Plant Reproduction - Lebanese University Fall 2021/2022 PDF

Lebanese University Lebanese University Faculty of Sciences Faculty of Sciences B1101 Plant Reproduction Department of Life and Earth Sciences Fall Semester...

Lebanese University Lebanese University Faculty of Sciences Faculty of Sciences B1101 Plant Reproduction Department of Life and Earth Sciences Fall Semester 2021/2022 ©opyright Reserved The following book was prepared and revised by the following instructors Prepared by: Revised by: Dr. Aline KADRI Dr. Jihad NOUN Dr. Mona TANNOURY Dr. Bouchra DOUAIHY Dr. Raghida DAMAJ This Course is compiled by Dr. Aline KADRI Dr. Hassane MAKHLOUF Dr. Mona TANNOURY Copyright Reserved for the Lebanese University ©2018 This material is only for academic use by the students at the Faculty of Sciences in the Lebanese University, it should not be distributed for purchase or photocopy anywhere under the threat of legal prosecution. 2018© ‫حقوق الطبع والنشر محفوظة للجامعة اللبنانية‬ ‫إ ن هذه النسخة موضوعة بتصرف طالب كلية العلوم في الجامعة اللبنانية و لهدف اكاديمي فقط ال غير وعليه يمنع‬ ‫توزيع اي نسخة (ورقية او الكترونية) الي جهة اخرى او التصوير والبيع في كافة المكتبات تحت طائلة المالحقة‬.‫القانونية‬  Plant Reproduction – Lebanese University, Faculty of Sciences Table of Contents INTRODUCTION - GENERAL PRINCIPLES............................................................... 1 1. ASEXUAL REPRODUCTION......................................................................................... 1 2. SEXUAL REPRODUCTION............................................................................................ 1 3. ASEXUAL VERSUS SEXUAL REPRODUCTION........................................................ 2 CHAPTER 1: REPRODUCTION OF BACTERIA......................................................... 3 1. MODE OF REPRODUCTION OF BACTERIA............................................................... 3 1.1. Binary fission or scissiparity.................................................................................. 3 1.2. Budding.................................................................................................................. 3 1.3. Endospores............................................................................................................. 3 2. ASEXUAL REPRODUCTION OF CYANOBACTERIA................................................ 4 2.1. Hormogonia (s. hormogonium)................................................................................... 4 2.2. Heterocyst.................................................................................................................... 5 2.3. Akinetes....................................................................................................................... 5 3. GENETIC RECOMBINATION OF BACTERIA............................................................. 5 3.1. Conjugation................................................................................................................. 5 3.2. Transformation............................................................................................................ 6 3.3. Transduction................................................................................................................ 7 CHAPTER 2: SEXUAL REPRODUCTION - GENERAL CONCEPTS..................... 8 INTRODUCTION.................................................................................................................. 8 1. STRUCTURES AND STRATEGIES - TYPES OF SEXUAL REPRODUCTION......... 8 2. LIFE CYCLES................................................................................................................... 9 2.1. Characteristic patterns and variations....................................................................... 9 2.2. Basic definitions........................................................................................................ 12 CHAPTER 3: REPRODUCTION OF ALGAE.............................................................. 13 (KINGDOM OF PROTISTA).......................................................................................... 13 1. ASEXUAL REPRODUCTION....................................................................................... 13 2. SEXUAL REPRODUCTION.......................................................................................... 14 2.1. Monobiontic Haplontic life cycle (monogenetic)...................................................... 14 2.2. Monobiontic Diplontic life cycle (monogenetic): Fucus vesiculosis........................ 15 2.3. Haplodiplontic life cycle (digenetic)......................................................................... 17 2.4. Triplobiontic life cycle (Trigenetic): Polysiphonia, Antithamnion, Nemalion.......... 19 CHAPTER 4: REPRODUCTION OF MYCOTA.......................................................... 21 1. ASEXUAL REPRODUCTION....................................................................................... 21 2. SEXUAL REPRODUCTION.......................................................................................... 22 2.1. ZYGOMYCOTA......................................................................................................... 22 2.2. ASCOMYCOTA......................................................................................................... 23 2.3. BASIDIOMYCOTA.................................................................................................... 24 2.4. DEUTEROMYCOTA................................................................................................. 24 CHAPTER 5: GENERAL CHARACTERISTICS OF PLANTS................................. 25 1. The Embryophyte Condition............................................................................................ 25 2. Alternation of Generations............................................................................................... 25 CHAPTER 6: REPRODUCTION OF BRYOHYTES................................................... 28 i  Plant Reproduction – Lebanese University, Faculty of Sciences NON-VASCULAR PLANTS, HOMOSPOROUS.......................................................... 28 1. ASEXUAL REPRODUCTION....................................................................................... 28 2. SEXUAL REPRODUCTION.......................................................................................... 28 2.1. Class of Bryopsida —Mosses (Mnium)..................................................................... 29 2.2. Class of Hepaticopsida — Liverworts (e.g. Marchantia polymorpha)..................... 30 2.3. Class of Anthoceropsida — Hornworts (e.g. Anthoceros punctatus)....................... 32 CHAPTER 7: REPRODUCTION OF PTERIDOPHYTES.......................................... 34 VASCULAR SEEDLESS PLANTS................................................................................. 34 1. ASEXUAL REPRODUCTION....................................................................................... 34 2. SEXUAL REPRODUCTION.......................................................................................... 34 2.1. Reproduction of Filicopsida – e.g. Ferns (Polypodium vulgare)- homosporous, homothallus...................................................................................................................... 34 2.2. Reproduction of Sphenopsida — e.g. Horsetails or Equisetum - homosporous heteroprothallus............................................................................................................... 35 2.3. Reproduction of Lycopodiopsida – e.g. Selaginella - heterosporous homothallus... 36 CHAPTER 8: REPRODUCTION OF GYMNOSPERMS............................................ 38 1. CONIFEROPHYTA – e.g. Pines..................................................................................... 38 CHAPTER 9: REPRODUCTION OF ANGIOSPERMS.............................................. 43 1. TYPICAL STRUCTURE OF A FLOWER..................................................................... 43 2. THE LIFE CYCLE OF AN ANGIOSPERM................................................................... 44 2.1. Gametogenesis: Formation of egg cells and pollen grains....................................... 44 2.2 Pollination and fertilization....................................................................................... 45 3. FRUIT FORMATION...................................................................................................... 47 CHAPITRE 10: ASEXUAL REPRODUCTION............................................................ 49 VEGETATIVE REPRODUCTION................................................................................. 49 1. NATURAL VEGETATIVE REPRODUCTION............................................................. 49 1.1. Bulbs..................................................................................................................... 49 1.2. Tubers................................................................................................................... 49 1.3. Rhizomes............................................................................................................... 50 1.4. Stolons (runners).................................................................................................. 50 1.5. Suckers................................................................................................................. 50 1.6. Adventitious Plantlets........................................................................................... 51 2. ARTIFICIAL VEGETATIVE REPRODUCTION.......................................................... 51 2.1. Cutting....................................................................................................................... 51 2.2. Layering.................................................................................................................... 51 2.3. Grafting..................................................................................................................... 52 2.4. Plant Tissue Culture.................................................................................................. 53 REFERENCES.................................................................................................................. 55 ii  Plant Reproduction – Lebanese University, Faculty of Sciences INTRODUCTION - GENERAL PRINCIPLES Reproduction is a biological process by which cells or living organisms give rise to offsprings. Species are maintained in existence through this vital process. Reproduction is not necessary to the life of an individual, yet it is a prerequisite for the continuity of any species. There are two basic types of reproduction: asexual and sexual. In asexual reproduction, offspring arise from a single parent even without special reproductive cells or organs. The new individual is a separated part of the parent organism. Sexual reproduction, however, involves the union of two nuclei from special cells, which are usually produced by two separate parents. Some organisms reproduce only asexually, others reproduce only sexually, and still others can reproduce by either of these methods. 