar to other algal species in being photosynthetic due to the presence of chlorophyll. Aside from this pigment, the blue-green algal cell also has phycobiliproteins that make them blue-green in color (thus, the name) Page 5 of 36 Reproduction in Algae Reproduction in Algae are categorized in to three, Vegetative, asexual and sexual. 1. Vegetative Reproduction The vegetative reproduction in algae takes place by following methods: (i) Fragmentation: Fragmentation is common in filamentous forms. In this process, filament breaks into fragments and each fragment give rise to a new filamentous thallus. The common examples are Ulothrix, Spirogyra, Oedogonium, Zygnema, Oscillatoria, Nostoc etc. (ii) Fission: This process is common in desmids, diatoms, and other unicellular algae. The cell divides into two by mitotic division and then separation occurs through septum formation. (iii) Adventitious branches- Protonema develops in certain algae like Chara and give rise to new thalli when detached from parent thallus. These adventitious branches develop mainly on the rhizoids. Other examples include Dictyota and Fucus. vi) Budding- In some algae like Protosiphon, budding takes place which results in new individuals. (v) Hormogonia- In some cyanobacteria like Nostoc, Cylindrospermum hormogonia develop that may give rise to new thalli. These hormogonia are of varying lenths and may develop at the place of heterocysts in the thallus. These hormogones are produced by breakage of filament into two or more cells. 2. Asexual Reproduction: In a large number of algae asexual reproductions takes place with the help of different kind of spores and other structures. Basically, spores are meant for asexual reproduction and each spore can grow into a new thallus. Spores are one cellwd structure and are produced internally in the case of algae. They are produced within the vegetative cell (Chlamydomonas) or in a specialized structure called sporangia. They may be motile or non- motile. Motile spores are called zoospores and non-motile as aplanospores. Different types of asexual spores and structures are as follows: (i) Akinetes: In filamentous forms, certain vegetative cells become thick walled elongated structures called as akinetes. Akinetes are perrenating bodies that can survive under unfavourable Page 6 of 36 conditions and can give rise to new individual on occurrence of favourable conditions. e.g., Anabaena (ii) Hypnospores: Hypnospores are thick walled, non flagellated spores with plenty of food reserves. They are perrenating structures. Hypnospores are produced under unfavourable conditions by some green algae. They germinate into new plants with return of favourable environmental conditions. e.g., Chlamydomonas, Protosiphon. In Chlamydomonas nivalis the walls of hypnospores become red due to the presence of pigment Haematochrome due to which snow becomes red. (iii) Zoospores: These are flagellated asexual spores which are formed in zoosporangium or directly from the vegetative cells. The zoospores may be bi, quadric or multiflagellate. The multiflagellate zoospores are of again two typesflagella arranged on entire length of body or arranged in a ring surrounding a beak like projection. e.g., Chlamydomonas (biflagellate), Ulothrix, Cladophora (quadriflagellate), Vaucheria, Oedogonium (multiflagellate). In Pediastrum, the zoospores do not germinate or divide but orientate themselves in a single plane and become opposed to form a colony just like the parent cell. This feature is not met in any other algae. (iv) Aplanospores: These are non flagellated thin walled asexual spores that are formed in majority of aquatic algae by the failure of flagella formation due to some unfavourable conditions. 3-Sexual Reproduction: Sexual reproduction is found in advanced algae as compared to less advanced forms where vegetative and asexual methods are main modes of reproduction. Sexual reproduction takes place by fusion of gametes of different sexuality. There is a wide range of variation in the nature of gametes and the mode of sexual reproduction. Any vegetative cell of thallus may produce gametes and thus behave as gametangium or a specialized gametangium may be developed. The gametangia may be morphologically similar (isogametangia) or dissimilar (heterogametangia). The gametes are produced in the gametangia by simple mitotic division or by reduction division. The haploid gametes fuse to make diploid zygote that give rise to the thallus. Depending upon the morphological and physiological characteristics of gametes, sexual reproduction can be of the following types- Page 7 of 36 (i) Isogamy: When fusing gametes are morphologically similar and physiologically different (+ and -) then the sexual reproduction is called as isogamous. E.g., Chlamydomonas, Ulothrix, Zygnema, Spirogyra. (ii) Anisogamy: In anisogamous sexual reproduction fusing gametes are morphologically as well as physiologically different. The gametes are produced in different gametangia. The microgametes are male gametes while macrogametes are female gametes. e.g., Chlamydomonas. (iii) Oogamy: Oogamy is the most advanced type of sexual reproduction in which microgamete or male gamete fuses with a large female gamete or egg. Male gametes are produced in antheridium while female gamete or egg is produced within a structure called as oogonium. During fertilization male gamete reaches the oogonium to fertilize the egg and a diploid zygote is formed. e.g., Chlamydomonas. (iv) Hologamy: In certain unicellular algae whole thallus behaves like gamete and in this process fusion takes place between opposite strained gametes or thalli that after fusion make diploid zygote. e.g., Chlamydomonas. (v) Autogamy: In autogamy fusion between two gametes of opposite strains from same mother cell takes place. Since both the fusion gametes comes from same mother cell there is no genetic recombination e.g., Diatom Economic Importance of Algae 1. Agricultural importance: Blue green soil algae are very important in agriculture as they are capable of nitrogen fixation in the soil. Some important soil cyanobacteria are Tolipothrix tenius, Aulosira fertilissima, Anabaenopsis, Oscillatoria, Anabaena, Nostoc, Spirulina and Cylindrospermum. 