Plant reproduction and growth Chapter 34

Angiosperm Reproduction Reproduction in flowering plants, the angiosperms, can be asexual or sexual. Asexual reproduction is common in stable environments. This vegetative reproduction results when new individuals are simply cloned from parts of the parent. Asexual reproduction allows individuals to reproduce with a lower investment of energy than sexual reproduction.

Angiosperm Reproduction There are many forms of vegetative reproduction: Runners are slender stems that grow along the soil surface. Rhizomes are underground horizontal stems that create a network, giving rise to new shoots. Suckers are produced by roots and give rise to new plants. Adventitious plantlets arise from meristematic tissue located in the notches of leaves.

Vegetative reproduction Runner Rhizome

Angiosperm Reproduction Sexual reproduction in plants involves an alternation of generations. The diploid sporophyte generation gives rise to a haploid gametophyte generation, which is enclosed within the tissues of the sporophyte.

Angiosperm Reproduction Parent sporophyte tissue (main plant body and outer whorls of flowers) Female gametophyte (embryo sac) is inside base of flower Male gametophytes (pollen grains) are on these filaments

Structure of the Flower Most flowers contain male and female parts. The male parts are called stamens. The female part is called the carpel. Flowers that contain only male or only female parts are known as imperfect.

The structure of a flower Anther Stamen Stigma Style Carpel Ovary (b) (a) (c) b: © David Sieren/Visuals Unlimited; c: © Barbara Gerlach/Visuals Unlimited

Structure of the Flower Pollen formation occurs in the anthers. Inside the anthers are pollen sacs, which contain microspore mother cells. Each microspore mother cell undergoes meiosis to form four haploid microspores. These microspores then undergo mitosis to form pollen grains that contain a generative cell and a tube cell nucleus. The generative cell will later divide to form two sperm cells.

Structure of the Flower Egg formation occurs in the ovary, which contains the ovule. Each ovule contains a megaspore mother cell that undergoes meiosis. Only one megaspore survives to undergo repeated mitotic divisions that produce eight haploid nuclei. These nuclei are enclosed in an embryo sac, where the nuclei are precisely arranged.

Formation of pollen and egg Pollen sacs Pollen grains (n) 1 Anther 2 3 Meiosis Mitosis Microspore mother cell (2n) Microspores (n) Tube cell nucleus Generative cell Stamen Carpel Ovary Megaspore mother cell (2n) Megaspores (n) Surviving megaspore Antipodals Ovule 2 4 1 3 Polar nuclei Meiosis Mitosis Synergids Egg cell Degenerated megaspores 8-nucleate embryo sac (n)

Gametes Combine Within the Flower Pollination is the process by which pollen is transferred from the anther to the stigma. 2 3 pollen tube Tube cell Sperm cells Tube cell nucleus Pollen tube Growth of Pollination Pollen grain Stigma Style Ovary Carpel Embryo sac 1 Tube cell Ovule Generative 4 5 Antipodals Egg cell Synergids Polar nuclei Release of sperm cells Double fertilization Zygote (2n) Endosperm (3n)

Fertilization Pollen grains adhere to the sticky surface of the stigma and begin to grow a pollen tube. The pollen tube pierces the style and grows until it reaches the ovule in the ovary. When the pollen tube reaches the entry to the embryo sac, it releases two sperm cells. One sperm fertilizes the egg while the other sperm fuses with the polar nuclei to form endosperm. This process of using two sperm cells in fertilization is called double fertilization.

Gametes Combine Within the Flower Many angiosperms use animals to carry pollen grains from flower to flower. These pollinators may be rewarded for their efforts with food (e.g., nectar). Coevolution has occurred between plants and pollinators.

Gametes Combine Within the Flower In some angiosperms and in all gymnosperms, pollen is dispersed by wind and reaches the stigmas passively. The individuals of a given plant species must grow where there is ample wind and grow relatively close together.

