Study Notes Exam 2
Characteristics of Plants (kingdom Plantae)
- plants are multicellular photosynthetic eukaryotes adapted to life on land
- plants are believed to have evolved from a freshwater green algal ancestor (possibly stoneworts) over 500 million years ago (Paleozoic era)
o both utilize chlorophylls a and b and various accessory pigments, store food as starch & have cell walls containing cellulose
- plants, from nonvascular to vascular, nourish a multicellular embryo within the body of the female plant; this distinguishes them from green algae
- vascular plants have vascular tissues, specialized elongated cells that conduct water and solutes through the plant
- the cone-bearing gymnosperms and flowering angiosperms both produce seeds
o seeds are mature ovules and stored food within protective seed coat
o seeds are resistant to drought and somewhat resistant to predators
- plants have a two-generation life cycle called alternation of generations
- the sporophyte generation is a diploid (2n) generation producing haploid spores by meiotic cell division
- the spores produce the gametophyte generation, a haploid generation producing haploid gametes by mitotic division
- mitosis occurs as a spore becomes a gametophyte, and also as a zygote becomes a sporophyte
- plants differ in which generation–gametophyte or sporophyte–is dominant
o in nonvascular plants, the gametophyte is dominant
o in the vascular plants, the sporophyte is dominant or more conspicuous
o the shift to sporophyte dominance is an adaptation to life on land; the gametophyte becomes microscopic and dependent on the sporophyte
- sporophyte dominance & adaptation for water transport and conservation
- vascular tissues transports water and nutrients in the body of the plant
- leaves and stems covered by a waxy cuticle that retains water & limits gas exchange
- leaves & other tissues have openings (stomata) that regulate gas and water exchange
Nonvascular Plants (³Bryophytes²)
- nonvascular plants lack true roots, stems, and leaves, although they have rootlike, stemlike, or leaflike structures
- the gametophyte is the dominant generation recognized in bryophytes
o the gametophyte produces eggs in archegonia, flagellated sperm in antheridia
- nonvascular plants are quite small because of lack of vascular tissue and the need for sperm to swim to the archegonia in water
o because sexual reproduction involves flagellated sperm, they are usually found in moist habitats
o mosses compete well in harsh environments because the gametophyte can reproduce asexually, allowing them to spread into stressful habitats
- Hornworts (phylum Anthocerophyta)
o the small sporophytes look like tiny green broom handles and are attached to a filmy gametophyte that is less than two cm in diameter
- Liverworts (phylum Hepatophyta)
o the thallus or body appears similar to lobes of the liver
o Marchantia has a flat, lobed thallus about a centimeter in length
o the upper surface of thallus is smooth; lower surface bears numerous rhizoids (root-like extensions) projecting into soil (absorb water & minerals)
o asexual reproduction involves gemmae in gemmae cups on upper surface of the thallus; gemmae can start a new plant
o sexual reproduction: antheridia are on disk-headed stalks and produce flagellated sperm; archegonia are on umbrella-headed stalks and produce eggs
§ the zygote develops into a tiny sporophyte with a foot, short stalk, & capsule
§ spores produced within the gametophyte capsule are dispersed by wind
- Mosses (phylum Bryophyta)
o mosses are found from the Arctic through the tropics to parts of the Antarctic
o moss prefers damp, shaded localities but some survive in deserts, bogs & streams
o some ³mosses² are not true mosses:
§ Irish moss is a red alga; Reindeer moss is a lichen; Club mosses are vascular plants; Spanish moss is a flowering plant related to pineapple
o most mosses can reproduce asexually by fragmentation
o the moss life cycle begins with algalike protonema developing from the germination of a haploid spore
§ upright shoots develop covered with leafy structures & anchored by rhizoids
§ the shoots bear antheridia and archegonia at their tips
§ the antheridia produce flagellated sperm which need external water to reach eggs in archegonia
§ the archegonium looks like a vase with a long neck; it has an outer layer of sterile cells with a single egg at the base
§ fertilization results in a diploid zygote that undergoes mitotic division to develop a sporophyte
o the sporophyte consists of a foot (which grows down into the gametophyte tissue starting at the former archegonium), a stalk, and an upper capsule (sporangium) where spores are produced
- Uses of Bryophytes
o sphagnum (bog or peat moss) has tremendous ability to absorb water and is important in gardening
o sphagnum does not decay in some acidic bogs; the accumulated dried peat can be used as fuel
Vascular Plants
- vascular tissue
o xylem is vascular tissue that conducts water & minerals upward from the roots
o phloem is vascular tissue that transports sucrose & hormones throughout the plant