2. Role of Algae as food and fodder: Algae synthesize organic food stuffs and are an important food source of fishes and other aquatic animals. As the flesh of the land is dependent upon the activities of the green leaf, so the fish and other aquatic forms of animal life are dependent, directly or indirectly, upon algae. Diatoms, filamentous and some planktonic green algae, and a number of blue-green algae are very often found in the guts of various species of fresh and brackish water fish and they appear to be directly utilized as fish food. Hence, algae are very important source for pisciculture Page 8 of 36 4. Role of algae in medicine: Algae has been used for medicinal purposes since time immemorial. Ancient literature of China revealed the use of Laminaria sp. For the treatment of goiter. Brown algae being rich source of iodine are employed in the preparation of medicines for goiter. Members of Laminariales have long been used as a surgical tool and also during child birth to expand the cervix. An antibiotic chlorellin is obtained from Chorella. Agar agar is an important algal product obtained from red algae used in the manufacture of pills and ointments by pharmaceutical industries. Carrageenin and alginic acid acts as blood coagulant. Extracts of Digenea, Codium, Alsidium and Durvillea are effective vermifuge. In Unani medicine system many algae are used in the treatment of lung, kidney and bladder ailments. Extracts of Cladophora and Lyngbya possess antiviral properties 5. Algae as the origin of petroleum and gases: It is an accepted fact that the fuels such as petroleum and gases have their origin in the organic matter in the marine environment. Planktons captured the energy from sunlight during photosynthesis and transferred to the marine animals in the form of food. Organic compounds derived from the planktons and the animals accumulated in the mud deposits in the shallow water of the ocean floor. In the source, materials were buried by sedimentary action in an oxygen free environment and converted gradually into oil and gas. 6. Algae and limestone formation: Many species of algae withdraw calcium from water, both fresh and salt and deposit it in the form of calcium carbonate, in their cell walls. The most significant forms for this purpose are the blue green algae, red algae and to some extent dinoflagellates. Fresh water blue green algae are chiefly responsible for the extensive formation of limestone deposits around hot springs and glaciers. The red algae are the most important calcareous algae of the seas and in particular they play an important role in the formation of coral reefs together with the nedarians (minute sedentary animals that are responsible for the construction of coral reefs). 7. Role of algae in industries: Algae being source of many commercial products are very useful in industries. The major products derived from algae are agar agar, carrageenin, algin, diatomite and kelp. The industrial utilization of algae particularly sea weeds have been known from hundred of years. 8. Toxic algae are those that causes sickness or death to aquatic life particularly animals. Some algae are known to secrete harmful/neurotoxic substances that causes illness or death of aquatic animals. Due to algal poisoning, several cases of death from different parts of world have been Page 9 of 36 recorded. Blue green algae Microcystis toxica and Anabaena flos-aquae are responsible for death of around thousands of cattles, sheeps and other animals in South Africa. Gymnodinium veneficum is known to be toxic to aquatic animals along the coasts of California and Washington. Not only the animals but human beings are also get affected by algal toxins. Gastrointestinal disorders, severe illness and even death have been reported. 2. FUNGI Fungi is defined as one of the five kingdom classification of living organisms. It includes microorganisms such as yeast and moulds. They are cosmopolitan and occur in air, water, soil and on animals and plants. Some fungi cause diseases in plants and animals whereas some are used for different economic purposes. Structure of Fungi The structure of fungi can be explained in the following points: 1. Almost all the fungi have a filamentous structure except the yeast cells. 2. They can be either single-celled or multicellular organisms. 3. Fungi consist of long thread-like structures known as hyphae. These hyphae together form a mesh-like structure called mycelium. 4. Fungi possess a cell wall which is made up of chitin and polysaccharides. 5. The cell wall comprises a protoplast, which is differentiated into other cell parts such as cell membrane, cytoplasm, cell organelles and nuclei. 6. The nucleus is dense, clear, with chromatin threads. The nucleus is surrounded by a nuclear membrane. Characteristics of Fungi Following are the important characteristics of fungi: 1. Fungi are eukaryotic, non-vascular, non-motile and heterotrophic organisms. 2. They may be unicellular or filamentous. 3. They reproduce by means of spores. 4. Fungi lack chlorophyll and hence cannot perform photosynthesis. 5. Fungi store their food in the form of glycogen. Page 10 of 36  6. The nuclei of the fungi are very small. 7. The mode of reproduction is sexual or asexual. 8. Some fungi are parasitic and can infect the host. Classification of Fungi Kingdom Fungi are classified based on different modes. The different classification of fungi is as follows: 1. Based on Mode of nutrition On the basis of nutrition, kingdom fungi can be classified into three (3) groups. 1. Saprophytic: The fungi obtain their nutrition by feeding on dead organic substances. Examples: Rhizopus, Penicillium and Aspergillus. 2. Parasitic: The fungi obtain their nutrition by living on other living organisms (plants or animals) and absorb nutrients from their host. Examples: Taphrina and Puccinia. 3. Symbiotic: These fungi live by having an interdependent relationship with other species in which both are mutually benefited. Examples: Lichens and mycorrhiza. Lichens are the symbiotic association between algae and fungi. Here both algae and fungi are mutually benefited as fungi provide shelter for algae and in reverse algae synthesis carbohydrates for fungi. Mycorrhiza is the symbiotic association present between fungi and plants. Fungi improve nutrient uptake by plants, whereas, plants provides organic molecules like sugar to the fungus. 2. Based on Spore Formation Kingdom Fungi are classified into the following based on the formation of spores: 1. Zygomycetes – These are formed by the fusion of two different cells. The sexual spores are known as zygospores, while the asexual spores are known as sporangiospores. The hyphae are without the septa. Example – Mucor. Page 11 of 36  2. Ascomycetes – They are also called sac fungi. They can be coprophilous, decomposers, parasitic or saprophytic. The sexual spores are called ascospores. Asexual reproduction occurs by conidiospores. Example – Saccharomyces. 3. Basidiomycetes – Mushrooms are the most commonly found basidiomycetes and mostly live as parasites. Sexual reproduction occurs by basidiospores. Asexual reproduction occurs by conidia, budding or fragmentation. Example- Agaricus. 