Flowers and Reproduction PDF

Chapter 9 Flowers and Reproduction  Concepts Reproduction can create. − Offspring with identical copies of the parent genes − Offspring genetically different from the parents In stable environments, asexual reproduction may be more selectively advantageous...

Chapter 9 Flowers and Reproduction  Concepts Reproduction can create. − Offspring with identical copies of the parent genes − Offspring genetically different from the parents In stable environments, asexual reproduction may be more selectively advantageous. In less stable environments, sexual reproduction can produce progeny that are more fit. Sexual reproduction − Progeny are genetically diverse. o Some are less adapted than the parent but others are more adapted. o Offspring cannot colonize a new site as rapidly because not all progeny are adapted for it, but some can colonize different sites with characteristics not suitable for parents. o Changes in habitat may adversely affect some progeny, but others may be adapted to the new conditions. o Isolated individuals cannot reproduce.  Concepts Asexual reproduction − All progeny are identical genetically to parent and to each other. − All are as adapted as parent, but none is more adapted. − Rapid colonization of a new site is possible. − All may be adversely affected by even minor changes in habitat. − Even isolated individuals can reproduce. Some plants reproduce both sexually and asexually. − Seeds are produced by sexual reproduction and can be dispersed over long distances. − New plants that are produced asexually are usually not capable of long-distance dispersal. What is Reproduction?  Reproduction Meiosis occurs in the anther (stamen) and ovule (carpel) Spore mother cells (which are diploid) divide meiotically to produce haploid spores (microspores - male; megaspore - female). In turn, the spores divide mitotically and develop into the male and female gametophytes, respectively. The microspores undergo one division to produce a two nucleate stage (tube and generative nuclei). The megaspore undergoes three mitotic divisions resulting in an eight nucleate stage (egg, 3 antipodals, 2 synergids, 2 polar nuclei). Pollen is the male gametophyte; actually the immature gametophyte The germinated pollen grain, with its pollen tube and two sperm nuclei represents the mature male gametophyte. Two sperms develop in the germinating pollen grain. The pollen tube follows chemical signals on its trip through the stigma and style to the ovule. The female gametophyte is housed in the ovule There may be from one (i.e., cherry) to many (i.e., watermelon) ovules per flower, depending on the species. In one type of female gametophyte, there are several cells, with a total of 8 nuclei. One nucleus, near the micropyle (opening into ovule), serves as the egg, and two others in the middle of the gametophyte (also called embryo sac) are called polar nuclei. Amitosis Mitosis Cell Cycle Meiosis  Asexual Reproduction Fragmentation is one of the most common methods of asexual reproduction. − A large vining plant grows to several meters in length. − Individual parts become self-sufficient by adventitious roots. − If middle portions of the plant die, ends become separated and act as individuals. Sexual Life of a Plant  Sexual Reproduction: Plant Life Cycle Sporophytes are diploid and produce haploid spores meiotically. Gametophytes produce gametes by mitosis. The gametes undergo syngamy to produce a diploid zygote that grows into a new sporophyte. Oogamous plants’ microspores develop into microgametophytes that produce sperm. Eggs produce megagametophytes that develop from megaspores. This life cycle is an alternation of generations. Sexual Reproduction: Plant Life Cycle Pollination Fertilization Sexual Variety  Flower Structure A flower is a stem with leaf-like structures. Complete flowers have all four floral appendages: sepals, petals, stamens, carpels. Incomplete flowers lack at least one appendage. Sepals are the outermost floral appendage. − Are modified leaves that surround maturing flower parts. − Protect the flower bud as it develops. − May be colorful. − All the sepals together are referred to as the calyx.  