o lignin strengthens the walls of conducting cells in xylem
o the cuticle and stomata are also characteristics of a dominant sporophyte
o seedless plants are mostly homosporous, using spores for dispersal
o all seed plants are heterosporous, using pollen grain and seeds
Seedless Vascular Plants
- Club Mosses (division Lycopodophyta)
o common in temperate woodlands where they are called ³ground pine.²
o a branching rhizome sends up aerial stems less than 30 cm tall
o tightly packed, scalelike microphylls cover stems and branches; each contains one strand of vascular tissue
o sporangia are borne on the surface of leaves called sporophylls which are grouped in club-shaped stroboli; spores germinate into independent gametophytes
o most club mosses live in tropics or subtropics as epiphytes, plants that live on trees without harming them
o closely related are spike mosses (Selaginella) and quillworts (Isoetes)
- Ferns and Allies
o Phylum Sphenophyta today contains one genus, Equisetum (horsetails)
o a rhizome produces aerial stems that stand about 1.3 meters tall
o whorls of slender side branches & small scalelike leaves encircle nodes of a stem, resembling a horse¹s tail
o the tough, rigid stems have silica in the cell walls; early Americans used them as ³scouring brushes.²
- Whisk Ferns (phylum Psilotophyta)
o whisk ferns occur in the southern United States and in the tropics
o whisk ferns have no leaves or roots; a branched rhizome with rhizoids and a mycorrhizal fungus helps gather nutrients
o other genera including Tmesipteris have true leaves that are microphylls
- Ferns (phylum Pterophyta)
o ferns are widespread, and especially abundant in warm, moist tropical regions
o ferns range in size from low-growing mosslike forms to tall trees
o fronds are leaves that are variable in size and shape
o ferns are the only group of seedless plants to have well-developed megaphylls; megaphylls may have evolved by fusion or branching of stems
o life cycle of a fern: spores produced by meiotic cell division within sporangia, located in sori on underside of leaflets
§ spores are released and disperse largely by wind
§ a spore germinates into a prothallus which grows to develop antheridia and archegonia underneath
§ fertilization occurs if water is present; flagellated sperm swim from antheridia to archegonium; the resulting zygote begins its development inside archegonium but embryo soon outgrows the space
§ a sporophyte becomes visible as the first leaf grows above and as roots develop below the prothallus
§ the young sporophyte develops a root-bearing rhizome from which fronds project.
Seed Plants
- seeds are mature ovules containing embryonic sporophyte and stored food enclosed in a protective seed coat
- seeds disperse the sporophytes
- seeds are resistant to adverse conditions (dryness and temperature extremes)
- food reserve supports the emerging seedling until it can exist on its own
- there are separate male female gametophytes
- pollen grains are drought resistant & become multicellular male gametophytes
- pollination is the transfer of pollen to the vicinity of the female gametophyte
o the whole male gametophyte, not just the sperm, moves to the female gametophyte
o sperm is delivered to an egg through a pollen tube; no external water is required for fertilization
- the female gametophyte develops within an ovule which, after fertilization, becomes an embryonic plant or ³seed.²
- in gymnosperms, the ovules are not completely enclosed by sporophyte tissue at pollination
- in angiosperms, the ovules are completely enclosed within diploid sporophyte tissues which becomes a fruit
- Gymnosperms
o the Gymnosperms include the conifers, cycads, ginkgo, and gnetophytes
o all have ovules exposed on the surface of sporophylls or similar structures
o Conifers: phylum Coniferophyta (~ 575 species)
§ conifers are cone-bearing trees and shrubs: pines, hemlocks, and spruces
§ conifers usually have evergreen needlelike leaves well adapted to withstand extremes in climate
§ pine needles have a thick cuticle and recessed stomata
§ pine uses: construction wood; resin is insect & fungal deterrent (turpentine)
§ the pine life cycle:
§ the sporophyte is dominant and its sporangia are borne in cones
§ two types of cones are pollen cones (small and near the tips of lower branches) and seed cones
§ each scalelike sporophyll of a pollen cone has two or more microsporangia on the underside
§ within the sporangia, each microsporocyte undergoes meiosis and produces four microspores
§ each microspore develops into a male gametophyte which is the pollen grain
§ a megasporangium is within an ovule; a megasporocyte undergoes meiosis producing four megaspores
§ only one spore develops into a female gametophyte with 2–6 archegonia, each containing a single large egg
§ once a pollen grain is enclosed within the seed cone, it develops a pollen tube that digests its way toward a female gametophyte and discharges two nonflagellated sperm
§ fertilization takes place one year after pollination
§ the ovule matures and becomes the seed, composed of embryo, reserve food and seed coat