4. Deuteromycetes – They are otherwise called imperfect fungi as they do not follow the regular reproduction cycle as the other fungi. They do not reproduce sexually. Asexual reproduction occurs by conidia. Example – Trichoderma. Reproduction Fungi reproduce sexually and/or asexually. Perfect fungi reproduce both sexually and asexually, while imperfect fungi reproduce only asexually (by mitosis). In both sexual and asexual reproduction, fungi produce spores that disperse from the parent organism by either floating on the wind or hitching a ride on an animal. Fungal spores are smaller and lighter than plant seeds. The giant puffball mushroom bursts open and releases trillions of spores. The huge number of spores released increases the likelihood of landing in an environment that will support growth. Asexual Reproduction Fungi reproduce asexually by fragmentation, budding, or producing spores. Fragments of hyphae can grow new colonies. Mycelial fragmentation occurs when a fungal mycelium separates into pieces with each component growing into a separate mycelium. Somatic cells in yeast form buds. During budding (a type of cytokinesis), a bulge forms on the side of the cell, the nucleus divides mitotically, and the bud ultimately detaches itself from the mother cell. The most common mode of asexual reproduction is through the formation of asexual spores, which are produced by one parent only (through mitosis) and are genetically identical to that parent. Spores allow fungi to expand their distribution and colonize new environments. They may be released from the parent thallus, either outside or within a special reproductive sac called a sporangium. There are many types of asexual spores. Conidiospores are unicellular or multicellular spores that are released directly from the tip or side of the hypha. Other asexual spores originate in the fragmentation of a hypha to form single cells that are released as spores; some of these have a thick wall surrounding the fragment. Page 12 of 36 Sexual Reproduction Sexual reproduction introduces genetic variation into a population of fungi. In fungi, sexual reproduction often occurs in response to adverse environmental conditions. Two mating types are produced. When both mating types are present in the same mycelium, it is called homothallic, or self-fertile. Heterothallic mycelia require two different, but compatible, mycelia to reproduce sexually. Although there are many variations in fungal sexual reproduction, all include the following three stages. First, during plasmogamy (literally, ―marriage or union of cytoplasm‖), two haploid cells fuse, leading to a dikaryotic stage where two haploid nuclei coexist in a single cell. During karyogamy (―nuclear marriage‖), the haploid nuclei fuse to form a diploid zygote nucleus. Finally, meiosis takes place in the gametangia (singular, gametangium) organs, in which gametes of different mating types are generated. At this stage, spores are disseminated into the environment. Fig 1: Generalized Life cycle of Fungi Page 13 of 36 ECONOMIC IMPORTANCE OF FUNGI 1. Mold and yeast are important in food spoilage. It is a fact that, molds are involved in the spoilage of many foods, some are important in food manufacture especially in mold ripened cheese and in preparation of oriental foods. 2. Some species of yeast Saccharomyces cerevisae are used in fermentation processes especially in the production of beer from grapes germinating grains of some cereals. Yeast is involved in the production of alcohol from palm wine. 3. Other fungi are used in the manufacture of foods. For example, when yeast is added to fruit juice it ferment the juice to produce wine. Yeasts also are used in the manufacturing of beer, and they are added to dough to make bread rise, producing more volume and lighter texture in the final baked product. 4. Some fungi such as Agaricus bisporus and Valuariella volvacea are used as food, some others such as Yeast contain high proportion of protein and vitamins are sources of food supplement for man and animals. 5. Many fungi produce biologically active compounds that are useful in manufacturing industries. These compounds include alcohols- such as ethanol and glycerol produced during fermentation and plant growth regulators such as giberellic acid which is used in the promotion of plant and fruit development. Fungi are extremely important in the production of antibiotics for examples penicillin, Griseofulvin, cyclosporine and cephalosporin are used to fight bacterial and fungal diseases world wide. 6. Fungi such as Saccharomyces cerevisae, Aspergillus, Rhizopus are used in the production of industrial alcohol, citric acid, fumaric acid, gluconic acid, lactic acids and enzymes such as lipases, proteases, pectinase etc. 7. Fungi are becoming an important tool in cleaning the environment. The accumulation of pesticides and other chemicals in the environment is destroying many ecosystems and placing many animal and plant species at risk. A number of fungi are used in the bioremediation in which the fungi are mixed with polluted water or soil where they decompose the organic material in pollutants and in the process detoxify them. Fungi employed in this effort include many that are commonly found in soils such as Aspergillus, Fusarium, Rhizopus, Mucor, Penicillin and Trichoderma. 8. Fungi also have been used successfully to control insects, fungus pathogens, round worms and other organisms that cause damage and disease to agricultural crops. Page 14 of 36 9. Fungi cause a number of human respiratory diseases. Coccidioidomycosis is caused by the yeasts Coccidioides immitis. Although typically contracted by the inhalation of dust containing yeast spores, the fungus may also be introduced through the skin from infected soil. 10. Candida albicans is the cause of various diseases in man. Various types of candidiasis of humans are skin candidiasis, bronchocandidiasis, pulmonary candidiasis and vulvovaginal candidiasis. 11. Mycotoxins are poisons produced by fungal growth in cereals, nut, fruits and vegetables. More than 100 species of fungi produce these toxin, produced by Aspergillus flavus and Aspergillus parasiticus. Commonly found on corn, peanuts, and tree nuts, the toxins also can be transmitted to humans through the milk, meat or eggs of animals fed contaminated grains. 12. Ring worm infection of the skin, hair ir nails caused by various kinds of fungi that belong to the genera Trichophyton, Epidermophyton and Microsporum. Ring worm tends to infect moist areas of the body such as groin between the toes and under the arms. Ring worm may also affect the finger nails causing thickening and deformation. When found on the feet, ringworm is called athletes foot. 13. Fungi also cause disease in plants. Fungus diseases were responsible for several catastrophe in various parts of the world. Fungus attack leaf, stem, roots and fruits including seeds. 