1. ASEXUAL REPRODUCTION Asexual reproduction (also known as agamogenesis) is the biological process by which an organism creates a genetically-similar or identical copy of itself without a contribution of genetic material from another individual (lack of genetic recombination). It is a form of reproduction which does not involve meiosis, gamete formation, or fertilization. Asexual reproduction may be as simple as cell division, resulting in two separate individual cells, as in Prokaryotes and simpler Eukaryotes, or it may involve the production of spores, special reproductive cells, each capable of producing a new organism, common in Algae and Fungi. At a more complex level, as in higher plants (also reproduce sexually), asexual reproduction involves the formation of a complete multicellular individual, which becomes detached from its parent. This way of reproducing is called vegetative reproduction as it is accomplished without seeds or spores. Asexual reproduction involves only mitotic cell division, therefore each offspring has exactly the same hereditary information as its parent: each daughter cell receives an exact copy of the chromosomes of the mother cell (clones). Thus asexual reproduction results in stable characteristics within a species from generation to the next: the same chromosome number is retained from generation to generation. Asexual reproduction is efficient in that it is generally rapid and often results in the production of large numbers of identical offsprings. 2. SEXUAL REPRODUCTION Sexual reproduction is a biological process by which organisms generate heterogeneous offsprings (descendants) that have a combination of genetic material contributed from two different members of the species (typically one of each sex), thus leading to genetic diversity. This type of reproduction is characterized by two processes. The first, meiosis, involves the halving of the number of chromosomes. The second process, fertilization, leads to the fusion of two gametes and the restoration of the original number of chromosomes. 1  Plant Reproduction – Lebanese University, Faculty of Sciences 3. ASEXUAL VERSUS SEXUAL REPRODUCTION Many organisms can reproduce sexually as well as asexually. Bryophytes, Pteridophytes and many plants are examples. When environmental factors are favorable, asexual reproduction is employed to exploit suitable conditions for survival such as an abundant food supply, adequate shelter, favorable climate, optimum pH or a proper mix of other lifestyle requirements. Populations of these organisms increase exponentially via asexual reproductive strategies to take full advantage of the rich supplied resources. When food sources have been depleted, the climate becomes hostile, or individual survival is jeopardized by some other adverse change in living conditions, these organisms switch to sexual forms of reproduction. Sexual reproduction ensures a mixing of the gene pool of the species. The variations found in offspring of sexual reproduction allow some individuals to be better suited for survival and provide a mechanism for selective adaptation to occur. In addition, sexual reproduction usually results in the formation of a life stage that is able to endure the conditions that threaten the offspring of an asexual parent. Thus, seeds, spores, cysts or other “dormant or quiescent” and resistant form ensure the survival during unfavorable times and the organism can “wait out” adverse situations until a swing back to suitability occurs in order to carry on its asexual reproduction. 2  Plant Reproduction – Lebanese University, Faculty of Sciences CHAPTER 1: REPRODUCTION OF BACTERIA Bacteria are considered as the most ancient living forms on earth. They can colonize many environments even the harshest ones. They are divided into Eubacteria and Archaebacteria. Bacteria are prokaryotes; they are unicellular organisms that lack membrane-bound organelles and a defined nucleus. This chapter includes a part dedicated to the reproduction of Cyanobacteria which belong to the kingdom Eubacteria (Cyanobacteria = Blue-green algae). Initially, according to Linnaeus (1735), they were classified in the Plant Kingdom (due to their ability to photosynthesis) but later they were moved to the Kingdom Eubacteria (Oxygenic photosynthetic bacteria, Whittaker, 1969). 1. MODE OF REPRODUCTION OF BACTERIA Bacteria are organisms which reproduce through asexual reproduction; they are not able to reproduce by mitosis or meiosis because they lack a true nuclei. They do not have different sexes, and they are capable of “splitting” themselves into two or more individuals. 1.1. Binary fission or scissiparity Most bacteria reproduce by a process called binary fission or scissiparity. Binary fission is basically an asexual mode of cell division where a single parent bacterial cell replicates its DNA before splitting into two new daughter cells. Under ideal conditions, bacteria divide (reproduce) rapidly, doubling their numbers every 20 minutes. Figure 1: Binary fission 1.2. Budding Rare form of bacterial division where a cell bulges and forms a protuberance that grows to equal volume of the mother cell, and then becomes detached to form a daughter cell. Figure 2 : Bacteria reproducing by budding 1.3. Endospores Several types of bacteria are able to form resistant cells within the cell wall of a parent cell known as endospores. Endospores are resistant resting cells that are formed at the end of the period of active growth. Endospores are protected by several layers and possess low metabolic activity. Only one spore is formed inside each bacterial cell during sporulation. 3  Plant Reproduction – Lebanese University, Faculty of Sciences Generally, the spore will remain dormant for a period of time before it can be induced to germinate and to revive the vegetative state in hospitable environments. Bacteria increases its ability to survive by producing endospores. Figure 3: Endospore formation 2. ASEXUAL REPRODUCTION OF CYANOBACTERIA All cyanobacteria are unicellular, though many grow in colonies or filaments, often surrounded by a gelatinous or mucilaginous sheath. Cyanobacteria present several forms of asexual reproduction: - Unicellular cyanobacterial cells divide and reproduce by binary fission and spore formation. - Filaments may fragment, often at weak points where a cell has collapsed (hormogonia) or near a nitrogen-fixing site (heterocysts). 2.1. Hormogonia (s. hormogonium) Filamentous cyanobacteria reproduce by fragmentation of their filaments (trichomes) at more or less regular intervals to form short pieces of filaments. These pieces of filaments (surrounded by a mucilaginous sheath) are called hormogonia. The latter show gliding motility and develop into new filaments. Figure 4: Hormogonia formed during asexual reproduction in filamentous cynobacteria 4  Plant Reproduction – Lebanese University, Faculty of Sciences 2.2. Heterocyst Within the filament, heterocysts, specialized enlarged thick wall cells, fix nitrogen and pass it to surrounding cells through microplasmodesmata, fine holes in their walls. When environmental conditions trigger reproduction, certain heterocysts in the filament form large holes in their walls and then die; the walls and mucilage sheath tear at these spots, so the filaments are broken into multicellular fragments, each capable of growing into a new filament. 2.3. Akinetes Under unfavorable conditions, some Cyanobacteria form resistant spores called akinetes - enlarged vegetative cells with a thickened outer wall. Akinetes are dormant cells. They are resistant to cold and drought (desiccation). They also accumulate and store various essential materials, thus allow the Cyanobacteria to survive under unfavorable environmental conditions. Once conditions become more favorable for growth, the akinete can then germinate back into a vegetative cell. Figure 5: Trichome of cyanobacteria possessing heterocyst and akinetes 3. GENETIC RECOMBINATION OF BACTERIA All bacteria are haploid and contain about 1/1000 as much DNA as ordinary eukaryotic cells. Most bacteria’s DNA is a single double strand that attaches to the cell membrane and replicates just before the cell divides. Some kinds of bacteria also contain circular DNA called plasmids. Bacteria can undergo genetic recombination, a non-reproductive means by which bacteria acquire new combinations of genes. Bacteria can exchange genes by one of three special means: conjugation, transformation, or transduction. 3.1. Conjugation - The process of exchanging genetic material through cell-to-cell contact (conjugation bridge). During conjugation, plasmid (circular DNA) moves from one bacterial cell to another, this allows the DNA to change and provide variations and diversity of the generations of bacteria to follow. It increases the chances that some bacteria will survive the environment changes. The bacteria attached together using special hairlike structures called pili, a bridge of cytoplasm (conjugation bridge) forms between two bacteria cells, and the DNA passes from one cell to another. 5  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 6: In conjugation, a donor bacterium transfers plasmid DNA to a recipient bacterium. 3.2. Transformation - The process by which bacterial cells pick up and incorporate released DNA (in the environment or medium) from dead bacteria cells. When bacteria die, they release DNA that can be taken in by other bacteria. In transformation, a bacterial cell takes up fragments of foreign DNA (or RNA) released by another bacterium. Once it enters the bacterial cell, some of the DNA is incorporated into the host’s own genome by reciprocal recombination (the DNA is exchanged) between the new DNA and the host chromosome. Bacterial transformation is used in biotechnology and genetic engineering and has practical applications in different sectors (medicine, agriculture, food…): It is used to genetically engineer bacteria to produce medicines, important proteins... Insulin, which is used to control diabetes, was the first medication for human use to be produced by genetic engineering. Some vaccines, as well as enzymes are also made using transformed bacteria. Figure 7: Transformation: In this process, foreign DNA from a bacterium that has died enters a host bacterium. DNA is exchanged by recombination. 6  Plant Reproduction – Lebanese University, Faculty of Sciences 3.3. Transduction - The process of transferring DNA from one bacteria to another via a virus Sometimes a bacteriophage (bacterial virus) incorporates some of the bacterial DNA of its host. Then, when the phage infects another bacterium, it transfers that DNA to its new host. Phage therapy has many potential applications in human medicine as well as dentistry, veterinary science, and agriculture. As example, bacteriophages (lytic phages) are currently being used therapeutically to treat bacterial infections that do not respond to conventional antibiotics. Figure 8: In transduction, a phage transfers bacterial DNA from one bacterium to another. Transduction is an important means of gene transfer. 