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Plant structure, Growth, and Reproduction, fruit is a mature ovary that…

- - - - Botanists (plant biologists) have used a variety of meth-ods to determine the time and place of the domestication of other crops. Many tropical plants—such as bananas, yams, and chilies—pose particular difficulties because they rot quickly in their humid climates, leaving little behind.
        
        Genetic analysis can also be used to pinpoint the time and place of domesticationSuch a genetic analysis indicates that corn was first domesticated about 9,000 years ago in southern Mexico.
        
        By combining all these types of evidence—archaeological, anatomical, microscopic, genetic—plant scientists have been able to piece together an overall map and timeline for crop domestication
  - - - Most flowering plants are eudicots, including many food crops (such as nearly all our fruits and vegetables), the major-ity of ornamental plants, and most shrubs and trees (except for the gymnosperms, naked seed plants such as cone-bearing conifers).
        
        Monocots:one cotyledon,veins usually parallel,
        Vascular tissue scattered in complex arrangement,Floral parts usually in multiples of three Fibrous root system
        
        Eudicots:two cotyledons,Veins usually branched Vascular tissue usually arranged in rings Floral parts usually in multiples of four or five Taproot usually present
        
        Annuals emerge from seed, mature, reproduce, and die in a single year or growing season. Our most important food crops (including grains and legumes) are annuals, as are a great number of wildflowers.
        Biennials complete their life cycle in two years, with flower-ing and seed production usually occurring during the second year. B
        
        Perennials are plants that live and reproduce for many years.
        
        Plants can keep growing because they have perpetually dividing, unspecialized tissues called meristems that divide when conditions permit, producing new cells that elongate and become specialized.
        
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  - - - Collenchyma cells (Figure 31.6C) resemble paren-chyma cells in lacking secondary walls, but they have unevenly thickened primary walls. T
        
        Sclerenchyma cells (Figure 31.6d) have thick second-ary cell walls usually strengthened with lignin, an import-ant chemical component of wood
        
        two types of scleren-chyma cells. One, called a fiber, is long and slender and is usually arranged in strands.
        Some plant tissues with abundant fiber cells are commercially important;
        hemp fibers, for example, are used to make rope and clothing, and flax fibers are woven into linen. Sclereids, which are shorter than fiber cells, have thick, irregular, and very hard secondary walls. Sclereids impart the hardness to nutshells and seed coats and the gritty texture to the soft tissue of a pear.
        
        Tracheids are long, thin cells with tapered ends. Vessel elements are wider, shorter, and less tapered.
        
        Food-conducting cells, known as sieve-tube elements (or sieve-tube members), are also arranged end-to-end, form-ing tubes as part of phloem tissue
        
        end walls between sieve-tube elements, called sieve plates, have pores that allow fluid to flow from cell to cell along the sieve tube. Alongside each sieve-tube ele-ment is a companion cell, which is connected to the sieve-tube element by numerous plasmodesmata.
        
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  - - - Secondary xylem makes up the wood of a tree, shrub, or vine.
        
        epidermis is shed and replaced with a new outer layer called cork (brown). Mature cork cells are dead and have thick, waxy walls that protect the underlying tissues of the stem from water loss, physical damage, and pathogens. Cork is produced by meristem tissue called the cork cambium (light brown), which first forms from parenchyma cells in the cortex
        
        Everything external to the vascular cambium is called bark.
        
        Wood rays consist of parenchyma cells that transport water and nutri-ents, store organic nutrients, and aid in wound repair. The heartwood, in the center of the trunk, consists of older lay-ers of secondary xylem.
        
        The lighter-colored sapwood is younger secondary xylem that does conduct xylem fluid (sap).
  - - - You can think of a seed as an “escape pod” in which a plant embryo lies dormant, surrounded by a supply of food and pro-tected from the elements. At germination, the plant resumes the growth and development that were suspended during seed dormancy. Safely enclosed in a seed, a plant embryo can survive for quite a long time. Recently scientists germinated the 2,000-year-old seeds of a Middle Eastern date palm found in an archeological excavation of Herod the Great’s palace, thereby resurrecting a species that had been extinct for more than 1,500 years. In an even more ancient example, Siberian wildflower seeds were successfully germinated after remaining dormant in permafrost for 32,000 years.
        