14. Equally large numbers of fungi in other groups produce large array of diseases characterized by leaf spots, ulcerous lesions, blights, powdery and downy mildews, cancers, wood rots and strains, root, rots wilts, club root, and various other symptoms. 15. Aspergillosis is an infection of skin, nasal sinuses, and lungs or other internal organs caused by molds of the genus Aspergillus. The disease, contracted by inhalation of spores, occurs most often among agricultural workers. Itching and pain are frequent symptoms if the scratching is prolonged, the skin may thicken and become gray or black. Page 15 of 36 BRYOPHYTES Bryophytes are plants that are found growing in moist and shady places. Something unique about these plants is that they can survive on bare rocks and soil. They play an important role in plant succession on bare rocks. They show alternation of generations and have a unique nickname. So they are called the amphibians of the plant kingdom. Though they grow in a terrestrial environment, they are dependent on water for the reproduction process. Bryophytes thrive in a wide variety of habitats, range of elevations, temperatures and moisture. They can be seen growing in shady and damp environments, in extreme and diverse habitats such as arctic and deserts regions. It can grow where vascularized plants cannot as it does not depend on roots for the uptake of nutrients from the soil. Some of the bryophytes remarkably tolerate lengthy periods of freezing and dry conditions and when moisture returns, the process of photosynthesis resumes. Several bryophytes thrive on persistent remains of its own growth or on soil and on decomposing or living matter of other plants. A few grow on bare surface of rock and some are aquatic in nature. Characteristics of Bryophytes 1. Bryophytes live in humid and shaded places. But, they can be also found in arid forests, rainforests, apart from the alpine habitats. 2. They are widely distributed throughout the world. You can see them growing on varied surfaces like rocks, soil, tree trunks, bones, rotting wood etc. 3. The tissue organization is not complex. But, they do display some amount of diversity. Some bryophytes maybe just over a millimetre long and some are a meter long too. 4. They contain chlorophyll and can synthesize food through the process of photosynthesis. 5. The plant body is slightly differentiated, though they do not have true roots, stems, and leaves. They generally have something similar to roots called the rhizoids. The main body of the bryophytes is more thallus like and haploid. Page 16 of 36 6. Bryophytes are spore-producing plants that do not have a vascular tissue. They reproduce through gametes and hence have earned the name gametophytes. The sex organs of bryophytes are multicellular. 7. Anthredium is the male sex organs while the female sex organ is called archegonium. The archegonium is a flask-shaped organ and produces a single egg. The anthredium produces antherozoids, which have two flagella. Classification of Bryophytes 1. Hepaticopsida (Liverworts): The word ―hepatic‖ means ―liver,‖ hence the term Hepaticopsida means ―liverworts.‖ Liverworts are included in this category. Liverworts, which are a kind of bryophyte, are included in this group. It has almost 900 species. The most primitive bryophytes are liverworts. They like living near wet rocks and soil. Because they dwell near water, they have a far lesser chance of drying out. It can be thalloid (flat) or ribbon-like, and it is frequently dichotomously branched (ribbon-like). Marchantia, for example, is connected to the soil through rhizoids. Other species, such as Porella, grow erect and appear to be leafy, with a phoney stem and leaves. The gametophyte nourishes and protects the sporophyte. 2. Anthocerotopsida (Hornworts): There are around 300 species in this class. The common term for them is hornworts. It is the only member of the Anthocerotales order. Examples include Anthoceros, Megaceros, and Notothylas. This bryophyte clade is slightly more advanced than Bryopsida and Hepaticopsida in some regions. From a distance, the gametophyte seems lobed and uneven. The sporophyte is not reliant on the gametophyte for sustenance or protection, except in the early stages of development. Antheridia and archegonia are partially surrounded by gametophytic tissue. 3. Bryopsida (Mosses): There are around 1400 species in the Bryophyta class. Mosses is a common moniker for them. Mosses, like liverworts, like damp environments. They survive in relatively dry environments, unlike other bryophytes. Mosses, on the other hand, need water to reproduce, which is why they often create cushions or mats. Examples include Funaria, Polytrichum, and Sphagnum. Page 17 of 36  Fig 3: Structure of Moss Life cycle of Bryophytes Bryophytes alternate a gametophytic generation with a sporophytic one (a sporic meiosis, a life cycle in which meiosis gives rise to spores, not gametes). Each of the haploid (1 n) spores is capable of developing into a multicellular, haploid individual, the gametophyte. The first structure formed from spores in most mosses and many liverworts is a filamentous, algal-like, green protonema (plural,protonemata). In some mosses the protonemata are long lived with rhizoids and aerial filaments and they often form dense green mats in suitable sites. Cells in the protonema, probably stimulated by red light and kinetin, give rise to shoots, which enlarge and become the mature gametophytes. In the bryophytes, these are the dominant, independent (photosynthetic) plants. The gametophytes initiate gametangia on special branches or at the tip of the main shoot. In these structures the gametes eggs and sperms are produced during the sexual portion of the cycle. The female gametangium called an archegonium and the male antheridium may be produced on the same plant or on different plants. In both kinds of gametangia, a protective layer of non-reproductive tissue a sterile layer surrounds the inner reproductive cells. (A sterile layer is absent in algal gametangia and is considered an upward evolutionary step towards the protective seed coats of flowering plants.) Mature sperm, released from the tip of the antheridia when dew or rainwater is present on the surface of the plants, swim Page 18 of 36 to the archegonia and down the necks to reach the eggs. One fuses with the single egg in each archegonium the process of fertilization thus combining the sperm and egg nuclear and cytoplasmic material. The resulting cell, a zygote, has a diploid (2 n) chromosome number and is the beginning of the sporophytic generation. This reproduction is termed oogamy a large, nonmotile egg is fertilized in the archegonium by a small, motile sperm that swims to the egg. In the bryophytes, an external film of water on the surface of the plant is the passageway for the biflagellate sperm; in more