7  Plant Reproduction – Lebanese University, Faculty of Sciences CHAPTER 2: SEXUAL REPRODUCTION GENERAL CONCEPTS INTRODUCTION Sexual reproduction generally requires two parents; each of the parent organisms contributes half of the offspring’s genetic makeup by giving haploid gametes. Offspring inherit one allele for each trait from each parent, thereby ensuring that offspring have a combination of the parents’ genes. Sexual reproduction always involves two complementary processes: meiosis and fertilization. Haploid gametes are produced by the process of meiosis. The single diploid cell formed by the union of two gametes (egg and sperm) is called a zygote. The zygote contains all the genetic information necessary for growth, development, and eventual reproduction of the organism. In sexual reproduction, the offsprings produced are not identical to either parent, showing variations in structure and / or function. Increasing the amount of variation in members of a species increases the possibility that some individuals of that species will be better adapted than others to survive both short- term and long- term changes in the environment. Variations may also enable certain individuals in a population to compete for new environments. 1. STRUCTURES AND STRATEGIES - TYPES OF SEXUAL REPRODUCTION The complexity of the systems and devices used to achieve sexual reproduction has resulted in proposing numerous terms to describe structures and strategies. The simplest form of reproduction, in which two eukaryotes unicellular organism or some multicellular algae (e.g. Spirogyra), link together joined by the outgrowth of conjugation tubes, exchange genetic information, and then separate. This type of reproduction is termed conjugation (cystogamy) when there is a complete transfer of one organism’s (or cell’s) contents to the other organism ( Fig. 9). If the process involves only the transport of the sperm through a tube it is then termed siphonogamy. Syngamy is the process of union (fusion) of two gametes (of different sexes, male and female) to produce diploid zygote which later becomes the new individual. Syngamy can be divided into two stages named plasmogamy (fusion of cell membranes and cytoplasm of two cells) and karyogamy (fusion of two haploid nuclei to produce a diploid cell). Plasmogamy occurs first and is followed by karyogamy. In some organisms, these two occur simultaneously while in some species karyogamy is delayed for a considerable time duration. Whereas in somatogmy, fusion occurs between two somatic cells (non sexual) and involves only plasmogamy. The cells are designated as + and -. e.g. Basidiomycete. Sexual reproduction involves formation of gametes in gametangia. When these gametes are capable of moving - flagellated - they are called planogametes (zoogametes); planogamy refers to the type of reproduction that involves such gametes. Aplanogamy refers to the type of reproduction that involves non motile gametes (aplanogametes). In isogamous species, the gametes (the female and the male, or the + and the – gametes as for example in the green algae, Chlamydomonas) are similar or identical in form and size (Fig. 9). Such type of reproduction is called isogamy. 8  Plant Reproduction – Lebanese University, Faculty of Sciences Most organisms form two different types (and sizes) of gametes (anisogametes). In these anisogamous species, (e.g. Ulva) the two sexes are referred to as male (producing sperm) and female (producing ovules). The female gamete is larger than the male gamete, a state called heterogamy or anisogamy (Fig. 9). a. b. c. Figure 9: Reproduction types: Isogamy, anisogamy and oogamy (a), conjugation and cystogamy (b and c) The evolutionary tendancy progresses further towards oogamy (Fig. 9). Oogamous organisms - e.g. some Algae such as Fucus, Laminaria) produce a nonmotile female macrogamete (egg or oogonium) and a flagellated male microgamete (spermatozoid). In oogamy, more male than female gametes are produced. 2. LIFE CYCLES 2.1. Characteristic patterns and variations A life cycle represents the series of changes that an organism undergoes as it passes from the beginning of a given developmental stage to the inception of that same developmental stage in a subsequent generation. The life cycle of every sexually reproducing group of organisms has a characteristic pattern. Alternation of meiosis and fertilization is common to all sexually reproducing organisms; however, the timing of these two events in the life cycle varies among species. When fertilization is followed by mitosis, a diploid sporophyte is produced. When meiosis is followed by mitosis, a haploid gametophyte is produced. Accordingly, there are three types of cycles: monobiontic, haplodiplontic and triplobiontic. 9  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 10 : Life cycles. a. haplontic cycle (Chlamydomonas, Spirogyra). b. haplo-diplontic cycle (Ulva). c. diplontic cycle (Fucus). d. Triplobiontic cycle (red algae). F, Fertilization; g, gametes; RC, Chromatic reduction; s, spore; Z, zygote.  A monobiontic cycle It describes a life cycle in which there is only a single independent generation (monogenetic). A single generational cycle is termed:  Haplontic when it encompasses a single generation of organisms whose cells are haploid. In this case, meiosis occurs immediately after the zygote forms (zygotic meiosis) and mitosis produces a multicellular haploid organism. In the whole cycle, zygotes are the only diploid cell; mitosis occurs only in the haploid phase. The individuals or cells as a result of mitosis are haplonts; e.g.: some fungi and some green algae - e.g. Chlamydomonas, Spirogyra. Gametophyte Sporophyte Figure 11: Single generation life cycles: haplontic (left) and diplontic (right)  Diplontic when the cycle encompasses a single generation of organisms whose cells are diploid. Organisms produce sex cells that are haploid, and each of these gametes must combine with another gamete in order to obtain the double set of chromosomes necessary to grow into a complete organism. Meiosis only occurs during the formation of gametes (gametic meiosis) and the gametes undergo no further cell division until fertilization occurs. 10  Plant Reproduction – Lebanese University, Faculty of Sciences In the whole cycle, gametes are the only haploid cells. The diploid multicellular individual is a diplont; this is the case of some brown algae, e.g. Fucus.  A haplo-diplontic cycle In some protists and some fungi as well as in plants, the life cycle is, by contrast, multigenerational, characterized by an alternation of generations - it includes both a multicellular sporophyte (diploid generation) and a multicellular gametophyte (haploid generation). Life cycles that present two phases are called digenetic (also known as diplobiontic, diplohaplontic, haplodiplontic, or dibiontic):  A diploid (2n) sporophyte undergoes meiosis to produce haploid (n) reproductive cells, called spores. An individual plant begins with the germination of a haploid spore, which undergoes mitosis and grows into a gametophyte (gamete-producing organism). The gametophyte reaches maturity and produces haploid (n) gametes, which, following fertilization, grow into a zygotic sporophyte (spore-producing organism).  Such life cycle is characterized by a sporic meiosis, also known as intermediary meiosis, whereas mitoses occur in both the diploid and haploid phases. Figure 12: Digenetic life cycle (Haplo-diplontic cycle)  A triplobiontic cycle In some cases (e.g. Ascomycetes, Basidiomycetes and Rhodophytes) the life cycle includes three phases. Such multigenerational cycles are said to be “trigenetic” because they produce three successive generations. 11  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 13: Trigenetic life cycle 2.2. Basic definitions Monoecious: Male and female organs are on the same individual. Dioecious: Male and female organs are on different individuals. Heterothallic: having two or more morphologically similar haploid phases or types of which individuals from the same type are mutually sterile, but individuals from different types are cross-fertile (heterothallic fungi). Gametes produced by one type of thallus are compatible only with gametes produced by the other type. Male gametes fuse with female gametes from different individuals. Homothallic: having a haploid phase that produces two kinds of gametes capable of fusing to form a zygote, (especially some algae and fungi). Sex organs produced by a single thallus. Male gametes fuse with female gametes from the same individual. Isomorphic: having mature sporophytic and gametophytic generations alike in size and shape (some species of algae, e.g. Ulva lactuca). Heteromorphic: the alternating generations have different forms; the gametophyte and the sporophyte are distinct in appearance (e.g. Laminaria). This is true for the bryophytes and for all vascular plants, both angiosperms (flowering plants) and gymnosperms (conifers and allies). Homosporous, Isosporous: producing spores of one kind only. A homosporous life cycle occurs in nearly all Bryophytes and in most Pteridophytes. It is characterized by morphologically identical spores that germinate to produce bisexual (both male and female) gametophytes in Pteridophytes, or more usually unisexual (either male or female) gametophytes in some Bryophytes. Heterosporous: is characterized by morphologically dissimilar spores [microspores, or male spores, and megaspores (macrospores), or female spores] produced from two types of sporangia. The spores produce two types of gametophytes. A heterosporous life history occurs in some Pteridophytes and in all seed plants. 12  Plant Reproduction – Lebanese University, Faculty of Sciences CHAPTER 3: REPRODUCTION OF ALGAE (Kingdom of Protista) Algae (Protists) comprise a large heterogenous assemblage of photoautotrophic organisms which vary vastly in size, shape and mode of life. Algae show morphological, cytological, physiological differences. The reproductive systems of algae also show similar diversity. Algae reproduce in astoundingly diverse ways; some reproduce asexually, others use sexual reproduction, and many use both mode of reproduction. 1. ASEXUAL REPRODUCTION Asexual reproduction may be by: - Mitotic division for single-celled forms - Fragmentation for multicellular organisms. Some multicellular algae, e.g. Sargassum reproduce asexually through fragmentation, in which fragments of the parent develop into new individuals. Figure 14: Fragmentation - Budding, a process similar to fragmentation, where the parent organism divides into two unequal parts. New individuals develop as buds on the outer surface of the parent organism. The buds may break off and live independently or they may remain attached, forming colony. Figure 15: Budding - Spore formation. Many algae produce spores asexually by mitosis. If these spores are flagellated and motile, they are called zoospores or planospores. The majority of algae produce zoospores except red algae. Many algae carry out both sexual and asexual reproduction. This is well demonstrated in the life cycle of the alga Chlamydomonas. The mature alga is a single haploid cell - that is, it contains only one set of chromosomes. During asexual reproduction the algae absorbs its flagellum. The algal cell then undergoes mitosis, 2 to 3 times. Four to eight daughter cells are created that emerge from the enclosing parent cell as spores. The spores develop into mature haploid cells that are genetically identical to the parent cell. 13  Plant Reproduction – Lebanese University, Faculty of Sciences 2. SEXUAL REPRODUCTION 2.1. Monobiontic Haplontic life cycle (monogenetic) 2.1.1. Chlamydomonas Recall that Chlamydomonas is one of many algae that incorporate both sexual and asexual modes of reproduction. The mature alga is a single haploid cell. During the sexual phase haploid cells divide mitotically to produce either “plus” or “minus” motile