        In the wild, only a small fraction of fragile seedlings endure long enough to reproduce. Production of enormous numbers of seeds compensates for the odds against individual survival.
        
        If this were possible, the result would be a clone, an asexually produced, genetically identical organism or group of organisms
        
        In nature, asexual reproduction in plants often involves fragmentation, the separation of a parent plant into parts that develop into whole plants
        
        Aside from the issues raised by GMO plants, modern agriculture faces some potentially serious problems. Nearly all of today’s crop plants have very little genetic variability.
        In fact, we grow most crops in monocultures, cultivating a single plant variety on large areas of land. Given these con-ditions, plant scientists fear that a small number of diseases could devastate large crop areas. In response, plant breeders are working to maintain “gene banks,” storage sites for seeds of many different plant varieties that can be used to breed new hybrids.
        
        Some plants can survive a very long time. For example, coast redwoods (Sequoia sempervirens), found only in northern California and southern Oregon, are estimated to be 2,000 to 3,000 years old. However, those redwoods are mere youngsters compared with some other trees. The tree shown in Figure 31.16 is a 4,600-year-old bristlecone pine (Pinus longaeva). Bristlecone pines are found only in six western U.S. states (including the White Mountain region of California, pictured here). Another bristlecone pine, named Methuselah, is believed to be Earth’s oldest-living organism. Its location is kept secret to protect the tree.
  - - - being used to improve the nutritional quality of crops. Golden Rice, a transgenic vari-ety with a few foreign genes that increase the synthesis of vitamin A, is currently under development and evaluation
        
        About 80% of living plant species gain even more absorptive surface by teaming up with fungi.
        
        Together, the root and the fungus form a mutually beneficial (mutualistic) association called a mycorrhiza
        
        nodules, plant cells have been “infected” by nitrogen-fixing bacteria of the genus Rhizobium, which means “root living.” Rhizobium bacteria reside in cytoplasmic vesicles formed by the root cell (visible in the micrograph on the left side of the figure).
        Each legume is associated with a particular strain of Rhizobium.
        Other nitrogen-fixing bacteria are found in the root nodules of certain plants that are not legumes, such as alder trees.
        
        Examples of epiphytes include staghorn ferns and many kinds of orchids (Figure 32.15a).
        Epiphytes produce their own food and absorb water and minerals from rain.
        
        Unlike epiphytes, parasitic plants absorb water, sugars, and minerals from their living hosts. Many species have roots that tap into the host plant.
        
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- - - - A stem has nodes, the points at which leaves are attached, and internodes, the portions of the stem between nodes. The leaves are the main photo-synthetic organs in most plants, although green stems also perform photosynthesis.
        
        When a plant stem is growing in length, the terminal bud (also called the apical bud) at the apex (tip) of the stem has developing leaves and a compact series of nodes and internodes. The axillary buds, one in each of the crooks formed by a leaf and the stem, are usually dormant. In many plants, the ter-minal bud produces hormones that inhibit the growth of the axillary buds, a phenomenon called apical dominance
        
        Stolons enable a plant to reproduce asexually as plantlets form at nodes along their length.
        You’ve seen a different stem modifica-tion if you have ever dug up an iris plant or cooked with fresh ginger; the large, brownish, root-like structures of these plants are actually rhizomes, horizontal stems that grow just below the soil surface.
        
        A potato plant has rhizomes that end in enlarged structures specialized for storage called tubers (the potatoes we eat
        
        Some eudicots, such as celery, have enormous petioles—the stalks we eat. Modified leaves called tendrils help vines, such as pea plants, cling to solid structures. (Some tendrils, such as those of grapevines, are modified stems.)
        