§ the woody seed cone opens to release winged seeds in the fall of 2nd season
o Cycads: phylum Cycadophyta (~100 species)
§ the trunk is stout and unbranched; the large leaves are compound giving a palmlike appearance
§ cycads have pollen and seed cones on separate plants, pollinated by insects
§ the pollen tube bursts in the vicinity of the archegonium and multiflagellated sperm swim to reach an egg
§ today, cycads are endangered because of their very slow growth
o Ginkgoes (maidenhair trees): phylum Ginkgophyta (1 species)
§ it is called the maidenhair trees because its forked-veined, fan-shaped leaves resemble the maidenhair fern
§ ginkgo ovules are at the end of short, paired stalks; female trees produce seeds with a fleshy covering and foul odor
§ similar to cycads, the pollen tube of Gingko bursts to release multiflagellated sperm that swim to the egg produced by the female gametophyte in an ovule
o Gnetophytes: phylum Gnetophyta (~70 species)
§ gnetum consists of trees and climbing vines with broad leaves; they live mainly in the tropics (Ephedra & Welwitschia in deserts of US & Africa)
§ the xylem and stroboli are uniform across genera, and all lack archegonia
§ angiosperms also lack archegonia, suggesting that gnetophytes are the gymnosperms most closely related to angiosperms
§ some gnetophytes produce nectar in their reproductive structures, recruiting insects in pollination
- Angiosperms (flowering plants): phylum Anthophyta (240,000 known species)
o unlike gymnosperms, angiosperms enclose their ovules within diploid tissues
o flowering plants became the dominant plants in the late Cretaceous and early Tertiary periods, and probably arose ~ 200 million years ago
o Monocots and Eudicots
§ most flowering plants belong to one of two classes: Monocotyledones (65,000 species) or the Eudicotyledones (175,000 species)
§ the term eudicots is preferred to the earlier dicots; some former dicots are now know to have split off before the rise of these two major classes
§ monocot produce one cotyledon (seed leaf) at germination and have flower parts mostly in threes or multiples of threes
§ dicots produce two cotyledons (seed leaves) at germination and have flower parts mostly in fours or fives, or multiples of these numbers
o The Flower
§ flowers have several kinds of highly modified leaves arranged in rings and attached to a receptacle
§ receptacle is a modified stem tip to which flower parts are attached
§ sepals are outer ring of modified leaves of flowers; usually green, they enclose flower before it opens
§ petals (collectively a corolla) are a ring of modified leaves inside of sepals; large and colorful, they help attract pollinators
§ stamens form a whorl inside the petals and around a pistil; each slender filament has an anther at its tip
§ the anther produces pollen
§ the pistil contains one or more fused carpels; it consists of a stigma, style, and ovary
· carpels are modified sporophylls that contain ovules in which megasporangia are located
· a stigma is a landing platform for pollen and the site where the pollen tube enters the style
· the style is a slender column that holds up the stigma to receive pollen
· pollen grains develop a pollen tube that takes sperm to the female gametophyte in the ovule
· glands located in the region of the ovary produce nectar, a nutrient gathered by pollinators as they go flower to flower
o the angiosperm life cycle:
§ a megaspore located in an ovule within an ovary of a carpal develops into an egg-bearing female gametophyte called the embryo sac
§ usually, the embryo sac has seven cells; one is an egg and one contains two polar nuclei
§ microspores produced in anthers become pollen grains which mature into sperm-bearing male gametophytes
§ the mature male gametophyte consists of three cells; the tube cell and two sperm cells
§ pollination brings the male gametophyte to the stigma where it germinates
§ during germination, the tube cell produces a pollen tube that carries the two sperm to the micropyle opening of an ovule
§ in double fertilization, one sperm fertilizes egg and one sperm unites with polar nuclei to form the triploid endosperm
§ the ovule becomes the seed and contains the embryo (the sporophyte of the next generation) and stored food enclosed within a seed coat
§ A fruit is derived from an ovary and possibly accessory parts of the flower; some fruits are fleshy and some are dry
o Flowers and Diversification
§ inconspicuous flowers disperse pollen by wind; colorful flowers attract specific pollinators (e.g., bees, wasps, flies, butterflies, moths, and even bats) which carry only a particular pollen
§ flowers promote efficient cross pollination; they also aid in dispersal through production of fruits
§ there are fruits that utilize wind, gravity, water, and animals for dispersal
Chapter 25: Structure And Organization Of Plants
Plant Organs
- Diverse Flowering Plant Structure
o structures of flowering plants are well-adapted to varied environments including water
o flowering plants usually have three vegetative organs: root, stem and leaf
o the flower itself contains a number of organs
