Plant Reproduction and Growth The Living World Fifth Edition George B. Johnson Jonathan B. Losos Chapter 24 Plant Reproduction and Growth Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

24.1 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 lower investment of energy than sexual reproduction

24.1 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

Figure 24.1 Vegetative reproduction

24.1 Angiosperm Reproduction Sexual reproduction in plants involves an alternation of generations diploid sporophyte generation gives rise to a haploid gametophyte generation the male gametophytes are pollen grains that come from microspores the female gametophyte is the embryo sac, which develops from a megaspore these gametophytes are produced in separate, specialized structures of the angiosperm flower but both usually occur together in the same flower they are produced seasonally

24.1 Angiosperm Reproduction 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 plants that contain imperfect flowers that produce only ovules or only pollen are known as dioecious plants that contain imperfect flowers of both male and female on the same plant are called monoecious

24.1 Angiosperm Reproduction When an anther is cut in half, one would see pollen sacs, containing microspores each microspore 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 tube cell nucleus forms the pollen tube the generative cell will later divide to form two sperm cells

24.1 Angiosperm Reproduction Pollination is the process by which pollen is transferred from the anther to the stigma if pollen from a flower’s anther pollinates the same flower’s stigma, then self-fertilization has occurred some plants cannot self-pollinate because of self-incompatibility the pollen and the stigma recognize each other as being genetically related and block fertilization

24.1 Angiosperm Reproduction 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) or deceived into doing it coevolution has occurred between plants and pollinators plants may be colored or shaped in ways that attract pollinators for pollination by animals to be effective, a particular insect or animal must visit plant individuals of the same species

Figure 24.3 Insect pollination

24.1 Angiosperm Reproduction 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 the flowers of the wind-pollinated angiosperms are small, green, and odorless with inconspicuous or absent petals

24.1 Angiosperm Reproduction Eggs develop in the ovules of the angiosperm flower, which form the base of the carpel each ovule contains a megaspore mother cell each megaspore mother cell undergoes meiosis to produce four haploid megaspores 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

24.1 Angiosperm Reproduction 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 goes on to form endosperm this process of using two sperm cells in fertilization is called double fertilization

Figure 24.2 Formation of pollen and egg

24.2 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 the cells, the embryo early in the development of the embryo, the embryo stops developing and becomes dormant as a result of drying this arrestment of development is usually at a point soon after apical meristems and the seed leaves (called cotyledons) have developed

24.2 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

Figure 24.4 Development in an angiosperm embryo

24.3 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 fruits with fleshy covering are normally dispersed by bird and other vertebrates the animals carry seeds from place to place before excreting them as solid waste 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

Figure 24.5 Types of fruits and common modes of dispersion

24.4 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 the coleoptile emerges from underground in in monocots

Figure 24.6 Development of angiosperms

24.5 Plant Hormones Differentiation, the formation of specialized tissues, is largely reversible in plants some differentiated plant tissue are capable of expressing their hidden genetic information when provided with suitable environmental signals plant hormones control the expression of some plant genes all hormones in plants are produced in tissues that are not specialized for that purpose and carry out many other functions

24.5 Plant Hormones F.C. Steward successfully regenerated plants from isolated bits of phloem tissue Figure 24.7 How Steward regenerated a plant from differentiated tissue

Figure 24.8 Stages of plant differentiation

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

24.6 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

Figure 24.9 The Darwins’ experiment with phototropism

24.6 Auxin Frits Went worked out how auxin controls plant growth auxin causes the tissues on the side of the seedling into which it flowed 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

Figure 24.10 How Went demonstrated the effects of auxin on plant growth

Figure 24.11 Auxin causes cells to elongate

24.6 Auxin Synthetic auxins are used to control weeds the work by causing the plant to grow to death and reducing ATP production 2,4-D weedkiller affects broadleaf dicots a related herbicide, 2,4,5-T, (also known as Agent Orange) kills woody seedlings and weeds this product is easily contaminated with dioxin during the manufacturing process dioxin is an endocrine disruptor, a chemical that interferes with human development

24.7 Other Plant Hormones Gibberellins are synthesized in the apical portions of roots and shoots and affect stem elongation Cytokinins stimulate cell division in plants and determine the course of differentiation Ethylene, when applied to fruit, hastens ripening

Plant Hormones Figure 24.12 The effect of a gibberellin Figure 24.13 Cytokinins stimulate lateral bud growth in the absence of auxin

24.7 Other Plant Hormones Abscisic acid appears to stimulate ethylene synthesis, which promotes senescence and abscission it may also function in transpiration by stimulating the transport of potassium ions out of the guard cell

Figure 24.14 The effects of ethylene

Figure 24.15 Effects of abscisic acid

24.8 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

Figure 24.16 How photoperiodism works in plants

24.8 Photoperiodism and Dormancy Plants contain a pigment called phytocrome that influences flowering this pigment exists in two interconvertible forms Pr (inactive) and Pfr (active) in short-day plants, the presence of Pfr suppresses flowering the amount of Pfr steadily declines in darkness when the period of darkness is long enough, the suppression ceases and the flowering response is triggered

24.8 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

24.9 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

Figure 24.17 Tropism guides plant growth

Inquiry & Analysis What is the effect of auxin on the rate of pea root elongation over the range of concentrations studied? Is the inhibition of root elongation by high concentrations of auxin accompanied by corresponding increases in the concentration of ethylene? Graph of Effects of Auxin on Root Elongation