        The dermal tissue system (brown in the figure) is the plant’s outer protective covering. Like our own skin, it forms the first line of defense against physical damage and infectious organisms. In nonwoody plants, the dermal tissue system consists of a single layer of tightly packed cells called the epidermis. On leaves and most stems, der-mal cells secrete a waxy coating called the cuticle, which helps prevent water loss. The second tissue system found in all plant bodies is the vascular tissue system (purple).
        It is made up of xylem and phloem tissues and provides support and long-distance transport between the root and shoot systems. Tissues that are neither dermal nor vascular make up the ground tissue system (yellow). The ground tissue system accounts for most of the bulk of a young plant, filling the spaces between the epidermis and vascular tissue system. Ground tissue internal to the vascular tissue is called pith, and ground tissue external to the vascular tissue is called cortex. The ground tissue system has diverse func-tions, including photosynthesis, storage, and support.
        The close-up views in Figure 31.5 show how the three tis-sue systems are organized in the roots, stems, and leaves of a typical young plant. (We’ll examine older roots and stems later.) The view at the bottom left shows a cross section of the three tissue systems in a young eudicot root. Water and minerals that are absorbed from the soil must enter through the epidermis. In the center of the root, the vascular tissue system forms a vascular cylinder, with the cross sections of xylem cells radiating from the center like spokes of a wheel and phloem cells filling in the wedges between the spokes.
        The ground tissue system of the root, the region between the vascular cylinder and epidermis, consists entirely of cortex.
        The cortex cells store food as starch and take up minerals that have entered the root through the epidermis. The innermost layer of the cortex is the endodermis, a cylinder one cell thick
        
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- - - - Tissues that are neither dermal nor vascular make up the ground tissue system (yellow).
        
        Ground tissue internal to the vascular tissue is called pith, and ground tissue external to the vascular tissue is called cortex
        
        In the center of the root, the vascular tissue system forms a vascular cylinder, with the cross sections of xylem cells radiating from the center like spokes of a wheel and phloem cells filling in the wedges between the spokes.
        
        The innermost layer of the cortex is the endodermis, a cylinder one cell thick
        
        Both stems have their vascular tissue system arranged in numerous vascular bundles.
        
        The epider-mis is interrupted by tiny pores called stomata (singular, stoma), which allow the exchange of CO2 and O2 between the surrounding air and the photosynthetic cells inside the leaf. Also, most of the water vapor lost by a plant passes through stomata. Each stoma is flanked by two guard cells, specialized epidermal cells that regulate the opening and closing of the stoma.
        The ground tissue system of a leaf, called the mesophyll, is sandwiched between the upper and lower epidermis.
        
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- - - - An ovule contains a central cell (gold) surrounded by a protective covering of smaller cells (yellow). ➊ The central cell enlarges and undergoes meiosis, producing four haploid spores.
        Three of the spores usually degenerate, but the surviving one enlarges and ➋ divides by mitosis, producing a multicellular structure known as the embryo sac. Housed in several layers of protective cells (yellow) produced by the sporophyte plant, the embryo sac is the female gametophyte. The sac contains a large central cell with two haploid nuclei. One of its other cells is the haploid egg, ready to be fertilized.
        ➌ The first step leading to fertilization is pollination, the transfer of pollen from anther to stigma. Most angio-sperms depend on insects (mainly bees), birds, or other ani-mals to transfer pollen. But the pollen of some plants—such as grasses and many trees—is windborne (causing pollen allergies in some people).
        After pollination, the pollen grain germinates on the stigma. Its tube cell gives rise to the pollen tube, which grows downward into the ovary. Meanwhile, the generative cell divides by mitosis, forming two sperm. ➍ When the pollen tube reaches the base of the ovule, it enters the ovary and discharges its two sperm near the embryo sac. ➎ One sperm fertilizes the egg, forming the diploid zygote. The other con-tributes its haploid nucleus to the large diploid central cell of the embryo sac. This cell, now with a triploid (3n) nucleus, will give rise to a food-storing tissue called endosperm.
        The union of two sperm cells with different nuclei of the embryo sac is called double fertilization, and the result-ing production of endosperm is unique to angiosperms.
- - - - Plants require a constant supply of water and dissolved min- erals from the soil. This is provided as xylem sap, a solution of water and inorganic nutrients that flows from the roots through the shoot system to the leaves. Xylem sap flows through very thin tubes within xylem tissue, pulled by transpiration, the loss of water from the leaves by evaporation.
        Sap movement is aided by the cohesion and adhesion of water molecules and requires no energy expenditure by the plant.
        