gametes of the same size. A (+) gamete and a (-) gamete come into contact with one another, shed their cell walls, and fuse to form a diploid zygote (planogamy isogamous). This resting stage of a zygote is called a zygospore which withstands bad environmental conditions. When conditions become favorable, the zygospore undergoes meiosis and produce 4 haploid motile tetraspores called zoospores. The thick wall opens and the living zoospores emerge. Figure 16: Chlamydomonas life cycle 2.1.2. Spirogyra Spirogyra is a filamentous green alga which is common in freshwater habitats. Spirogyra lacks a motile variant at all stages of its life history; i.e. no motile gametes (ova or sperm), no zoospores etc. Sexual reproduction is done by a process called conjugation. Certain filaments in a loose parallel bundle of Spirogyra assume the female and others the male role. The cells of adjacent filaments develop bumps which grow towards one another and eventually fuse to form a conjugation tube between the cells. This type of reproduction is called cystogamy. Meanwhile the contents of each cell have detached themselves from their respective cell walls and have formed a round ball. Fertilization occurs when the contents of a (-) male cell moves through the tube and fuses to the contents of a (+) female cell. This fertilization takes place between somatic cells without differentiation of sexual cells, so it is a somatogamy. The resulting zygote forms a thick walled spore (zygospore) with a tough resistant outer covering within the chambers of the female filament (+) that breaks away from the parent. After a dormant period, these zygotes undergo meiosis and germinate, resulting in new haploid filaments of Spirogyra. Meiosis occurs immediately after karyogamy, it is a monobiontic haplontic cycle. 14  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 17: Conjugation tube Figure 18: Spirogyra zygote formation 2.2. Monobiontic Diplontic life cycle (monogenetic): Fucus vesiculosis Diplontic life cycles (gametic meiosis) are fairly unique among life history of algae. The genus Fucus vesiculosus is characterized by a dichotomous thallus composed of flat frond with paired air bladders. These air bladders help keep the brown algae afloat. Sporophytes are either homothallic (monoecious: Fucus vesiculosis monoica) or heterothallic (dioecious: Fucus vesiculosus dioica) depending on species. 15  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 19: Fucus vesiculosus morphology The life cycle contains a reduced haploid gametophyte and a well shown diploid sporophyte (monobiontic diplontic life cycle). Both forms are distinct one from the other with the sporophyte being the visible form (heteromorphic). At the tip of each frond a receptacle is formed containing conceptacles, it is within these conceptacles that sporangia, i.e. male and female gametangia, are formed. Conceptacles bear also filaments of sterile cells called paraphyses. The female gametangia, oogonia, use meiosis to produce haploid non motile female gametes (eggs). The male gametangia, antheridia, follow a similar procedure to produce haploid motile male gametes (sperm). Eggs and sperm are released simultaneously into water; the eggs release a pheromone that attracts the sperm (chemotaxis). Fertilization occurs externally (in water), by oogamy. The fertilized egg settles to the substrate where it becomes attached after just a few hours. The zygote germinates immediately. By mitosis and differentiation the zygote develops into a mature diploid plant. 16  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 20: Diplontic life cycle of a monoecious Fucus 2.3. Haplodiplontic life cycle (digenetic). Haplodiplontic (diplobiontic) life cycles include both a sporophyte (diploid generation) and a gametophyte (haploid generation), characteristic feature of what is known as alternation of generations. In some species (e.g. Ulva lactuca) the two mature forms of algae, alternating between diploid and haploid individuals, are identical in appearance; such generations are called isomorphic. When the gametophyte and the sporophyte are distinct in appearance (e.g. Laminaria), generations are termed heteromorphic. 17  Plant Reproduction – Lebanese University, Faculty of Sciences 2.3.1. Isomorphic haplodiplontic life cycle: Ulva All Ulva species alternate between gametophytic and sporophytic life stages (haplodiplontic life cycle n/2n) with similar morphologies (isomorphic). The gametophytes are haploid and the sporophytes are diploid. The gametophytes produce biflagellate haploid gametes through mitosis, and the sporophytes produce quadriflagellate haploid zoospores through meiosis. Both gametes are motile (zoogamy, planogamy) and the female is a little bit larger (anisogamous). Reproductive activities occur near the margins of Ulva fronds, with the fertile portions turning slightly brown. Figure 21: Ulva lactuca life cycle (isomorphic haplodiplontic) 2.3.2. Heteromorphic haplodiplontic life cycle: Laminaria Laminaria exhibits not only an alternation of generations but also heteromorphy. The sporophyte plant is a large multicelled alga whereas the microscopic female and male gametophytes are only one cell or a few cells in size. The sporophyte generation (2n) develops on the surface of the blades sori containing sporangia and paraphyses; sporangia produce by meiosis motile flagellated zoospores (n ch), which develop into male and female gametophytes (n ch). Male gametophytes release motile male gametes and female gametophytes produce eggs, which remain attached to the female gametophytes. Male gametes fertilize the eggs; the fertilization of Laminaria is oogamous. 18  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 22: Laminaria life cycle (heteromorphic haplodiplontic). (Unilocular sporangia are called so because they are not divided by cross walls or locules) 2.4. Triplobiontic life cycle (Trigenetic): Polysiphonia, Antithamnion, Nemalion Most red algae (Rhodophyta) have a trigenetic life cycle. A typical example of trigenetic life cycle of Algae is the one of Polysiphonia (Rhodophyta). Other red algae display the same pattern of life cycle, e.g. Antithamnion, Nemalion… The Trigenetic cycle include three successive generations:  The first generation : the gametophyte stage. The gametophytes male and female are isomorphic gametophytes presenting n chromosomes. The male sex structures called spermatangia produce non motile sperm cells called spermatia. The female sex structures are egg-like cells called carpogonia. Each carpogonium consists of a single large cell with a long tubular hair-like extension the trichogyne that basically acts as a receptor of drifting spermatia. Since reproductive organs of Rhodophyta are non motile, the spermatia are carried to the trichogyne of carpogonium by water currents. If a spermatium should brush against a trichogyne, it may become attached, and plasmogamy occurs. Then the nucleus of the spermatium migrates to the egg nucleus and fuses with it, forming a zygote. This type of 19  Plant Reproduction – Lebanese University, Faculty of Sciences fertilization is trichogamy. The produced zygote goes on to develop parasitically on the female gametophyte.  The second generation : the carposporophyte stage (parasitic on the gametophyte). The zygote, divides repeatedly by mitosis and gives birth to the carposporophyte with 2n chromosomes, producing then the carpospores (2n chromosomes) in the carposporangia. These carpospores are released and carried away by ocean currents.  The third generation : the tetrasporophyte stage. When a carpospore lodges in a suitable location, it germinates and grows into a tetrasporophyte. Tetrasporangia are formed along the branches of the tetrasporophyte. Each tetrasporangium undergoes meiosis, giving rise to four haploid tetraspores. When tetraspores germinate, they develop into male and female gametophyte, thereby completing the life cycle. In Polysiphonia, the three types of thalli (male gametophyte, female gametophyte and tetrasporophyte) all outwardly resemble one another. Figure 23: Trigenetic life cycle N.B.  Gametangium (pl. gametangia) any cell or structure in which gametes are produced (equivalent to both “gametocyste” and “gametangia” in french)  Sporangium (pl. sporangia) a structure in which spores are produced; it may be either unicellular or multicellular (equivalent to both “sporocyste” and “sporangia” in french) 20  Plant Reproduction – Lebanese University, Faculty of Sciences CHAPTER 4: REPRODUCTION OF MYCOTA Fungal reproduction occurs in a variety of ways asexually and sexually. Sexual reproduction in fungi is somewhat different from that of animals or plants, and each fungal division reproduces using different strategies. Fungi that are known to reproduce sexually have all a haploid stage and a diploid stage (short-lived) in their life cycles. Ascomycetes and Basidiomycetes also go through a dikaryotic stage, in which the nuclei inherited by the two parents do not fuse right away, but remain separate in the hyphal cells. 1. ASEXUAL REPRODUCTION Asexual reproduction is carried out by budding, fragmentation, or, most commonly, by spore formation. - Budding: as in unicellular yeast (Saccaromyces, Ascomycota, yeast) - Fragmentation: Mycelial fragmentation occurs when a fungal mycelium separates into pieces with each component growing into a separate new mycelium. e.g. Basidiomycota - Spore formation: Spores are quite small and easily carried by wind. Reproductive structures involved in the production of spores are called sporangia, separated from hyphae by complete septa. Spores are the primary means of reproduction, dispersal and survival. Spores are resting resistant stages (cyst) under unfavorable conditions and are in an active stage under favorable conditions. Spores could be exogenous, called conidia, (e.g. Penicillium) or endogenous, produced inside sporangia (e.g. Rhizopus). Spores of fungi are non-flagellated (aplanospores). Deuteromycetes (Imperfect fungi) reproduce asexually by producing conidia. They have no known sexual reproduction. Figure 24: Conidiophore & Conidia 21  Plant Reproduction – Lebanese University, Faculty of Sciences 2. SEXUAL REPRODUCTION 2.1. ZYGOMYCOTA A common zygomycete is Rhizopus nigricans, agent of the “black bread mold” (Figure 25). The bread mold is heterothallic; sexual reproduction can occur only between a member of a plus (+) strain and one of a minus (-) strain (step 1). When hyphae of opposite mating types meet, their tips (the gametangia) come together forming cysts (step 2) (Cystogamy) and produce, by the way of plasmogamy, a heterokaryotic zygosporangium (steps 3 & 4). Zygosporangia are particularly resistant to dry and cold environmental conditions. These structures become separated from the rest of the thallus by the formation of septa. After karyogamy takes place, a zygospore forms (step 5), providing a thick protective covering around the zygote. Meiosis probably occurs at or just before germination of the zygospore, stage where an aerial hypha develops with a sporangium at the top (step 6). Haploid spores are then dispersed (step 7), which eventually germinate (step 8) to begin a new cycle. The gametophytic phase is the dominant phase over the two other ones (dikaryon and diploide). The life cycle is haplontic. Figure 25: Life cycle of Rhizopus nigricans (Zygomycota) 22  Plant