- Roots (root systems)
o the root system is the main root plus its lateral (side) branches; it is generally equal in size to the shoot system, the part above ground
o roots anchor a plant in soil and give support
o roots absorb water and minerals from soil; root hairs are central to this process
§ root hair cells are in a zone near root tip
§ root hairs are numerous to increase absorptive surface of a root
§ transplanting plants damages a plant when the root hairs are torn off
§ roots produce hormones that are distributed along with water and nutrients to the rest of the plant
o perennials ³die back² to regrow the next season; roots of herbaceous perennials store food (e.g., carrots, sweet potatoes)
- Stems
o the shoot system of a plant consists of the stem, the branches, & leaves
o the stem forms the main axis of the plant, along with lateral branches
o upright stems produce leaves and array them to be exposed to as much sun as possible
o a node occurs where a leaf attaches to the stem and an internode is the region between nodes; nodes and internodes identify a stem even if it is underground
o the stem has vascular tissue to transport water and minerals from roots and sugar from leaves
o nonliving cells form a continuous pipeline through vascular tissue
o a cylindrical stem expands in girth and length; trees use woody tissue to strengthen stems
o stems function in storage: cactus stems store water and tubers are horizontal stems that store nutrients
- Leaves
o a leaf is the major organ of photosynthesis in most plants
o leaves receive water from roots by way of the stem
o broad, thin leaves have a maximum surface area to absorb CO2 and collect solar energy
o a blade is the wide portion of a leaf with most photosynthetic tissue
o petiole is a stalk that attaches a leaf blade to stem
o the leaf axil is the upper acute angle between petiole and stem where an axillary (lateral) bud originates
o some leaves protect buds, attach to objects (tendrils), store food (bulbs), or capture insects
Monocot Versus Eudicot Plants
- cotyledons are embryonic seed leaves providing nutrition from the endosperm before the mature leaves begin photosynthesis
- Monocots: **
o 1 cotyledon in seed
o root xylem & phloem in a ring
o vascular bundles scattered in stem
o parallel leaf veins
o flower parts in multiples of 3
o usually one aperture in pollen grain
o includes grasses, lilies, orchids, rice, wheat, corn
- Eudicots: **
o 2 cotyledons in seed
o root xylem & phloem in a cross or star-shaped pattern
o vascular bundles arranged in a ring in stem
o net pattern to leaf veins
o flower parts in multiples of 4 & 5
o usually 3 aperture in pollen grain
o includes dandelions to oak trees
Plant Tissues
- Meristem Produces Tissue: plants continually grow due to meristem (embryonic tissue) in stem and root tips (apexes)
- 3 types of primary meristem continually produce three types of specialized tissue
o protoderm is outermost primary meristem giving rise to epidermis
o ground meristem is inner meristem producing ground tissue
o procambium produces vascular tissue
- 3 specialized tissues are produced
o epidermis forms outer protective covering
o ground tissue fills the interior
o vascular tissue transports water and nutrients and provides support
- Epidermal Tissue
o epidermis is an outer protective covering tissue of plant roots, leaves, and stems of nonwoody plants containing closely packed epidermal cells
o waxy cuticle covers the walls of epidermal cells, minimizing water loss and protecting against bacteria
o in roots, certain epidermal cells are modified into root hairs that increase surface area of the root for absorption of water and minerals and help to anchor plants in the soil
o epidermal cells are modified as glands to secrete protective substances
o on the lower epidermis of eudicot leaves, and both surfaces of monocot leaves, special guard cells form microscopic pores called stomata that regulate gas exchange and water loss
o in older woody plants, the epidermis of the stem is replaced by cork tissue
§ cork is outer covering of the bark of trees; composed of dead cork cells that may be sloughed off
§ cork cambium is lateral meristem that produces new cork cells
§ mature cork cells produce the lipid suberin making them waterproof & inert
§ cork protects a plant and is resistant to attack by fungi, bacteria, and animals
- Ground Tissue
o ground tissue fills the inside of plants with parenchyma, collenchyma and sclerenchyma cells
o parenchyma are the least specialized of all plant cell types
§ contain plastids (e.g., chloroplasts or colorless storage plastids)
§ found in all organs of a plant & divide to form more specialized cells (e.g., roots develop from stem cuttings in water)
o collenchyma resemble parenchyma but has thicker primary cell walls & are uneven in the corners
§ usually occur as bundles of cells just beneath epidermis
§ give flexible support to immature regions of plants (e.g., a celery stalk is mostly collenchyma)
o sclerenchyma cells are mostly nonliving & have thick secondary cell walls
§ they are impregnated with lignin that makes the walls tough and hard