        The leaf stomata, which can open and close, are evolu-tionary adaptations that help plants adjust transpiration rates to changing environmental conditions. A pair of guard cells flanking each stoma control the opening by changing shape
        
        What actually causes guard cells to change shape and thereby open or close stomata? A stoma opens (left side of Figure 32.4) when its guard cells gain potassium ions (K+, red dots) and water (blue arrows) from neighboring cells (light gray). The guard cells actively take up K+. Following this active transport of solute, water enters by osmosis. As the vacuoles in the guard cells gain water, the cells become more turgid and bowed. When a guard cell becomes turgid, the structure of its cell wall causes it to buckle outward, away from its companion guard cell. The result is an increase in the size of the gap (stoma) between the two cells. Conversely, when the guard cells lose K+, they also lose water by osmosis and become flaccid and less bowed, closing the space between them
- - - - sugar maple (Acer saccharum)—although small-scale producers can use other maple species
        
        tropical regions, the rubber tree (Hevea brasil-iensis) can be tapped to col-lect latex, also called natural rubber. The latex, which reduces herbivory by making the plant less desirable to eat, is transported in special vessels that lie outside the phloem.
        The milky white latex is collected by cutting into the bark (Figure 32.6B) and is then chemically treated to produce commercial latex. Other kinds of commercially valuable latex are produced by the chicle tree (used in chewing gum) and the opium poppy (used to produce medications such as morphine).
        
        A nutrient is considered an essential element if a plant must obtain it from its environment to complete its life cycle—that is, to grow from a seed and produce another generation of seeds
        
        Nine essential elements are called macronutrients because plants require relatively large amounts of them
        
        Fertilizers are compounds given to plants via the soil to promote the plant’s growth. There are two basic types of fertilizers: inorganic and organic. Inorganic fertilizers (also called mineral fertilizers) may contain natu-rally occurring inorganic compounds (such as mined lime-stone or phosphate rock) or synthetic inorganic compounds (such as ammonium nitrate). Inorganic fertilizers come in a wide variety of formulations, but most emphasize their “N-P-K ratio,” the relative amounts of the three nutrients most often deficient in depleted soils: nitrogen (N), phos-phorus (P), and potassium (K). For example, a 100-pound bag of 5–6–7 fertilizer contains 5 pounds of nitrogen (often as ammonium or nitrate), 6 pounds of phosphorus (usually as phosphoric acid), and 7 pounds of potassium (usually as the mineral potash), plus 82 pounds of filler
        
        Organic fertilizers are composed of biologically derived products such as compost, a soil-like mixture of decomposed organic matter.
        
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- - - - showing That Light Is detected by the shoot Tip In the late 1800s, Charles Darwin and his son Francis conducted some of the earliest experiments on phototropism. They observed that grass seedlings could bend toward light only if the tips of their shoots were present.
        The five grass plants in Figure 33.1B summarize the Darwins’ findings. First, they verified that a control plant bends toward the light. When they removed the tip of a grass shoot, the shoot did not curve toward the light. Next, the Darwins found that the shoot also remained straight when they placed an opaque cap on its tip. However, the shoot curved normally when they placed a transparent cap on its tip or an opaque shield around its base (as seen in the last two shoots on the right). The Darwins concluded that the tip of the shoot was responsible for sensing light. They also recognized that the growth response, the bending of the shoot, occurs in cells that are below the tip. Therefore, they hypothesized that some chemical signal must be transmitted from the tip downward to the growth region of the shoot.
        