Seeds Development is the entire series of events that occurs between fertilization and maturity. The first stage of development is active cell division to form an organized mass of cells, the embryo.

Development in an angiosperm embryo 1 2 3 4 Polar nuclei Triploid endosperm mother cell Endosperm (3n) Suspensor Egg (n) Basal cell Pollen tube Zygote (2n) Micropyle First cell division Sperm (n) Hypocotyl Shoot apical meristem Procambium Cotyledons Cotyledon Ground meristem Embryo Protoderm Endosperm 7 6 5 Cotyledons Root apex (radicle) 8 Root apical meristem

Seeds The integuments that form the outermost covering of the ovule develop into a seed coat. This layer is relatively impermeable and encloses the dormant embryo within the seed, together with a source of food. Germination cannot take place until water and oxygen reach the embryo. This assures that the seed will germinate when conditions are favorable for a plant’s survival.

Fruit During seed formation, the flower ovary begins to develop into fruit. Fruits form in many ways and exhibit a wide array of modes of specialization. Fleshy fruits are normally dispersed by birds and other vertebrates. Some fruits are dispersed by wind or by attaching themselves to the fur of mammals or the feathers of birds. Some fruits are dispersed by water.

Types of fruits and common modes of dispersion

Germination When a seed encounters conditions suitable for its germination: It first absorbs water. Once the seed coat ruptures, aerobic respiration begins. The roots emerge first. Cotyledons emerge, in dicots, from underground along with the stem.

Development of angiosperms DICOT MONOCOT First leaf First leaf Cotyledon Withered cotyledons Hypocotyl Coleoptile Adventitious root Cotyledon Radicle Seed coat Primary roots Secondary roots Primary root

Growth and Nutrition Plants require a number of nutrients. Macronutrients are needed in large amounts. Micronutrients are needed in trace amounts. These nutrients are called essential because the plant cannot manufacture them.

Plant Hormones Plant hormones control the expression of some plant genes. Hormones in plants are produced in tissues that are not specialized for that purpose and carry out many other functions.

Plant Hormones auxin gibberellins cytokinins ethylene abscisic acid At least five major kinds of hormones are found in plants auxin gibberellins cytokinins ethylene abscisic acid

Auxin Phototropism is the growth of plants toward light. Charles Darwin and his son Francis performed experiments that suggested that a substance caused the plant to bend if exposed to light. The substance was later identified to be auxin.

THE DARWINS’ EXPERIMENT WITH PHOTOTROPISM 1 2 Light Light Charles and Francis Darwin found that a young grass seedling normally bent towards the light. If the tip of the seedling was covered with a light proof cap, the seedling did not bend toward the light. 3 4 Light Light When the tip of the seedling was covered with a transparent cap, the bending did occur. When the Darwins placed a lightproof collar below the tip, the seedling bent toward the light.

Auxin Frits Went worked out how auxin controls plant growth. Shaded side of seedling Light Lighted side Frits Went worked out how auxin controls plant growth. Auxin causes the tissues on the side of the seedling into which it flows to grow more than those on the opposite side. The side of the plant in the shade has more auxin and divides more, causing the plant to bend towards the light.

Photoperiodism and Dormancy Photoperiodism is a mechanism by which organisms measure seasonal changes in relative day and night length. Plants’ flowering responses fall into three basic categories in relation to day length: Long-day plants flower when days become longer in the summer. Short-day plants flower when days become shorter in the fall. Day-neutral plants produce flowers without regard to day length.

Photoperiodism and Dormancy Plants have the ability to stop growing altogether when conditions are not favorable. This is called dormancy. In temperate zones, dormancy is generally associated with winter when low temperature and the freezing of water make it impossible for plant growth.

Tropisms Tropisms are directional and irreversible growth responses to external stimuli Gravitropism causes stems to grow upward and roots to grow downward. Thigmotropism is the response of plants to touch.