advanced plants, sperm move internally within special structures (pollen tubes) to reach the eggs. After fertilization, the zygote remains in the archegonium and divides by mitosis repeatedly to form a multicellular, diploid embryo, the young sporophyte. Fig 2: Life cycle of Bryophytes showing alternation of generation Page 19 of 36 PTERIDOPHYTES Pteridophytes are plants that do not have any flowers or seeds. Hence another name for it is Cryptogams. They include ferns and horsetails. In fact, they can be considered as the first terrestrial vascular plants, showing the presence of the vascular tissue, xylem, and phloem. Mostly, we find these plants in damp and shady places. Also, most ferns are grown as ornamental plants. Pteridophytes display differentiation. The plant body can be divided into true root, stem, and leaves. A saprophyte is the main plant body here. Some of the species belonging to this division have small leaves called the microphylls. For example, Selaginella. Megaphylls are the large leaves that some pteridophytes have. For example, fern plants. The main plant bears the sporangia. These bear some leaf-like appendages called the sporophylls. In a few species such as Selaginella and Equisetum, the sporophylls form compact structures called cones or strobili. Characteristics of Pteridophyta Following are some of the general characteristics of the division Pteridophyta; 1. They are found mainly in shady or damp places. 2. The plant body is made up of roots, stems, and leaves. 3. They have well developed vascular system(xylem and phloem) for the conduction of water and other essential substances, from one part of the plant body to another. 4. These plants have no flowers and do not produce seeds. 5. Multicellular sex organs are present. 6. A fertilized egg develops into an embryo. 7. Water is essential for fertilization i.e. transfer of gamete. 8. They show a typical heteromorphic alternation of generations. 9. They show much variation in their form, size, and habit. 10. They range from small annual plants to large tree-like perennials. Classification of Pteridophytes nother botanist Smith, classified Pteridophytes into Psilophytineae, Lycopodineae, Equisetineae and Filicineae. Pteridophytes were classified by Oswald and Tippo in 1942 into the following four classes: Psilopsida, Lycopsida, Sphenopsida and Pteropsida Page 20 of 36 Features of Psilopsida  Most of the species are now fossils. This group includes some of the oldest vascular plants.  The plant body does not have roots. The rhizome is subterranean, and it has an aerial shoot.  The stem has dichotomous branches.  Small rhizoids come out of the rhizome. These help in the absorption of water and salts.  On the shoot, the leaves are either scale-like arranged in a spiral as in Psilotum or leaf- like appendages as in Tmesipteris.  There is no secondary growth in the stem.  The sporophyte is homosporous as the spores are similar.  The sporangia are present either at the tip or laterally directly on the stem; i.e they are cauline in position.  The gametophyte is not photosynthetic as it is not green in colour. It grows as a saprophyte with a fungus.  The antherozoids are flagellated and spirally coiled.  An example of it is Psilotum. Features of Lycopsida  The plant body is differentiated into well-defined roots, stem and leaves.  The leaves are small, that is they are microphyllous.  The branching is dichotomous.  The sporangia are present in the axil of sporophylls.  The sporophylls are arranged as strobili.  The sporophyte is either homosporous as in Lycopodium or heterosporous as in Selaginella.  The gametophyte develops independently.  Some of the examples include Selaginella and Lycopodium. Page 21 of 36 Features of Sphenopsida  This class has only one living genus called Equisetum.  The stems have distinct nodes and internodes.  Branches are arranged in whorls.  Very small leaves are arranged in whorls at the nodes of stems and branches.  There is a special appendage called sporangiophore in which the sporangia are formed.  Equisetum is homosporous.  The gametophyte is photosynthetic.  There are multiple flagella present on antherozoids.  An example of it is Equisetum. Features of Pteropsida  The most prominent members of this class are ferns.  There are more than 9000 species in this group, and it is the largest group of Pteridophytes.  These plants are highly evolved than other Pteridophytes.  Members of this class are found in all kinds of habitats; terrestrial, damp and shady places, even in water and some are epiphytes too.  The plant body is differentiated into well-defined roots, stem and leaves. The leaves are arranged spirally.  The rhizome is thick and short.  The leaves are large in size (megaphylls). They are pinnately compound and are called a frond.  Young fronds are coiled.  Except for aquatic ferns, others are all homosporous.  Sporangia are located at the margins and tip of leaves on the ventral side of leaves. These are present in clusters called sori.  Some of the examples are Dryopteris and Pteris. Life Cycle of Pteridophytes The life cycle is a continuous reproductive process dominated by the sporophyte (sexual) stage of a generational alternation. Fern spores are launched into the air, developing into heart-shaped haploid gametophytes with both male and female sex organs. The sex organs of the young Page 22 of 36 gametophyte become active as it matures. The antheridium is the male reproductive organ in ferns, and it contains and releases sperm. The archegonium is the female reproductive organ, with the egg at its base. Water is needed for fern reproduction since sperm must swim to the archegonium and fertilize the egg. The sporophyte stage of the fern's life cycle is the most significant. The sporophyte is diploid, which means that all of its chromosomes are duplicated twice. The sporophyte produces haploid spores from its cells (one copy of chromosomes). The haploid gametophyte develops from these spores. Female structures (archegonia) and male structures (archegonia) are found in the gametophyte (antheridia). Fertilization is how the male sex cells (sperm) and female sex cells (eggs) join together. The genetic materials of both sperm and ovule combine to create a diploid zygote, which then develops into the sporophyte. The presence of water is needed for sporophytes to remain genetically stable. Many gametophytes release a significant amount of sperms that swim in the same water as the archegonium. As a result, sperm cells from the same gametophyte do not always fertilize the egg of the same gametophyte. Inbreeding could increase harmful recessive alleles if this opportunity for cross- fertilization is lost. A fertilized egg, or zygote, develops roots, stems, and a new sporophyte through mitosis (cell division). Embryonic sporophytes emerge as fiddleheads, tightly curled structures that unfurl as they mature into fronds (leaf-like structures). The sporophyte is the mature frond, which has several clusters of sporangia, which are sacs that bear asexual spores. Meiosis produces spores, which are then released into the air, continuing the life cycle. Page 23 of 36  Fig 4: Life cycle of Pteridophytes Similarities Between Ptreridophytes and Bryophytes Following are some of the characteristics that connect Bryophytes to Pteridophytes; 1. Both show heteromorphic alternation of generation i.e. haploid gametophyte in one generation and a diploid sporophyte in another generation. 