        А few decades later, Danish plant biologist Peter Boysen-Jensen further tested the Darwins’ chemical signal idea (Figure 33.1C, on the facing page). In one group of seedlings, Boysen-Jensen inserted a block of gelatin between the tip and the lower part of the shoot. The gelatin prevented direct contact but allowed chemicals to diffuse through. The seed-lings with gelatin blocks behaved normally, bending toward light. In a second set of seedlings, Boysen-Jensen inserted a thin piece of the mineral mica under the shoot tip. Mica is an impermeable barrier, and the seedlings so treated with mica had no phototropic response. These experiments supported the hypothesis that the signal for phototropism is a chemical that diffuses through the plant body
        
        Isolating the Chemical signal In 1926, Frits Went, a Dutch graduate student, modified Boysen-Jensen’s tech-niques and extracted the chemical messenger for photo-tropism in grasses. As shown in Figure 33.1d, Went first removed the tips of grass seedlings and placed the tips on blocks of agar, a gelatin-like material. He reasoned that the chemical messenger (pink in the figure) from the shoot tips would diffuse into the agar and that the blocks would then be able to substitute for the shoot tips. Went tested the effects of the agar blocks on tipless seedlings, which he kept in the dark to eliminate the effect of light. ➊ First, he centered the treated agar blocks on the cut tips of a batch of seedlings.
        These plants grew straight upward. They also grew faster than the decapitated control seedlings, which hardly grew at all.
        Went concluded that the agar had absorbed the chemical messenger produced in the shoot tip and that the chemical had passed into the shoot and stimulated it to grow. ➋ He then placed agar blocks off center on another batch of tip-less seedlings. These plants bent away from the side with the chemical-laden agar block, as though growing toward light.
        ➌ Control seedlings with blank agar blocks (whether offset or not) grew no more than the control.
        
        Plant biologists initially identified five major types of plant hormones (Table 33.2), all of which stimulate or inhibit cell division and elongation, thereby influencing growth.
        
        Auxins (33.3) Stimulate stem elongation; affect root growth, differentiation, branching; development of fruit; apical dominance; phototropism and gravitropism (response to gravity); retard leaf abscission Meristems of apical buds, young leaves, developing seeds and fruits Cytokinins (33.4) Affect root growth and differentiation; stimulate cell division and growth; stimulate germination; delay leaf aging Made in the roots and transported to other organs Gibberellins (33.5) Promote bud development, stem elongation, and leaf growth;
        stimulate flowering, fruit development, and seed germination Meristems of apical buds and roots, young leaves, developing seeds Abscisic acid (ABA) (33.6) Inhibits growth; closes stomata during dry spells; helps maintain seed dormancy; promotes leaf aging Every major organ: leaves, stems, roots, fruits Ethylene (33.7) Promotes fruit ripening and leaf abscission; opposes some auxin effects; promotes root formation; promotes flowers in some species Ripening fruits, nodes of stems, aging leaves and flowers, and wounds
- - - - Auxin promotes cell elongation in stems only within a specific concentration range. Above a certain concentration, auxin may inhibit cell elongation by inducing the production of ethylene, a hormone that generally counters the effects of auxin
        
        Cytokinins are hormones produced in actively growing tissues—particularly in roots, embryos, and fruits—that promote cytokinesis, or cell division, and cell differentiation.
        They also retard the aging of leaves and flowers by inhibiting protein breakdown
        
        Farmers in Asia have long noticed that some rice seedlings in their paddies grow so spindly that they topple over before they produce grain. In 1926, Japanese scientists discovered the cause: a fungus of the genus Gibberella.
        By the 1930s, scientists had determined that the fungus produced hyperelongation of rice stems by secreting a chemical they named gibberellin. In the 1950s, researchers discovered that gibber-ellin exists naturally in plants, where it is a growth regulator. Rice plants infected with the Gibberella fungus suffer from too much gibberellin.
        Since the 1950s, biologists have identi-fied more than 100 different gibberellins in plants, although any particular species uses only a few. Young roots and leaves are major sites of gibberellin production.
        One of the main effects of gibberellin is to stimulate cell elongation and cell division in stems and leaves.
        