Reproduction – Lebanese University, Faculty of Sciences 2.2. ASCOMYCOTA In Ascomycetes, sexual reproduction is characterized by the formation of multinucleate gametangia called “antheridia” and “ascogonia” (1) on the same mycelium that produces conidia (Figure 26). The male nuclei of the antheridium pass into the ascogonium via the trichogyne, which is an outgrowth of the ascogonium (2). Plasmogamy, the fusion of the two protoplasts, has now taken place. New dikaryotic hyphae (= dikaryon) develop from this fused structure (3) and form structures called asci (sing. ascus) (4), in which karyogamy (nuclear fusion) occurs. These asci are embedded in an ascocarp, or fruiting body, of the fungus. Karyogamy in the asci is followed immediately by meiosis (5) and the production of ascospores (6, 7). The ascospores are disseminated (8) and germinate (9) to form new haploid mycelium. Asexual conidia may be produced by the haploid mycelium (10). Many Ascomycetes appear to have lost the ability to reproduce sexually and reproduce only via conidia. In this life cycle, plasmogamy and karyogamy are separated in time and space by a dikaryotic phase. Figure 26: Ascomycota life cycle 23  Plant Reproduction – Lebanese University, Faculty of Sciences 2.3. BASIDIOMYCOTA Sexual reproduction in Basidiomycetes is similar to that of Ascomycetes. Somatic cells of sexually compatible haploid hyphae fuse to produce a dikaryotic mycelium. This leads to the production of a fruiting body termed basidiocarp. On the undersurface of the basidiocarp develop many thin perpendicular plates called gills. Club-like structures known as basidia develop on the surface of these gills and generate haploid basidiospores following karyogamy and meiosis. These basidiospores then germinate to produce new haploid mycelium. As in Ascomycota, the life cycle of Basidiomycota presents also a plasmogamy and a karyogamy separated in time and space by a dikaryotic phase. Figure 27: Life cycle of a typical Basidiomycete 2.4. DEUTEROMYCOTA They are referred to as imperfect fungi; their sexual form of reproduction has never been observed. Imperfect fungi reproduce asexually by producing conidia on specialized hyphae called conidiophores. 24  Plant Reproduction – Lebanese University, Faculty of Sciences CHAPTER 5: GENERAL CHARACTERISTICS OF PLANTS Plants appeared on land about 425 million years ago, and the evolutionary history of the plant kingdom reflects increasing adaptation to the terrestrial environment. Plants are multicellular eukaryotes that are photosynthetic autotrophs. Structural and reproductive adaptations made the colonization of land possible: 1. The Embryophyte Condition In contrast to the reproductive style of algae, in some groups of vascular plants additional adaptations to living on land evolved, including the increasing dominance of the diploid sporophyte in the alternation of generations, the replacement of flagellated sperm with pollen as a mean of delivering gametes in a non-aquatic environment, the protection of embryo against desiccation and the production of seeds. With the move from aquatic to terrestrial environment, different life cycles modifications and a new mode of reproduction was necessary: 1. The gametophytes of seed plants become even more reduced than in ferns and other seedless plants. Rather than developing in the soil as an independent generation, the minute gametophytes of seed plants are protected from desiccation by being retained within the moist reproductive tissue of the sporophyte generation. 2. Pollination replaced swimming gametes as the mechanism for delivering sperm to eggs. Gametes must be dispersed in nonaquatic environment. Plants produce gametes within gametangia, organs with protective jackets of sterile (non reproductive) cells that prevent gametes from drying out. Pollen, which contains sperm cells (produced in antheridium), is disseminated by wind or by insects and other animals. The egg is fertilized within the female organ (archegonia). Accordingly, the group of organisms bearing archegonia are called archegoniate, they regroup Bryophyta, Pteridophyta and Gymnosperms. 3. Embryos must be protected against desiccation. The zygote develops into an embryo that is retained for a while within the female gametangia's jacket of protective cells. Emphasizing this terrestrial adaptation, plants are often referred to as embryophytes. 4. The seed evolved. Instead of the zygote developing into an embryonic sporophyte that fends for itself; the zygote of a seed plant develops into an embryo that is packaged along with a food supply within a seed coat. This protects the dormant embryo from drought, cold, and other harsh conditions. Seeds also function in overland dispersal; they may be carried far from their parents by wind, water or animals. In seed plants, the seed has replaced the spore as the stage in the life cycle that disperses offsprings. 2. Alternation of Generations The life cycle of land plants (as well as many other plants) includes both diploid and haploid multicellular stages. This type of life cycle is characterized by an alternation of generations and results in two different multicellular body plans over the life cycle of an individual. 25  Plant Reproduction – Lebanese University, Faculty of Sciences All plants have the same basic life cycle (Figure 28) with a clearly defined alternation of generations, they spend: - part of their lives in the haploid stage (gametophyte generation) because it gives rise to haploid gametes by mitosis, and - part in the diploid stage (sporophyte generation) which gives rise to spores immediately following meiosis. Figure 28: Basic plant life cycle During the haploid gametophytic generation, male (antheridia) and female (archegonia) sex organs (gametangia) are produced by the gametophyte. Recall that archegonium produces a single egg and that sperm are produced in the antheridium. One sperm will fuse with the egg; this process, known as fertilization, results in a fertilized egg, or zygote (2n). Because the zygote is diploid, it is the first stage in the sporophyte generation. It divides by mitosis and develops into a multicellular embryo, which is supported and protected by the gametophyte. Eventually, the embryo matures into the sporophyte. The sporophyte has special cells that are capable of dividing by meiosis. These cells, called spore mother cells, undergo meiotic division and form haploid spores. Because the spores are haploid, they represent the first stage in the gametophyte generation. Each spore is capable of growing by mitosis into a multicellular gametophyte, and the cycle continues as previously discussed. Thus, plants have an alternation of generations, alternating between a haploid (gametophyte) stage and a diploid (sporophyte) stage. The basic plant life cycle occurs in all plants; however, differences in structures can be observed in the various phyla (Figure 29-30): - The sporophyte is larger and more visible than the gametophyte in all plants except mosses and their relatives. 26  Plant Reproduction – Lebanese University, Faculty of Sciences - A comparison of life cycles among plant divisions emphasizes an important trend in plant evolution: reduction of the haploid gametophyte generation and dominance of the diploid sporophyte. Certain life cycle features are adaptations to a terrestrial environment, e.g. the replacement of flagellated sperm by pollen. The evolutionary trend has been toward a reduction in the size of the haploid generation. Figure 29: Variations in plants Gametophyte / Sporophyte structure Figure 30: Evolutionary trend in the size of plant haploid generation. 27  Plant Reproduction – Lebanese University, Faculty of Sciences CHAPTER 6: REPRODUCTION OF BRYOHYTES Non-vascular plants, homosporous Bryophytes are non-vascular plants that reproduce via spores. Bryophytes display two adaptations that first made the move onto land possible. They are covered by a waxy cuticle that helps the body retain water, and their gametes develop within gametangia. Bryophytes include mosses, liverworts and hornworts. 1. ASEXUAL REPRODUCTION Asexual reproduction in Bryophyta (Mosses, Liverworts and Hornworts) may occur by fragmentation or by gemmae. - Fragmentation: Pieces of a gametophyte can break off and form new moss plants. - Gemmae (e.g. Marchantia: division heptophyta) are tiny, multicellular propagules, cup- shaped structures on the gametophytes. Raindrops separate gemmae from the parent plant so they can spread and form new gametophytes. Figure 31: Gemmae 2. SEXUAL REPRODUCTION The gametophytes are the dominant stage of the life cycle: that is, the normal plant is the haploid gametophyte, with the only diploid structure being the sporophytes in season. The gametophytes are usually larger and longer-living than the sporophytes. Gametophytes can form multiple gametangia, each of which produces gametes. Male gametophytes develop reproductive structures called antheridia (sing. antheridium) that produce sperm by mitosis. Female gametophytes develop archegonia (sing. archegonium) that produce eggs by mitosis. Each archegonium produces one egg, whereas each antheridium produces many flagellated sperm. Bryophyte could be monoecious or dioecious depending on species. Fertilization occurs only when water covers the reproductive organs: Sperm cells, which have flagella, are transported from antheridium to archegonium (into the neck canal of the archegonium), by flowing water. Within the archegonium, one of the sperm fuses with the egg cell, producing the zygote. This type of reproduction is oogamy. The diploid zygote develops into a mature moss sporophyte that grows parasitically on the female gametophyte. 28  Plant Reproduction – Lebanese University, Faculty of Sciences 2.1. Class of Bryopsida —Mosses (Mnium) Each individual moss plant (a few centimeters tall) consists of two parts: - The gametophyte structures: an upright stemlike structure bearing leaflike blades and anchored by tiny hairlike absorptive rhizoids, - The sporophyte structures: a foot, which anchors the sporophyte to the gametophyte, a seta and a capsule, which contains sporogenous cells (spore mother cells). Though green and photosynthetic when young, the sporophyte turns tan or brownish red when ready to release spores. Mosses have an alternation of generations (haplodiplontic cycle), the sporophyte grows on the female gametophyte and remains attached and nutritionally dependent on the gametophyte throughout its existence. Archegonia Antheridial head Archegonial head Figure 32: Moss reproductive structures Fertilization occurs when one of the sperm cells fuses with the egg within the archegonium. Sperm cells, which have flagella, travel to a neighbouring plant via a water droplet and are chemically attracted to the entrance of the archegonium, and fertilization results. The diploid zygote formed as a result of fertilization grows into a multicellular embryo by mitosis, and matures into a moss sporophyte. The embryonic sporophyte develops within the archegonium, and the mature sporophyte stays attached to the female gametophyte throughout its existence. The sporophyte is not photosynthetic. Thus both the embryo and the mature sporophyte are nourished by the photosynthetic gametophyte. Meiosis within the capsule of the sporophyte (which contains a sporogenous tissue) yields haploid spores. When the spores are mature, the capsule opens and the spores are dispersed by wind or rain. If a moss spore lands on a suitable spot, it germinates and grows into a filamentous thread of cells called protonema. The protonema forms buds, each of which grows into a leafy gametophyte plant, and the life cycle continues. 