§ provide strong support to mature regions of plants
§ form fibers (used in linen and rope) and shorter sclereids (found in seed coats, nut shells, and gritty pears)
- Vascular Tissue
o xylem passively conducts water and mineral solutes upward through a plant from roots to leaves
§ xylem contains tracheids and vessel elements
§ both are hollow, nonliving cells
§ vessel elements form a continuous pipeline for water and mineral transport
§ vascular rays conduct water and minerals across the width of the plant
o phloem is vascular tissue that conducts the organic solutes in plants, from the leaves to the roots; it contains sieve-tube cells and companion cells
§ sieve-tube cells contain cytoplasm but no nucleus
· they are arranged end to end & have channels in their end walls (³sieve-tubes²), through which plasmodesmata extend from one cell to another
§ companion cells are connected to sieve-tube cells by many plasmodesmata
· they are smaller and more generalized than sieve-tube cells & have a nucleus which may control & maintain the function of both cells
· they are also thought to be involved in the transport function of phloem
o vascular tissue extends from root to leaves as vascular cylinder (roots), vascular bundles (stem) and leaf veins
Organization of Roots
- Eudicot Root Tip: the eudicot root tip, a site of primary growth, is organized into zones of cells in various stages of differentiation
o cells are continuously added to a root cap below and zone of elongation above by contributions from the zone of cell division
o the root cap is a protective cover; its cells are replaced constantly because they are soon ground off
o the zone of elongation is above the zone of cell division where cells become longer and more specialized
o the zone of cell division contains meristematic tissue and adds cells to root tip and zone of elongation
o the zone of maturation is above the zone of elongation; cells are mature and differentiated with root hairs
- Tissues of a Eudicot Root
o epidermis is a single layer of thin-walled, rectangular cells that forms the protective outer layer of the root
§ root hairs in the region of maturation project as far as 5–8 mm into the soil
o cortex is a layer of large, thin-walled, irregularly shaped parenchyma cells
§ these cells contain starch granules; the cortex functions in food storage
§ the cells are loosely packed; water and minerals can diffuse through the cortex
o endodermis is single layer of rectangular cells that forms the boundary between cortex and inner vascular cylinder
§ it regulates the entrance of minerals into the vascular cylinder
§ the Casparian strip is an impermeable lignin and suberin layer that excludes water and mineral ions
o vascular cylinder is an arrangement of vascular tissues as a cylinder
§ the pericycle is the first layer of cells within vascular cylinder
§ its cells have retained the capacity to divide; it can start the development of branch or secondary roots
§ vascular tissue forms main portion of a vascular cylinder
· it is composed of xylem, whose cells are arranged in a star-shaped pattern; and phloem, whose cells are located in regions between arms of xylem
- Organization of Monocot Roots: monocot roots have the same zones as a eudicot root but do not undergo secondary growth
o the monocot root has a ring of vascular tissue where alternating bundles of xylem and phloem surround pith
o monocot roots also have pericycle, endodermis, cortex, and epidermis
- Root Diversity
o roots have adaptations to help anchor plants, absorb water and minerals, and store carbohydrates
o there are three general root types
§ taproot is common in eudicots; root adapted to store food (e.g., carrots, beets)
§ the fibrous root system of monocots is a mass of slender roots and lateral branches that hold the plant secure in the soil
§ adventitious roots develop from underground stems or from the base of above-ground stems
· prop roots (corn and mangrove plants) anchor the plant
· ivy has holdfast roots to anchor aerial shoots
o haustoria are rootlike projections from stems on parasitic plants (e.g., dodders and broomrapes)
o mycorrhizae are an association between fungus and roots
§ in this mutualism, fungus receives sugars and amino acids from plant & plant receives water and minerals from the fungus
o legumes (e.g., peas and beans) have root nodules containing nitrogen-fixing bacteria
§ bacteria extract nitrogen from air and reduce it to a form that can be used by plant tissues
Organization of Stems
- Primary Growth
o the stem tip is the site of primary growth where cell division extends length of stems or roots
o shoot apical meristem produces new leaves and primary meristems, increasing stem length & is protected within a terminal bud of leaf primordia (immature leaves)
o bud scales are scalelike coverings protecting terminal buds during winters when bud growth stops
- Herbaceous Stems
o herbaceous stems are mature nonwoody stems that exhibit only primary growth
o the outermost tissue of herbaceous stems is epidermis covered by a waxy cuticle to prevent water loss.