        In the 1960s, one research group studying bud dormancy in trees and another team investigating abscission (dropping) of cotton fruits independently isolated the same compound, abscisic acid (ABA). Ironically, ABA is no longer thought to play a primary role in either bud dormancy or leaf abscis-sion (for which it was named), but it is a plant hormone of great importance in other functions. In contrast to the growth-stimulating hormones we have studied so far—auxin, cytokinins, and gibberellins—ABA slows growth. ABA often counteracts the actions of growth hormones; it is the ratio of ABA to one or more growth hormones that often determines the response of a plant.
        
        During the 1800s, leakage from coal gas streetlights caused the leaves on nearby trees to drop prematurely. In 1901, scientists demonstrated that this effect was due to ethylene, a gaseous by-product of coal combustion.
        We now know that plants pro-duce their own ethylene, which triggers a variety of aging responses, including fruit rip-ening and programmed cell death. Ethylene is also pro-duced in response to stresses such as drought, flooding, injury, and infection.
        
        Auxin prevents abscission and helps to maintain the leaf’s metabolism. But as a leaf ages, it produces less auxin.
        Meanwhile, cells begin producing ethylene, which stimu-lates formation of the abscission layer. The ethylene primes the abscission layer to split by promoting the synthesis of enzymes that digest cell walls in this layer.
        
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- - - - In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by the length of continuous darkness, not the length of continuous daylight. That is, “short-day” plants will flower only if it stays dark long enough, and “long-day” plants will flower only if the dark period is short enough. Therefore, it would seem more appropriate to call the two types “long-night” plants and “short-night” plants. However, the day-length terms are embedded firmly in the literature of plant biology and so will be used here.
        
        Notice that the top short-day plant will not flower until it is exposed to a continuous dark period exceeding a critical length (about 14 hours, shown in the middle bar).
        
        he effect of night length on a long-day plant. In this case, flowering occurs when the night length is shorter than a critical length (less than 10 hours, in this example, as seen in the top bar of this group). A dark interval that is too long will prevent flow-ering (middle bar in this group). In addition, flowering can be induced in a long-day plant by a flash of light during the night (bottom bar).
        Most species of plants have a critical night length, but how that critical night length affects flowering varies with the type of plant. In short-day plants, the critical night length is the minimum number of hours of darkness required for flow-ering; less darkness prevents flowering. In long-day plants, this critical night length is the maximum number of hours of darkness required for flowering; less darkness promotes flowering.
        
        remains to be learned, but photoreceptors called phytochromes are part of the answer. Phytochromes are proteins with a light-absorbing component.
        Phytochromes were discovered during studies on how different wavelengths of light affect seed germination. Many types of seeds germinate only when light conditions are near optimal, an adaptation that increases the chances of survival for the seedlings. Red light, with a wavelength of 660 nm, was found to be most effective at increasing germination. Light in the far-red range (730 nm, near the edge of human visibil-ity) inhibited germination. Furthermore, researchers found that the last flash of light a seed is exposed to determines the seed’s response. In other words, the effects of red light and far-red light are reversible.
        
        The bot-tom two bars indicate that no matter how many flashes of red or far-red light a plant receives, only the wavelength of the last flash of light affects the plant’s measurement of night length.
        Plants also have a group of blue-light photoreceptors that control such light-sensitive plant responses as phototropism and the opening of stomata at daybreak. Light is an especially important environmental factor in the lives of plants, and diverse receptors and signaling pathways have evolved that mediate a plant’s responses to light.
        
        most terrestrial ecosystems, plants are primary producers that sit at the base of the food web. Plants are therefore subject to attack by a wide range of herbivores (plant- eaters).
        
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