2. The sexual reproduction in both groups is oogamous and the male and female reproductive structures are known as antheridia and archegonia. 3. Water is essential for the opening of mature sex organs and fertilization in both groups. 4. Certain pteridophytes are homosporous like bryophytes. Differences between Bryophytes and Pteridophytes Following are some of the differences between Bryophytes and Pteridophytes; 1. The plant body of pteridophytes is differentiated into root, stem, and leaves, whereas in bryophytes it is thalloid or foliose i.e. without distinct root, stem, and leaves. 2. Vegetative reproduction is more common in bryophytes than in pteridophytes. Page 24 of 36 3. Bryophytes are always homosporous, whereas many pteridophytes show heterospory. GYMNOSPERMS They are plants which are reproduced by seed. They bear naked seeds which are borne in cones, like pines. The term "gymnosperm" means "naked seed". Characteristics of Gymnosperms 1. The living gymnosperms are woody, evergreen (except Larix and a Taxodium) perennials grow as trees or shrubs.. 2. The living members are founding in colder regions of earth where snow (not rain) is the source of water. Only the members of cycadales and gnetales thrive in warm dry climate. 3. The dominant plant body is sporophyte (2n) which may be dioecious or monoecious. Gametophytes are inconspicuous and endosporic i.e. develop with the spores. 4. Sporophyte differentiated into root, stem and leaves. 5. Gymnosperms are xerophytes in nature due to presence of thick bark, thick hypodermis, thick cuticle, scales leaves, sunken stomata, transfusion tissue, etc. In some cases leaves modified into needle-like, scale like or small leathery. These are the adaptations to combat water stress in air and colder regions. 6. The gymnosperms are heterosporous, means 2 types of spores produced i.e. haploid microspores and megaspores. Microspores produced within micro-sporangia while megaspores produce within megasporangia (nucellus) of ovules. Both types of sporangia are formed on special leaf-like structures called sporophylls (microsporophylls and megasporophylls). Classification of Gymnosperm Classes of gymnosperms are placed into four divisions: 1. Cycadophyta e,g Cycads 2. Corniferophyta e.g Cornifers Page 25 of 36 3. Ginkgophyta e.g Ginkgo biloba 4. Gnetophyta e.g Gnetum spp. Class Cycadophyta (Cycads) Cycad, is any of the palmlike woody plants that constitute the order Cycadales. The order consists of three extant families—Cycadaceae, Stangeriaceae, and Zamiaceae—which contain 10–11 genera and 306 species. Some authorities use the term cycad to refer to all members of the division Cycadophyta. Plants of this division are known to have existed in the Mesozoic Era, about 252.2 million to 66 million years ago. Only the order Cycadales contains living species. Cycads are gymnosperms distinguished by crowns of large pinnately compound leaves and by cones typically borne at the ends of the branches. Characteristics of Cycads 1. Cycads are either male or female. Plant that are one specific sex are known as dioecious. 2. Each sex produces a type of cone containing the pollen or seeds that allow the plant to reproduce. 3. The seeds and pollen are spread by windy breezes, insects or birds and rodents that are attracted to outer covering of the seeds. 4. Male seed cones are typically smaller than female cones and the female cones are more brightly colored than the males. Although the stems of cycads initially appear thick and woody, they are really soft, fibrous storage areas for the starches that feed the plants. 5. The pachycaul stems are either hidden underground or rising above it to assume a trunk- like appearance. Attached to each stem is a sizable taproot with smaller roots known as coralloid roots that sprout from it. 6. The secondary roots do not run deep, but sprout on or near soil's surface. 7. Cycads have large, oversize leaves that produce a crown-like appearance atop the trunk of the plant. The majority of the leaves are either pinnate or bipinnate, meaning the are divided once or twice, respectively. Many people mistake the leaves on cycads for palm or fern leaves, due to their showy tropical appearance. 8. Cycad leaves are typically a vivid glossy green that command attention as they unfurl from the plant. Page 26 of 36  9. Cycads grow in tropical and subtropical regions of the world. At one time, this now small group of plants covered a considerable part of the earth. According to the Union County College Biology Department, cycads are currently found in limited quantities in Australia, Central and South America, Japan, Africa and Madagascar. Extremely limited in North America, the only native specimens are located in southern Florida. Class Corniferophyta (Cornifers) The cornifers are larger in number than the cycads. They are also far more important economically than the cycads. The cornifers occur widely in both hemispheres of the globe. They are said to constitute up to 80% of the world’s forest trees. They are the dominant trees of the cold temperate regions of the world. In tropical Africa however, they are restricted to the mountains, even though they are now being planted in the lowlands in many parts of the regions of Africa. Examples of cornifers are the pines (Pinus species). Characteristics of Cornifers 1. Cornifers have small leaves described as microphylls. These are usually needle like in their shapes. 2. The stems of cornifers are large and much branched. 3. Cornifers have large wood which implies that there is much secondary xylem in their stems. 4. The stems of cornifers have a small cortex together with pith and rays. 5. Cornifers seeds are bilaterally symmetrical and are usually wind dispersed. 6. Cornifers are typically monoecious plants, that is their plants Class Ginkgophyta (Ginkgo biloba) Ginkgo biloba, commonly known as ginkgo or gingko also known as the ginkgo tree or the maidenhair tree, is the only living species in the division Ginkgophyta, all others being extinct. It is found in fossils dating back 270 million years. Native to China, the tree is widely cultivated, and was cultivated early in human history. It has various uses in traditional medicine and as a source of food. Page 27 of 36 Characteristics of Ginkgo biloba 1. Tall, well-branched trees with short and long shoots. However, some earliest fossil members were without short and long shoots. 