o xylem and phloem are in distinctive vascular bundles
§ in each bundle, xylem is found to the inside of the stem; phloem is found to the outside
§ in the eudicot herbaceous stem, vascular bundles are arranged in a ring towards outside of the stem and separating the cortex from the central pith
§ in monocot stem, vascular bundles are scattered throughout the stem; there is no well-defined cortex or pith
§ cortex sometimes carries on photosynthesis; pith may function as storage site
- Woody Stems
o woody plants have both primary and secondary tissues
o primary tissues are new and form each year from primary meristem right behind the apical meristem
o secondary tissues develop from second year onward from lateral meristem growth
o primary growth increases length of a plant; secondary growth increases its girth
o as secondary growth continues, it is not possible to distinguish individual vascular bundles
o the woody eudicot stem has a different organization with three distinct areas: bark, wood, and pith
o bark of a tree contains cork, cork cambium, and phloem
o cork cambium is meristem beneath the epidermis that produces new cork cells when needed
§ cork cells become impregnated with suberin, causing them to die but making them waterproof
§ consequently, cork forms an impervious barrier, even to gas exchange, except at lenticels
o wood is secondary xylem which builds up each year; the vascular cambium is dormant during the winter
§ spring wood is composed of wide xylem vessel elements with thin walls, necessary to conduct sufficient water and nutrients to supply abundant growth that occurs during spring
§ summer wood forms when moisture is scarce; composed of a lower proportion of vessels, it contains thick-walled tracheids and numerous fibers
§ an annual ring is one ring of spring wood followed by a ring of summer wood; this equals one year¹s growth
§ sapwood is outer annual rings where transport occurs
§ heartwood is inner annual rings of older trees & help to support a tree
· vessels no longer function in transport; they become plugged with resins and gums that inhibit growth of bacteria and fungi
o Woody Plants
§ the first flowering plants were probably woody shrubs; herbaceous plants evolved later
§ woody plants have an advantage when there is adequate rainfall; they can grow taller and have adequate tissue to support and service leaves
- Stem Diversity
o stolons are stems that grow along the ground; new plants grow where the nodes contact the soil (e.g.: strawberries)
o the succulent stems of cacti are modified for water storage
o tendrils of grapes and morning glories are stems adapted for wrapping around support structures
o rhizomes are underground horizontal stems
§ rhizomes are long and thin in grasses and thick and fleshy in irises
§ some rhizomes have tubers that function in food storage (e.g.: potatoes)
o corms are bulbous underground stems that lie dormant during winter, like rhizomes (e.g.: crocus, gladiolus)
o humans use stems: sugarcane is primary source of table sugar, cinnamon and quinine are from bark, wood is from paper, etc.