Flower Structure Petals are located above the sepals on the receptacle. − Are leaf-like but contain pigments other than chlorophyll. − Attract pollinators. − Collectively are a corolla. − Sepals and petals collectively are a perianth. − Absent in wind-pollinated species. Stamens occur above the petals. − Are collectively the androecium. − Two parts: anther and its supporting filament. − Diploid anther cells (microsporocytes) undergo meiosis to produce four microspores. − Microspores form a resistant cell wall and become  Flower Structure Carpels, collectively, are the gynoecium. − Stigma catches pollen grains. − Style elevates the stigma. − Ovary where megaspores are produced. Within the ovary are placentae bearing small structures called ovules. The ovule has a central mass of parenchyma called a nucellus. − There the megasporocyte undergoes meiosis to produce four megaspores (three degenerate). An ovule develops into a seed after it is fertilized.  Gametophytes Microgametophytes develop from microspores by mitosis. − In angiosperms, the microgameto-phyte consists of at most three cells. − Located within the original pollen cell wall. The microspore nucleus divides into Courtesy of Alan Prather, Michigan State University − A large vegetative cell. − A small generative cell, which divides to form two sperm cells. Pollen lands on a stigma and germinates. − It produces a pollen tube that penetrates into the stigma and makes its way to the ovule.  Gametophytes A megagametophyte is produced through division of the megaspore nucleus. − It divides to form eight cells, the embryo sac. − Most nuclei migrate to opposite ends. − Walls form around the nuclei. The large, eight-nucleate megaspore becomes a megagametophyte with seven cells. − One is binucleate. The seven cells are: − One large central cell with two polar nuclei. − Three small antipodal cells.  Syngamy of sperm and egg involves Fertilization − Plasmogamy: fusion of the protoplasts of the gametes − Karyogamy: fusion of the nuclei The pollen tube − Penetrates the nucellus. − Reaches the egg apparatus. − Enters one synergid. The two sperm are released. − One sperm moves through the synergid, loses its protoplasm along the way, to fuse with the egg nucleus. In angiosperms, the second sperm nucleus migrates into the central cell. − It undergoes karyogamy with both polar nuclei. − This establishes a large triploid endosperm nucleus. This is called double fertilization because both sperm nuclei undergo fusions. Endosperm nucleus undergoes very rapid mitosis. − The central cell enlarges into a huge cell with hundreds or thousands of nuclei; walls form later. − The resulting endosperm nourishes the development of the zygote.  Embryo and Seed Development The zygote grows and forms a short stalk-like suspensor. − It pushes the embryo deep into the endosperm. Divisions of the zygote form the embryo. − The end opposite from the suspensor develops two primordia that develop into one or two cotyledons. The embryo elongates and a short axis is established. − Radicle (embryonic root) − Epicotyl (embryonic stem) − Hypocotyl (the root/shoot junction)  Embryo and Seed Development Cotyledons store nutrients used during and after germination. − Endosperm nutrients may be transferred to the cotyledons as the seed matures. In monocots, the endosperm remains in the mature seed. − Cotyledon is thin. A mature seed in which endosperm is abundant is an albuminous seed. − If endosperm is sparse or absent at maturity, the seed is exalbuminous. The integuments that surround the nucellus expand and mature into the seed  Fruit Development As the ovule develops into a seed, the ovary matures into a fruit. Often three layers form. − The exocarp is the outer layer —the skin or peel. − The middle layer is the mesocarp, or flesh. − The innermost layer, endocarp, can be either tough or quite (c) Sarah Marchant/Shutterstock thin. The entire fruit wall is the pericarp.  Cross-pollination is by pollen from a different Flower Structure and Cross-Pollination individual. Self-pollination is by pollen from the same flower or another on the same plant. Results of each are similar to those of sexual versus asexual reproduction. Mechanisms have evolved to: − Decrease the probability of self-pollination. − Increase the chances of cross-pollination. – Self-fertilization in flowers that have both stamens and carpels is prevented if anthers and stigmas mature at different times. – On plants with many flowers that do not open simultaneously, older flowers could be self-  Flower Structure and Cross-Pollination Self-pollination may be inhibited by compatibility barriers. − Chemical reactions between pollen and carpels that prevent pollen tube growth Monoecious and dioecious species − Flowers that lack either stamens and carpels (essential organs) are imperfect flowers. − If a flower has both, it is a perfect flower. Some species have distinct individuals that produce only staminate flowers and others that produce only carpellate flowers. Courtesy of J. B. and M. E. Nasrallah, Cornell University − The species (not the flower or the plant) is said to be dioecious. − Dioecy ensures cross-pollination.  