29  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 33: Mosses life cycle 2.2. Class of Hepaticopsida — Liverworts (e.g. Marchantia polymorpha) Liverworts sexual reproduction involves the production of archegonia and antheridia on the haploid gametophyte. Their life cycle (haplodiplontic) is basically the same as that of mosses, although some structures look quite different. They both have the dominance of the gametophyte generation, and the sporophyte is parasitic on the gametophyte. Most liverwort gametophytes develop directly from spores (absence of protonema), but some genera first form a protenema-like filament of cells, from which the mature gametophyte develops (rudimentary protonema). In some liverworts (Marchantia), gametangia are born on stalked structures; archegonial receptacle (archegoniophores) bears archegonium and antheridial receptacle (antheridiophores) bears antheridia. Raindrops transport sperm cells to the archegonia, where fertilization takes place. The resulting zygote develops into a multicellular embryo that is totally dependent on the gametophyte. The developed sporophyte is anchored in the tissue of the archegoniophores. In its capsule, sporocytes undergo meiosis, producing haploid spores. Other cells inside the capsule do not undergo meiosis but remain diploid and develop instead into tubular structures with spiral thickenings, called elaters. Elaters help disperse spores by coiling and uncoiling in response to changes in humidity. 30  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 34: Marchantia polymorpha showing antheridium, archegonium, gemma cup, and photosynthetic thallus gametophyte Female gametophyte Male gametophyte Female umbrella (Archegonia) Male umbrella (Antheridia) Figure 35: Sexual structures (archegonia and antheridia) of Marchantia polymorpha 31  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 36: Life cycle of Marchantia. 2.3. Class of Anthoceropsida — Hornworts (e.g. Anthoceros punctatus) Hornworts mature sporophytes look like miniature, greenish to blackish rods. The sporophyte grow like horns from the gametophyte. Hornworts are less than 2 cm in diameter and are anchored by rhizoids. Sporophytes have neither setae nor calyptra, but have columella as in mosses, and elaters as in liverworts. They have a haplodiplontic life cycle, dominance of gametophytic phase, oogamy. In sexual reproduction, archegonia and antheridia are produced in rows just beneath the upper surfaces of the gametophytes. As growth occurs, sporocytes surrounding a central rodlike axis in the sporophyte undergo meiosis, producing spores. Diploid elaters that function similarly to those of liverworts are associated with spores. The tip of the sporophyte horn splits, releasing the spores. 32 Plant Reproduction – Lebanese University, Faculty of Sciences Figure 37: A Hornwort sporophyte 33  Plant Reproduction – Lebanese University, Faculty of Sciences CHAPTER 7: REPRODUCTION OF PTERIDOPHYTES Vascular seedless plants During the early stage of plant evolution, internal conducting tissues (xylem and phloem) began to develop, true leaves appeared, and roots that function in absorption as well as anchorage developed. The life cycle of pteridophytes displays an alternation of generations where the sporophyte is the dominant. Gametophytes are greatly reduced in size as well as in duration relative to the sporophyte. 1. ASEXUAL REPRODUCTION Pteridophytes reproduce asexually through the fragmentation of rhizomes. 2. SEXUAL REPRODUCTION 2.1. Reproduction of Filicopsida – e.g. Ferns (Polypodium vulgare)- homosporous, homothallus Ferns (Polypodium vulgare) follow a pattern of development similar to that of mosses, although most (but not all) ferns are homosporous. That is, the sporophyte produces only one type of spore within a structure called the sporangium. The gametophyte is a free-living organism. A single gametophyte can produce both male and female sex organs. The greatest contrast between mosses and ferns is that both the gametophyte and the sporophyte of ferns photosynthesize and are thus autotrophic; the shift to a dominant sporophyte generation is taking place. The life cycle of a typical fern (Polypodium vulgare) is as follows: 1. A sporophyte (diploid) phase (the conspicuous plant body of the fern) produces haploid spores by meiosis. 2. A spore grows by cell division into a gametophyte, which typically consists of a photosynthetic heart-shaped prothallus (mature fern gametophyte, which bears no resemblance to the sporophyte). 3. The gametophyte usually produces both archegonia and antheridia on its underside. Each archegonium contains a single egg, whereas numerous sperm are produced in each antheridium. Gametes are produced by mitosis. 4. A motile, flagellate sperm swims to the neck of the archegonium through a thin film of water on the ground underneath the prothallus and fertilizes an egg that remains attached to the prothallus. 5. The fertilized egg is now a diploid zygote and grows by mitosis into a multicellular embryo. At this stage in its life, the sporophyte embryo is attached to and dependent on the gametophyte, but as the embryo matures into a sporophyte plant (the typical “fern” plant), the prothallus withers and dies. 34  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 38: Fern life cycle The life cycle of ferns has a clearly defined alternation of generations between the diploid sporophyte and the haploid gametophyte (prothallus). The sporophyte generation is dominant not only because it is larger than the gametophyte, but also because it persists for an extended period of time (many ferns are perennial), whereas the gametophyte dies soon after reproducing. 2.2. Reproduction of Sphenopsida — e.g. Horsetails or Equisetum - homosporous heteroprothallus Small, cone-like strobili develop at the tips of the stems (fertile, non-photosynthetic or, in other species, at the tips of regular photosynthetic stems). When the sporocytes in the sporangia undergo meiosis, distinctive-appearing green spores are produced. The spores have four ribbon-like appendages, elaters. The elaters are very sensitive to changes in humidity and aid spores in their dispersal. While spores are being carried by air current, the appendages are slightly expanded like wings. Germination of spores gives rhizoids to gametophytes to anchor them to surfaces. When water contacts mature antheridia, sudden changes in water pressure cause the sperms produced within to be explosively ejected. They are homosporous heteroprothallus. 35  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 39: Life cycle of a horsetail. 2.3. Reproduction of Lycopodiopsida – e.g. Selaginella - heterosporous homothallus Sporangia of Selaginella develop on either microsporophylls or megasporophylls of the strobili. Microsporophylls bear microsporangia containing numerous microsporocytes that undergo meiosis, producing tiny microspores. The megasporangia of megasporophylls usually contain a megasporocyte that, after meiosis, becomes four comparatively large megaspores. They are heterosporous. Microspores and megaspores germinate inside their sporangia, endoprothally. Each microspore may become a male gametophyte with a spherical antheridium within the microspore wall (sperm cells with flagella are produced in each antheridium; oogamy). A megaspore develops into a female gametophyte. Archegonia and antheridia are on the same body structure, Selaginella is homothallic. Zygote develops into an embryo. 36  Plant Reproduction – Lebanese University, Faculty of Sciences Selaginella are heterosporous homothallus. The cycle is an haplo-diplontic life cycle with the dominance of the diploid stage. Figure 40: Life cycle of Selaginella. Figure 41: Dissected strobilus of a Spike Moss (Selaginella) 37  Plant Reproduction – Lebanese University, Faculty of Sciences CHAPTER 8: REPRODUCTION OF GYMNOSPERMS Spermatophytes, naked-seeded plants, without flowers, simple fertilization Gymnosperms are Spermatophytes characterized by the exposed nature of their seeds (gymnos=naked, sperma=seeds). They lack flowers and ovaries. In Gymnosperms the ovule, which becomes a seed, rests exposed on a scale (modified leaf, cones) and is not completely enclosed by sporophyte tissues at the time of pollination. The gametophyte of gymnosperms doesn’t grow independently but develops within sporophyte structures. The gametophyte stage is greatly reduced. In gymnosperms, the life cycle is digenetic, haplo-diplontic, with the dominance of the sporophyte generation. The most familiar gymnosperms are conifers (phylum Coniferophyta), which include pines, firs, cedars, cypresses, and others. Figure 42: General life cycle of Gymnosperms 1. CONIFEROPHYTA – e.g. Pines The term conifer comes from the reproductive structure of these plants: the cone. The pine tree, a representative conifer, is a sporophyte, with its sporangia located on cones. Like all seed plants, conifers are heterosporous. In conifers, the two types of spores are produced by separate cones: small pollen cones and large ovulate cones. Conifers are generally monoecious: male and female gametophytes are produced in separate cones but both are on the same tree. 38  Plant Reproduction – Lebanese University, Faculty of Sciences Pollen cones (male cones) consist of papery or membranous scales (microsporophyll) arranged in a spiral or in whorls around an axis; they are usually produced in the spring. In Lebanese species, the pollen cones usually develop toward the bases of the annual new shoots in clusters of up to 50 or more. In spring, male cones are relatively much numerous than female cones. Female cones typically are produced on the older branches of the same tree. Female cones are larger than male cones, and their scales, spirally arranged, become woody. In male cones, microsporangia develop in pairs toward the bases of the cone scales. Within female cones, two ovules develop toward the base (upper side) of each ovuliferous scale; a bract subtends each cone scale. Each ovule contains a megasporangium called the nucellus. The nucellus itself is completely surrounded by a thick layer of cells called the integument that has a small opening, the micropyle, toward one end. Figure 43: Structure of pine male and female cone (left). Series of micrograhs showing male and female gymnosperm gametophytes. (a) Cross section of a male cone showing microsporophylls, each of which produces hundreds of male gametophytes (pollen grains). (b) Pollen grains are visible in this single microsporophyll. (c) An individual pollen grain. (d) Croos section of a female cone showing megasporophylls. (e) The ovule can be seen in this single megasporophyll. (f) Within this single ovule are the megaspore mother cell (MMC), micropyle and a pollen grain.  