Organization of Leaves
- Leaf Structure
o leaves are organs of photosynthesis in plants; they have a flattened blade and a petiole
o the leaf veins contain vascular tissues that transport water and nutrients
o leaf veins have a net pattern in eudicots and a parallel pattern in monocots
o a petiole is a stalk that attaches a leaf blade to the plant stem
o epidermis is the layer of cells that covers the top and bottom sides of a leaf
§ epidermis often bears protective hairs or glands that irritating substances
§ epidermis is covered by a waxy cuticle that keeps the leaf from drying out
§ the epidermis, particularly lower epidermis, contains stomata that allow gases to move into and out of the leaf
o mesophyll is the inner body of a leaf and the site of most of photosynthesis
§ palisade mesophyll is the layer of mesophyll containing elongated parenchyma cells with many chloroplasts
§ spongy mesophyll contains loosely packed parenchyma cells that increase the surface area for gas exchange
- Leaf Diversity
o simple leaves have margins not deeply lobed or divided into smaller leaflets
o compound leaves are divided into smaller leaflets, and each leaflet may have its own stalk
o leaves are variously modified
§ shade plants have broad leaves while desert plants have reduced leaves with sunken stomata
§ cactus spines are modified leaves; succulents have fleshy leaves to hold moisture
§ onion bulbs have leaves surrounding a short stem
§ the tendrils of peas and cucumbers are leaves
§ the Venus¹s-flytrap has leaves to trap and digest insects
Chapter 26: Nutrition And Transport In Plants
Plant Nutrition and Soil
- Early Views
o ancient Greeks believed plants converted soil into plant tissues
o the 17th Century Dutchman Jean-Baptiste Van Helmont conducted an experiment from which he concluded the increase in tree weight came from water; he was unaware of substances in air
- Essential Inorganic Nutrients
o essential inorganic nutrients (e.g., carbon, hydrogen, oxygen) comprise 96% of plant dry weight
§ carbon dioxide is the source of carbon for a plant
§ water is the source of hydrogen
§ oxygen can come from either atmospheric oxygen, carbon dioxide, or water
o essential nutrients must fulfill the following criteria
§ they have an identifiable nutritional role
§ no other element can substitute and fulfill the same role
§ a deficiency of the element causes the plant to die
o beneficial nutrients: elements required or that improve growth of a particular plant
- Determination of Essential Nutrients
o when a plant is burned, most mineral elements (except nitrogen) remain in the ash
o hydroponics is the preferred method for determining plant mineral requirements
§ hydroponics is cultivation of plants in water
§ nutrient requirements of plants are determined by omitting a mineral and observing the effects
§ if plant growth suffers, it can be concluded that the omitted mineral is a required nutrient
- Soil Formation
o soil formation begins with weathering of rock by freezing, glacier flow, stream flow, and chemicals
o lichens and mosses grow on barren rock and trap particles and leave decaying tissues
o decayed organic matter (humus) takes time to accumulate; its acidity leaches minerals from rocks
o depending on parent material and weathering, a centimeter of soil may develop within 15 years
- The Nutritional Function of Soil
o soil consists of soil particles, decaying organic matter, living organisms, air and water
o the best soil includes particles of different sizes; this provides critical air spaces
o soil particles include sand, silt & clay
§ soil particles vary by size: sand particles are largest (0.05–2.0 mm in diameter); silt particles are medium sized (0.002–0.05 mm in diameter); clay particles are smallest: below 0.002 mm in diameter
§ sandy soils lose water too readily; clay packs tight to hold water and clumps
§ clay particles are negatively charged and attract positively charged ions (e.g., calcium [Ca2+] and potassium [K+])
§ in acidic soils, hydrogen ions replace positively charged nutrients and the nutrient ions float free and are leached; this is why acid rain kills trees
§ clay cannot retain negatively charged NO3-, and the nitrogen content of clay soil is low
§ loam (a mixture of the three soil particles) retains water and nutrients; roots take up oxygen in the air spaces
o humus: a mixture of 10-20% humus mixed with a top layer of soil particles is best for plants
§ humus keeps soil loose and crumbly, decreases runoff and aerates soil
§ humus is acidic and retains positively charged minerals for plants to use later
§ bacteria and fungi break down organic matter in humus and return inorganic nutrients to plants
o living organisms in soil
§ small plants play a major role in formation of soil from rock and in succession
§ roots of larger plants penetrate the soil and weather rocks
§ larger moles and badgers and smaller earthworms help turn over the soil
§ soil animals, from mites to millipedes help break down leaves and other plant remains
§ fungi, protozoa, algae and bacteria complete decomposition
§ soil bacteria make nitrate available to plants
§ some soil organisms (roundworms and insects) are crop pests that feed on roots
o soil erosion is caused by water or wind carrying away soil
§ erosion removes 25 billion tons of topsoil worldwide annually
§ deforestation and desertification contribute to erosion
Uptake of Water and Minerals
- Pathways
o minerals follow the path of water uptake
§ some mineral ions diffuse in between the cells
§ because of the impermeable Casparian strip, water must eventually enter the cytoplasm of endodermal cells
§ water can move directly into the cytoplasm of root hair epidermal cells and is transported across the cortex and endodermis of a root
§ in contrast to water, minerals are actively taken up by plant cells