Flower Structure and Cross-Pollination Monoecy is the condition of having staminate flowers located on the same plant as the carpellate. −The species is monoecious. −Some species can vary the ratio of staminate/carpellate (c) BKPrimus/iStock (c) mrhighsky/iStock flowers, depending on environmental conditions.  Animal and Insect Pollination Animal-pollinated flowers exhibit dramatic evolutionary changes. − Insect-flower associations increase the probability of pollination. Some animals and flowers have undergone coevolution. − A flower adapts for visitation by a particular animal and the animal for efficient exploitation of the flower. Most flowers are radially symmetrical. − Actinomorphic flowers Flowers that coevolved with animals are often bilaterally symmetrical, like their pollinators. − Zygomorphic flowers  Animal and Insect Pollination Zygomorphic flowers are easier for animals to interact with successfully. −Both the flower and insect receive their rewards. (c) kmartin457/iStock Some plants and animals have evolved tricks that rob one or  Pollination: Wind-Pollinated Wind-pollinated flowers usually have no petals and reduced or absent sepals. −Huge numbers of pollen grains are produced to increase the chance of pollination. −Large, feathery stigmas increase the area that can catch pollen grains. −Up to several thousand tiny flowers may be produced per plant. −Wind-pollinated plants tend to form dense populations that aid pollination.  Pollination: Ovary Position Long styles and stamens and buried ovaries help protect the ovary and ovules from pollinators. −Inferior ovaries have fused bases of stamens, petals, and sepals. −With superior ovaries, the ovary sits above the other floral parts. −Intermediate, partially buried ovaries are half-inferior.  Inflorescences and Pollination Reproductive success may be increased by increasing the number of ovules per flower. − Large flowers tend to have numerous ovules. Smaller flowers with fewer ovules may be grouped together in an inflorescence. − Visible to pollinators, but not as risky as one large flower. When grouped into inflorescences, two basic arrangements occur. − Determinate inflorescences: The apex is converted to a flower, which tends to open before the lower flowers. − Indeterminate inflorescences: The lowest or outermost flowers open first, and new flowers are being initiated at the apex.  Fruit Types and Seed Dispersal Fruits are adaptations that result in the protection and distribution of seeds. Tough fruits are well-protected but expensive to produce. − The seed must still be able to break out to germinate. − If animals are to disperse the seeds, part of the fruit must be edible or otherwise attractive. − The seed and embryo must be protected from consumption. Fruits can be described as “true” fruits or accessory fruits. − True fruits are fruits containing only ovarian (c) Alexander Chernyakov/iStock  Fruit Types and Seed Dispersal Fusion of carpels Classification of fruit affects the nature of − Dry fruits—inedible to fruits. animals −Simple fruit − Fleshy fruits—eaten to −Aggregate fruit distribute seeds −Multiple fruit Dry fruits are further classified based on fruit opening. − Dehiscent fruits break open, releasing the seeds. − Indehiscent fruits do not.  Fruit Types and Seed Dispersal In grasses, seed and fruit are fused together. −There is little protection and no attraction for animals. Fruits of beans and peas are called legumes. −Form from a single carpel. −They dehisce along two lines of weakness. Wind-dispersed seeds are lightweight. −They often have wings or parachutes that carry them in wind.  Fruit Types and Seed Dispersal Fruits and seeds that are transported by water must − Be buoyant. − Resist mildew and rot. Animals often disperse seed by interacting with the fruit. Pomes (apples, pears) develop from inferior ovaries and are accessory fruits. Drupes (peaches, cherries) provide maximum attraction to animals with minimum danger to the seed. There are benefits beyond dispersal for seeds distributed by animals. A seed may be “deposited” and find itself in a small (or large) mound of “organic fertilizer.”