The Life Cycle of Pine Each of the microsporocytes in the microsporangia undergoes meiosis, producing four haploid microspores. These then develop into pollen grains; each grain consists of four cells and a pair of external air sacs. The air sacs give the pollen grains added buoyancy that may result in the grains being carried great distances by the wind. 39  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 44: Pine pollen grain structure (left) and pollen tube (right) A single megasporocyte within the megasporangium of each ovule undergoes meiosis, producing a row of four relatively large megaspores. Three of the megaspores soon degenerate. Over a period of months, the remaining one slowly develops into a female gametophyte that ultimately may consist of several thousand cells. The nucellus is used as the food source for the growing gametophyte. As gametophyte development nears completion, two to six archegonia differentiate at the end facing the micropyle. Each archegonium contains a single large egg. Figure 45: A longitudinal section through a pine ovule. During the first spring, the immature cone scales spread apart, and pollen grains carried by the wind sift down between the scales. The pollen is drawn down through the micropyle to the top of the nucellus. After pollination, the scales grow together and close, protecting the developing ovule. Meiosis and megaspore development don’t occur until about a month after pollination. After a functional 40  Plant Reproduction – Lebanese University, Faculty of Sciences megaspore is produced, the female gametophyte and its archegonia don’t mature until more than a year later. Meanwhile, the pollen grain (immature male gametophyte) produces a pollen tube that slowly grows and digests its way through the nucellus to the area where the archegonia develop. While the pollen tube is growing, two of the original four cells in the pollen grain enter it. One of these, called the generative cell, divides and forms two more cells, called the sterile cell and the spermatogenous cell. The spermatogenous cell divides again, producing two male gametes, or sperms. The sperms have no flagella. The germinated pollen grain, with its pollen tube and two sperms, constitutes the mature male gametophyte. About 15 months after pollination, the tip of the pollen tube arrives at an archegonium, unites with it, and discharges the contents. One sperm unites with the egg, forming a zygote. The other sperm and remaining cells of the pollen grain degenerate. The sperms of other pollen grains present may unite with the eggs of other archegonia, and each zygote begins to develop into an embryo (the new sporophyte) that is nourished by the female gametophyte. Only one embryo completes development. The embryo develops a radicle and a shoot axis with cotyledons. While this development is occurring, one of the layers of the integument hardens, becoming a seed coat. The pine seed consists of an embryo and stored food supply, both of which are encased in a protective covering. From the time young pollen and ovulate cones appear on the tree, it takes nearly three years for the male and female gametophytes to be produced and brought together and for mature seeds to form from the fertilized ovules. The third year, the scales of each ovulate cone then separate, and the seeds are dispersed by the wind. A seed that lands in a suitable environment then germinates, its embryo emerging as a pine seedling. There are therefore some essential characteristics for Gymnosperms fertilization: - Fertilization is carried out by siphonogamy, pollen tube growth delivers the sperm (male gamete), which is no longer motile, to the egg on the female cone. - Pollination have replaced swimming as the mechanism for delivering sperm to eggs. Pollen, which contains sperm cells, is disseminated mainly by wind. - The zygote (seed plants) develops into an embryo that is packaged along with a food supply within a seed coat. This protects the dormant embryo from drought, cold and harsh conditions. At the time of their release, they contain a seedling. They are therefore anatomically and physiologically adapted to life on land. 41 Plant Reproduction – Lebanese University, Faculty of Sciences Figure 46: The life cycle of a pine. 42  Plant Reproduction – Lebanese University, Faculty of Sciences CHAPTER 9: REPRODUCTION OF ANGIOSPERMS Spermatophytes, flowering plants, double fertilization Commonly known as flowering plants, angiosperms are seed plants that produce the reproductive structures called flowers and fruits. The name angiosperm (from the Greek angion, container) refers to seeds contained in fruits, the mature ovaries. Angiosperms are divided into two classes: Monocotyledons and Dicotyledons. They reproduce asexually and sexually. The sexual reproduction of Angiosperms takes place in a complex host structure, the flower. Accordingly, Angiosperms are called “Anthophytes”. 1. TYPICAL STRUCTURE OF A FLOWER A typical flower has four main parts, or whorls: the calyx (sepals), corolla (petals), stamen (male reproductive structure), and carpel (female reproductive structure). Starting at the base of the flower are the sepals, which are usually green and enclose the flower before it opens. Interior to the sepals are the petals, which are brightly colored in most flowers and aid in attracting pollinators. In all angiosperms, the sepals and petals are sterile floral organs, they are not directly involved in reproduction. Within the whorl of petals are the reproductive organs, stamens and carpels. A stamen consists of a stalk called the filament and a terminal sac, the anther, where pollen is produced. At the tip of the carpel is a sticky stigma that receives pollen. A style leads from the stigma to the ovary at the base of the carpel. Protected within an ovary are the ovules, which develop into seeds after fertilization. Recall that the enclosure of seeds within ovary is one of the features that distinguish angiosperms from gymnosperms. Figure 47: Typical structure of Angiosperms’ flower 43  Plant Reproduction – Lebanese University, Faculty of Sciences 2. THE LIFE CYCLE OF AN ANGIOSPERM 2.1. Gametogenesis: Formation of egg cells and pollen grains Embryo sac (female gametophyte) develops inside ovules which are enclosed within ovary at the base of a carpel. While a flower bud is developing, a diploid megasporocyte cell differentiates from all the other cells in the ovule. This megasporocyte undergoes meiosis, producing four megaspores. In most flowering plants, three of the megaspores soon degenerate and disappear while the nucleus of the fourth undergoes mitosis, and the cell enlarges. While the cell is growing larger, its two haploid nuclei divide once more. The resulting four nuclei then divide yet another time. Consequently, eight haploid nuclei in all are produced. At the same time, two outer layers of cells of the ovule differentiate from maternal tissue. These layers, called integuments, later become the seed coat. As the integuments develop, they leave a pore at one end of the ovule, called the micropyle. At this stage, there are eight haploid nuclei in two groups, four nuclei near each end of the large cell. One nucleus from each group then migrates toward the middle of the cell. These two central cell nuclei may fuse to form a diploid nucleus. Plasma membranes and cell walls form around each of the remaining six haploid nuclei. In the group closest to the micropyle, one of the cells functions as the female gamete, or egg. The other two cells, called synergids, either are destroyed or degenerate. At the other end, the remaining three cells, called antipodals, have no apparent function, and later they also degenerate. The large sac, usually containing eight nuclei in seven cells, constitutes the female gametophyte (megagametophyte), formerly known as the embryo sac. Microspores develop in the microsporangium (pollen sacs in the anther) and form pollen grains (male gametophyte) Usually while the megagametophyte is developing, a parallel process that leads to the formation of male gametophytes takes place in the anthers. As an anther develops, many diploid microsporocyte cells, each of which undergoes meiosis, producing a tetrad of microspores. In most species the microspores of each tetrad separate. Each microspore divides once by mitosis to produce two cells: the generative (will produce the sperm cells) and the vegetative cells (will form the pollen tube). A two-layered wall develops around each microspore. When these events are complete, the microspores have become pollen grains. 44  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 48: Gametogenesis and gametophyte formation in flowering plants 2.2 Pollination and fertilization Pollination is simply the transfer of pollen grains from an anther to a stigma. After its release from the anther, the pollen is carried to the sticky stigma at the tip of a carpel. Although some flowers self-pollinate, most have mechanisms that ensure cross-pollination, which in angiosperms is the transfer of pollen from an anther of a flower on one plant to the stigma of a flower on another plant of the same species. The pollen grain germinates after it adheres to the stigma of a carpel. Under suitable conditions, the dense cytoplasm of the pollen grain absorbs fluids from the stigma and bulges out in the form of a tube. This pollen tube then grows down between the cells of the stigma and style until it reaches the micropyle of the ovule. The germinated pollen grain with its vegetative nucleus and two sperms within the tube cell constitutes the mature male gametophyte (microgametophyte). Figure 49: Pollination, pollen germination and fertilization in Angiosperms 45  Plant Reproduction – Lebanese University, Faculty of Sciences Fertilization involves the union of egg and sperm. After reaching the ovary, the pollen tube penetrates through the micropyle and discharges two sperm cells into the embryo sac (female gametophyte). One sperm nucleus unites with the egg, forming a diploid zygote. The other sperm fuses with the two nuclei in the center cell of the embryo sac, producing a triploid nucleus (3n). This phenomenon, known as double fertilization, is unique to angiosperms. Figure 50: Double fertilization After double fertilization, the ovule matures into a seed. The zygote develops into a sporophyte embryo with a rudimentary root and one or two seed leaves called cotyledons (monocots have one seed leaf and dicots have two). The triploid nucleus in the center of the embryo sac develops into a triploid tissue called endosperm, tissue rich in starch and other food reserves that nourish the developing embryo. The seed consists of the embryo, the endosperm, and a seed coat derived from the integuments. An ovary develops into a fruit as its ovules become seeds. After being dispersed, a seed may germinate if environmental conditions are favorable. The coat ruptures and the embryo emerges as a seedling, using food stored in the endosperm and cotyledons until it can produce its own food by photosynthesis. 46  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 51: Stages of sexual reproduction in flowering plants 3. FRUIT FORMATION At the end of double fertilization, the ovule becomes a seed, the integuments harden becoming a seed coat. The ovary matures into a fruit. The fruit layer, or pericarp, derives from the ovary wall. The fruit protects and provides a suitable environment for seed maturation and often could be a mechanism for the dispersa1 of mature seeds (Figure 54). 47  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 52: Life cycle of Angiosperms showing pollination, fertilization, fruit formation ad germination. 