2. Wood is pycnoxylic. 3. Leaves are large, leathery and fan-shaped or strap-shaped. They are often deeply divided. 4. Dichotomous venation is usually present in the leaves. 5. Un-branched, catkin-like male organs are axillary in position. 6. Male organs bear micro-sporangiophores. 7. Each micro-sporangiophore possesses 2-12 pendulous microsporangia. 8. Spermatozoids are motile and contain spiral bands of flagella. 9. Ovules are terminal in position on branched or un-branched axillary axes. They are 2-10 in number. 10. Seeds are large-sized. 11. Each seed contains a fleshy outer layer and a middle stony layer. Class Gnetophyta (Gnetum spp.) Gnetum is a genus of gymnosperms, the sole genus in the family Gnetaceae and order Gnetales. They are tropical evergreen trees, shrubs and lianas. Unlike other gymnosperms, they possess vessel elements in the xylem. Some species have been proposed to have been the first plants to be insect-pollinated as their fossils occur in association with extinct pollinating scorpionflies. Page 28 of 36 Characteristics of Gnetum 1. The ovules are naked, i.e not enclosed in the ovary 2. In pollination, the wind-borne pollen grains are lodge directly on the ovule, there being no style, stigma or ovary. 3. The male and female strobili are of gymnospermic types, although the flowers are more advanced with the development of perianth. 4. The generative cell divides and produce a prothallus cell and generative cell, the latter forming two male gametes. 5. Anatomically, there is a preponderance of gymnospermic tracheids with bordered pits. 6. It is the only type of gymnosperm that undergoes double fertilization. 7. Gnetum bear well developed broad leaves with distinct reticulate venation hardly distinguished from those in angiosperm. Life cycle of Gymnosperm In the gymnosperm life cycle, plants alternate between a sexual phase and an asexual phase. This type of life cycle is known as alternation of generations. Gamete production occurs in the sexual phase or gametophyte generation of the cycle. Spores are produced in the asexual phase or sporophyte generation. Unlike in non-vascular plants, the dominant phase of the plant life cycle for vascular plants is the sporophtye generation. In gymnosperms, the plant sporophyte is recognized as the bulk of the plant itself, including roots, leaves, stems, and cones. The cells of the plant sporophyte are diploid and contain two complete sets of chromosomes. The sporophyte is responsible for the production of haploid spores through the process of meiosis. Containing one complete set of chromosomes, spores develop into haploid gametophytes. The plant gametophytes produce male and female gametes which unite at pollination to form a new diploid zygote. The zygote matures into a new diploid sporophyte, thus completing the cycle. Gymnosperms spend most of their life cycle in the sporophyte phase, and the gametophyte generation is totally dependent upon the sporophyte generation for survival. Page 29 of 36 Fig 5: Life cycle of gymnosperms Page 30 of 36 ANGIOSPERMS Angiosperms are a group of plants that produce flowers which develop into fruits containing the seeds. These are also called "flowering plants". There are about 250.000 species of angiosperms. They appeared on Earth about 125 millions years ago in the early Cretaceous. Extended along all kind of habitats, due to their capability to adapt to the different types of soils and climates, they are considered superior to gymnosperms. General Characteristics of Angiosperms 1. The sporophyte which is the dominant plant in the life-cycle is differentiated into roots, stem and leaves. 2. The highest degree of perfection of the vascular system with true vessels in the xylem and companion cells in the phloem. 3. The organisation of the microsporophyll’s (stamens) and megasporophylls (carpels) into a structure called the flower, which is typical only of the angiosperms. 4. The presence of four microsporangia (pollen sacs) per microsporophyll (stamen). 5. The ovules are always enclosed in an ovary which is the basal region of the megasporophyll. 6. Production of two kinds of spores, microspores (pollen grains) and megaspores. Angiosperms thus are heterosporous. Presence of single functional megaspore which is permanently retained within the nucellus or mega-sporangium. 8. Adaptation of flower to insect pollination. 9. Pollination consists in the transference of pollen grains from anther to stigma. 10. Spore dimorphisim having resulted in the production of gametophytes, male and female. 11. Extreme reduction in size, duration of existence and complexity of the structure of the gametophytes which are entirely parasitic. Page 31 of 36 12. The seed or seeds remain enclosed in the ripened ovary called the fruit. 13. The phenomenon of double fertilization or triple fusion is the characteristic of the angiosperms. 14. The endosperm develops after fertilization. It is triploid. 15. The angiosperms are completely adapted to life on land. Classes of Angiosperms Class Dicotyledonae The dicotyledons, also known as dicots are one of the two groups into which all the flowering plants or angiosperms were formerly divided. The name refers to one of the typical characteristics of the group, namely that the seed has two embryonic leaves or cotyledons. There are around 200,000 species within this group. The other group of flowering plants were called monocotyledons or monocots, typically having one cotyledon. Characteristics of Dicotyledons  These are plant whose embryo of seeds has two cotyledons.  