48  Plant Reproduction – Lebanese University, Faculty of Sciences Chapitre 10: Asexual reproduction vegetative reproduction Plants asexual reproduction is referred to as “vegetative reproduction”, or “vegetative propagation”, because it doesn’t involve genetic recombination (absence of fertilization). It gives rise to a homogenous population (genetically identical individuals) genetically identical to the parent (clone). It involves vegetative organs or plant fragments, i.e. roots, stems, and leaves. Vegetative reproduction occurs naturally, and it can also be brought about artificially. 1. NATURAL VEGETATIVE REPRODUCTION Natural vegetative reproduction is mostly a process found in herbaceous and woody plants. Plants display several natural vegetative features, through specific organs: 1.1. Bulbs A bulb consists of a dormant, short, below ground, central stem and bud surrounded by layers of thick fleshy leaves (bud scales) that contain stored food. As the plant grows, new bulblets arise in the axils of the bulb’s leaves. They can develop into a new plant. Tulips (Tulipa) and onions (Allium cepa) reproduce by bulbs. Figure 53: Bulbs & Bulblet 1.2. Tubers A stem tuber is an enlarged portion of an underground stem that contains stored food. Potatoes (Solanum tuberosum) produce tubers. Along the surface of a tuber are indentations called “eyes”, which are actually tiny buds that produce new plants. Root tubers or tuberous root originate from roots that have been modified to store nutrients. These roots become enlarged and may give rise to a new plant. Sweet potatoes and dahlias are examples of root tubers. Figure 54: Potato tubers. 49  Plant Reproduction – Lebanese University, Faculty of Sciences 1.3. Rhizomes A rhizome is a stem that grows horizontally underground. It is usually thick and fleshy and contains stored food. Rhizomes produce erect, leaf-bearing rooted plantlets at their nodes all along their length. One rhizome can give rise to many plants. Ferns and many perennial grasses reproduce by rhizomes. Figure 55: Rhizomes 1.4. Stolons (runners) Stolons (or runners), are lateral slim stems that tend to grow horizontally and run along the surface of the soil. When a bud from a stolon touches the soil, it develops to form a new independent plant. Strawberry plants reproduce in this manner. Figure 56: Runners 1.5. Suckers A form of budding called suckering is the production or regeneration of plants from shoots that arise from an existing root system (around the mother tree). e.g. plum, poplar, sumac, jujube ‫عناب‬ Figure 57: Suckers 50  Plant Reproduction – Lebanese University, Faculty of Sciences 1.6. Adventitious Plantlets The formation of plantlets is a unique type of propagation. Some plants have the ability to form small new plantlets at their leaf margins or the bases of the plant. A. B. Figure 58: Adventitious plantlets (A. Kalanchloe – B. Saintpaulia) 2. ARTIFICIAL VEGETATIVE REPRODUCTION Artificial vegetative propagation is a type of plant reproduction that is accomplished through artificial means involving human intervention. They are utilized by horticulturists to propagate (at big scale) economically valuable plants. A number of commonly cultivated plants are propagated by vegetative means rather than by seeds e.g. avocado, banana, citrus (lemon, orange, grapefruit), fig, pome fruits (apple, pear) … 2.1. Cutting A cutting is any vegetative part of a plant used to produce a new individual. For many plant species, a leaf, section of stem, or piece of root cut from a plant and lightly covered in soil, peat moss, or another growth medium develops a new, independent plant by generating the missing parts. Stimulated by hormones called auxins, a partially buried leaf develops roots and stems, i.e. begonia. A piece of stem develops roots on the buried portion (e.g. roses, lavender), and a piece of root forms stems and leaves above the soil (e.g. olive tree). Figure 59: Cutting 2.2. Layering In this method, a stem is bent over so that part of it is covered with soil. After the covered part forms roots, the new shoot may be cut from the parent plant to form an independent individual. 51  Plant Reproduction – Lebanese University, Faculty of Sciences The flexible stems of some species spread naturally in this way such as trailing blackberries and raspberries. In some cases, layering can be performed directly on aerial parts of the plant; it is thus termed air layering. e.g. Gardenia. Other layering methods could be applied (tip, simple, serpentine, compound, air layering…) Figure 60: Types of layering. A. Tip layering: The shoot tip is inserted in the soil and covered. B. Simple layering: the stem is bent down and the target region buried in the soil. C. Compound or Serpentine layering is similar to simple layering, but several layers can result from a single flexible stem. D. Mound layering: The plant back is cut just above the ground in the dormant season. Soil is mounded over the emerging shoots in the spring to enhance their rooting. E. Aerial layering branches are scraped and covered with mosses or plastic to reduce moisture loss. Adventitious roots develop where the branches were scrapped and the branches are removed from the tree and planted. 2.3. Grafting In grafting, a stem or bud is removed from one plant and joined permanently to the stem of a closely related plant (same species, same genus, same family). The part of this combination providing the root is called the stock; the added piece (upper portion) which will develop the aerial part is called the scion. The scion is securely attached to the stock, and the tissues of the two plants grow into each other, forming a single plant. The scion produces the stems, leaves, and flowers on the new plant and the stock supports (provides the root system) and nourishes the scion. Grafting combines desirable qualities from two components of the new plant. The stock is selected to provide vigor, good adaptation to soil conditions and resistance to diseases and parasites. The scion produces the requested marketed quality and quantity (fruits, flowers, etc) and mainly preserves the performance of the selected varieties. 52  Plant Reproduction – Lebanese University, Faculty of Sciences Bud grafting is a form of grafting in which a single bud cut from a stem is grafted onto the stock. It can be carried out more rapidly than other forms of grafting and is used widely in the nursery industry for mass propagation of plants. Figure 61: Grafting 2.4. Plant Tissue Culture Plant Tissue Culture is a technique of growing new plants from plant’s material (cells, tissues or organs) which are cultured in vitro in sterilized nutrient media under controlled aseptic conditions (laboratory). A variety of tissue culture techniques are used to propagate plants. Tiny pieces of plant’s material are cut and placed in sterile Petri dishes, on a gel-like medium enriched with hormones and nutrients that stimulates their development into plantlets. The young plants are then removed and placed in pots with soil, acclimatized in specific greenhouses and then transferred outdoor, where they grow to maturity. Numerous identical plants (clones) could be generated year-round through this method.  Culture of plants in Laboratory - Totally artificial media (presence of nutrients and hormones) - Controlled conditions (temperature and light) - Aseptic conditions 53  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 62: Micropropagation (Tissue culture)  Objectives  Large scale propagation of plants (true to type, clones) in very short durations. Many important plants (food plants, ornementals,..) have been produced on the commercial scale using this method (e.g. tomato, banana, potatoes, roses, …)  Production of healthy plants (virus free)  Storing germplasm (gene banks).  Quick generation of numerous clones year-round in greenhouses. From a single rose plant, tissue culture allows the production of hundreds of thousands of plants within one year, while by cutting dozens of plants could be produced. 54  Plant Reproduction – Lebanese University, Faculty of Sciences REFERENCES Campbell, NA and JB Reece. 2008. Biology, eighth edition. Pearson Benjamin Cummings, San Francisco. Levetin E and K McMahon. 2008. Plants and Society, fifth edition. McGraw-Hill Higher Education, New York. Ozenda P. 2000. Les Végétaux. Organisation et diversité biologique. 2ème èdition, Dunod Purves, Orians, Heller, Sadava 2000. Le monde du Vivant. 2ème édition. Flammarion. Raven P., Johnson G., Mason K., Losos J., Singer S. 2014 Biology, 10th Edition, Mc-Graw- Hill Companies, New York, 2002, 1408 p Reece J. B., & Campbell, N. A. 2011. Campbell biology. Boston, Benjamin Cummings / Pearson. Reviers B. (2003). Biologie et phylogénie des algues. Tome 1 et Tome 2. Belin. Rost, TL, MG Barbour, CR Stocking and TM Murphy. 2006. Plant Biology, second edition. Thomson Brooks/Cole, USA. Solomon E., Berg L., Martin D. 2008. Biology. Thomson Brooks/Cole.1234 p. Stern, KR. 2000. Introductory Plant Biology, eighth edition. McGraw-Hill Higher Education, New York. 55  Plant Reproduction – Lebanese University, Faculty of Sciences Choose the right answer (20 points) 1- The asexual reproduction is characterized by: a. Offspring has a different hereditary information as its parent b. a contribution of genetic material from another individual c. Stable characteristics within a species from generation to the next d. all answers are correct e. all answers are false 2- The spore is a special haploid cell which is produced by: a. the gametophyte by mitosis b. the gametophyte by meiosis c. the sporophyte by mitosis d. all answers are correct e. all answers are false 3- The vegetative reproduction of Higher Plant can be done by: a. Cutting b. Layering c. Grafting d. all answers are correct e. all answers are false 4- When two gametes of two similar types and sizes meet we talk about: a. isogamy b. planogamy c. anisogamy d. all answers are correct e. all answers are false 5- Parthenogenesis means: a. the developing of an embryo after the union of male gamete and female gamete b. into the flower c. the developing of a fruit after the fertilization and the formation of the embryo d. the developing of an embryo from egg cells without being fertilized e. all answers are correct f. all answers are false 6- The haplontic life cycle is characterized by: a. in the whole cycle, zygotes are the only diploid cells b. meiosis occurs before the zygote forms c. the individuals or cells as a result of mitosis are diplonts d. all answers are correct e. all answers are false 56  Plant Reproduction – Lebanese University, Faculty of Sciences 7- Antheridia: a. are male gametangia b. are female gametangia c. produce gametes by meiosis d. all answers are correct e. all answers are false 8- Which pattern characterizes the reproduction of Fucus vesiculosis: a. eggs and sperm released simultaneously into the water b. eggs release a pheromone that attracts the sperm (chemotaxis) c. fertilization occurs externally d. all answers are correct e. all answers are false 9- The life cycle of Ulva lactuca is characterized by: a. alternation of generations b. reproductive activities are near the margins of fronds c. similar morphologies of the gametophytic and sporophytic life stages d. all answers are correct e. all answers are false 10- The sexual reproduction of the Ascomycota is characterized by: a. asci are embedded in an ascocarp b.