Their leaves are broad and have networks of veins  Cross section of stems reveals vascular bundles arranged in rings  They have taproot system  Centrally placed star-shaped xylem with phloem alternating with arms of the xylem  Their flowers have floral parts in five or fours Examples are herbs, shrubs, and trees. The herbs include plants with all stems such as beans, cabbages, tomatoes and black jack while the shrubs include plants with fairly thick stems such as coffee, tea and cocoa. Class Monocotyledonae Monocotyledons commonly referred to as monocots, are flowering plants (angiosperms) whose seeds typically contain only one embryonic leaf, or cotyledon. They constitute one of the Page 32 of 36 major groups into which the flowering plants have traditionally been divided, the rest of the flowering plants having two cotyledons and therefore classified as dicotyledons, or dicots. The monocots include about 60,000 species. The largest family in this group (and in the flowering plants as a whole) by number of species are the orchids (family Orchidaceae), with more than 20,000 species. About half as many species belong to the true grasses (Poaceae), which are economically the most important family of monocots. In agriculture the majority of the biomass produced comes from monocots. These include not only major grains (rice, wheat, maize, etc.), but also forage grasses, sugar cane, and the bamboos Characteristics of Monocotyledons  Their seeds have an embryo with one cotyledon  Relatively narrow leaves with parallel veins  The cross section of the stem reveals scattered vascular veins  No vascular cambium hence no secondary growth  They bear floral parts in threes Examples include Maize, grass, wheat, sorghum, sugarcane, coconuts, bananas and sisal. Reproduction in Angiosperms There are certain special features in the reproductive life cycle of an angiosperms, as compared with cryptogams. Some such features are the development of pollen tube, absence of antheridia and archegonia. To start with, the zygote grows rapidly in to an embryo, and the latter gradually in to a full- fledged plant with roots, stem, leaves and flowers. The flowers bears a stamen or microsporophyll and a carpel or megasporophyll. The stamen bears a pollen sac or microsporangium and the carpels bears a nucellus or megasporangium within the ovule which again develops within the ovary. The pollen sac and the nucellus in their turn produce the pollen mother cell and the embryo-sac mother cell. By reduction division, the pollen mother cell gives rise to pollen grains or microspores, and the embryo-sac mother cell to the megaspore. The pollen grain on the stigma of the carpel (pistil) and gives rise to the male gametophyte and the megaspore to the female gametophyte. Both gametophytes are extremely small. The female gametophyte is completely endosporous and entirely dependent on the mother sporophyte for protection and nutrition. Antheridia and archegonia are altogether absent in the gametophytes. The male gametophyte bears two male gametes and the female gametophyte bears female Page 33 of 36 gamete or egg cell. Pollination takes place mainly through the agency of insects and the wind, and is followed by fertilization. One of the male gametes of the pollen- tube fuses with the egg cell of the embro-sac and the other male gametes fuses with the two polar nuclei i.e the definitive nucleus with 2n chromosomes. As a result of this double fertilization, rapid changes takes place in the ovule and the ovary. The fertilized egg-cell becomes the zygote (oospore) which quickly grows in to the endosperm, and the the ovule and the ovary grow in to the seed and fruit respectively. It should be noted that, the zygote marks the the end of the life cycle. Sooner or later, the next cycle begins from it (the zygote) and the same series of changes is repeated. Fig 6: Reproduction in Angiosperm Page 34 of 36 THE FLOWER Flowers are modified leaves, or sporophylls, organized around a central stalk. Although they vary greatly in appearance, all flowers contain the same structures: sepals, petals, carpels, and stamens. The peduncle attaches the flower to the plant. A whorl of sepals (collectively called the calyx) is located at the base of the peduncle and encloses the unopened floral bud. Sepals are usually photosynthetic organs, although there are some exceptions. For example, the corolla in lilies and tulips consists of three sepals and three petals that look virtually identical. Petals, collectively the corolla, are located inside the whorl of sepals and often display vivid colors to attract pollinators. Flowers pollinated by wind are usually small, feathery, and visually inconspicuous. Sepals and petals together form the perianth. The sexual organs (carpels and stamens) are located at the center of the flower. Styles, stigmas, and ovules constitute the female organ: the gynoecium or carpel. Flower structure is very diverse. Carpels may be singular, multiple, or fused. Multiple fused carpels comprise a pistil. The megaspores and the female gametophytes are produced and protected by the thick tissues of the carpel. A long, thin structure called a style leads from the sticky stigma, where pollen is deposited, to the ovary, enclosed in the carpel. The ovary houses one or more ovules, each of which will develop into a seed upon fertilization. The male reproductive organs, the stamens (collectively called the androecium), surround the central carpel. Stamens are composed of a thin stalk called a filament and a sac-like structure called the anther. The filament supports the anther, where the microspores are produced by meiosis and develop into pollen grains. Fig 7: Structure of a typical flower Page 35 of 36 POLLINATION The stamens and pistils are directly involved with the production of seed. The stamen bears microsporangia (spore cases) in which are developed numerous microspores (potential pollen grains); the pistil bears ovules, each enclosing an egg cell. When a microspore germinates, it is known as a pollen grain. When the pollen sacs in a stamen’s anther are ripe, the anther releases them and the pollen is shed. Fertilization can occur only if the pollen grains are transferred from the anther to the stigma of a pistil, a process known as pollination. There are two chief kinds of pollination 1) Self-pollination: This is the transfer of from anther to the stigma of the same flower or another flower on the same plant. 2) Cross-pollination: The transfer of pollen from the anther of a flower of one plant to the stigma of the flower of another plant of the same species. Self-pollination occurs in many species, but in the others, perhaps the majority, it is prevented by such adaptations as the structure of the flower, self-incompatibility, and the maturation of stamens and pistils of the same flower or plant at different times. Cross-pollination may be brought about by a number of agents, chiefly insects and wind. Wind-pollinated flowers generally can be recognized by their lack of colour, odour, or nectar, while animal-pollinated flowers are conspicuous by virtue of their structure, colour, or the production of scent or nectar. After a pollen grain has reached the stigma, it germinates, and a pollen tube protrudes from it. This tube, containing two male gametes (sperms), extends into the ovary and reaches the ovule, discharging its gametes so that one fertilizes the egg cell, which becomes an embryo, and the other joins with two polar nuclei to form the endosperm. (Normally many pollen grains fall on a stigma; they all may germinate, but only one pollen tube enters any one ovule.) Following fertilization, the embryo is on its way to becoming a seed, and at this time the ovary itself enlarges to form the fruit. Page 36 of 36