o mineral ions cross plasma membranes by a chemiosmotic mechanism
§ plants absorb minerals in ionic form: nitrate (NO3-), phosphate (HPO4-), and potassium ions (K+) all have difficulty crossing a charged plasma membrane
§ it has long been known plants expend energy to actively take up and concentrate mineral ions
§ a plasma membrane pump called a proton pump hydrolyzes ATP to transport H+ ions out of cell; this sets up an electrochemical gradient that causes positive ions to flow into cells
§ negative ions are carried across the plasma membrane in conjunction with H+ ions as H+ ions diffuse down their concentration gradient
- Adaptations of Roots for Mineral Uptake
o two symbiotic relationships are known to assist roots in acquiring nutrients
o legumes have nodules infected with the bacterium Rhizobium
§ Rhizobium makes nitrogen compounds available to plants in exchange for carbohydrates by reducing atmospheric nitrogen (N2) to ammonium (NH4+) (nitrogen-fixation)
o most plants have mycorrhizae; those lacking mycorrhizae are limited in where they can grow
§ mycorrhizae are a mutualistic symbiotic relationship between soil fungi and plant roots
§ the fungus increases the surface area for mineral and water uptake and breaks down organic matter
§ in return the root furnishes the fungus with sugars and amino acids
o some plants have poorly developed roots or no roots; other mechanisms supply minerals and water
§ epiphytes take nourishment from air; their attachment to other plants gives them support
§ parasitic plants (e.g., dodders, broomrapes, pinedrops) send out haustoria (rootlike projections) that grow into host and tap into xylem and phloem of host
§ venus flytrap and sundew obtains nitrogen and minerals as leaves capture and digest insects
Transport Mechanisms in Plants
- Transport Tissues
o vascular plants have transport tissues as an adaptation to living on land
o xylem passively conducts water and minerals from roots to leaves; it contains two types of conducting cells: tracheids and vessel elements
o phloem conducts organic solutes in plants mainly from leaves to roots; contains sieve-tube cells and companion cells
o these transport systems rely on the mechanical properties of water
§ diffusion moves molecules from higher to lower concentrations
§ water potential considers both water pressure and osmotic pressure
- The Concept of Water Potential
o water flows from a region of higher water potential (the potential energy of water) to a region of lower water potential
o water potential is a measure of the capacity to release or take up water; in cells, water potential includes the following:
§ pressure potential, the effect that pressure has on water potential; water will move from a region of higher pressure to a region of lower pressure; and
§ osmotic potential, the effect that solutes have on water potential; water tends to move by osmosis from an area of lower solute concentration to area of higher solute concentration
o water flows by osmosis into a plant cell with greater solute concentration than a surrounding solution
§ as water enters, pressure increases inside the cell; the strong plant cell wall allows water pressure to build up
§ pressure potential inside the cell increases and balances the osmotic potential outside cell; water stops entering
§ turgor pressure is the pressure potential that increases due to process of osmosis; it is critical to plants, since plants depend on it to maintain the turgidity of their bodies
§ wilted plant cells have insufficient turgor pressure and the plant droops
- Water Transport
o water entering root cells creates a positive pressure called root pressure
§ root pressure (primarily at night) tends to push xylem sap upward in a plant
§ guttation is the appearance of drops of water along the edge of leaves, as a result of water being forced out of leaf vein endings; it is the result of root pressure
§ root pressure is not a sufficient mechanism for water to rise to the tops of trees
- Cohesion-Tension Model of Xylem Transport
o water and dissolved minerals must be transported upward from roots to xylem, perhaps as high as 90 meters
o transpiration is a plant¹s loss of water to atmosphere through evaporation at leaf stomata
o the cohesion-tension model states that transpiration creates a tension (i.e., a negative pressure) that pulls water upward in xylem
o water molecules are cohesive with one another, adhesive with xylem walls
o cohesion is the tendency of water molecules to cling together due to their forming hydrogen bonds
o adhesion is the ability of water (a polar molecule) to interact with molecules comprising the walls of xylem vessels; adhesion gives a water column extra strength and prevents it from slipping back down
o at least 90% of the water taken up by roots is lost through stomata by transpiration
o with plenty of water, stomata will remain open, allowing CO2 to enter the leaf and photosynthesis to occur
o transpiration exerts a tension that draws the water column up in vessels
o under water stress, more water is lost through a leaf than can be brought up and the stomata close
o photosynthesis requires CO2 to enter the leaf; there must be sufficient water so stomata can remain open and allow CO2 to enter
- Opening and Closing of Stomata
o each stoma has two guard cells with a pore between them
o stomata open from turgor pressure when guard cells take up water; when they lose water, turgor pressure decreases and stomata close
o guard cells are attached to each other at their ends; the inner walls are thicker than outer walls
o as they take up water, they buckle out, thereby creating an opening between cells
o since 1968, it has been known that when stomata open, there is accumulation of K+ ions in guard cells
o a proton pump run by breakdown of ATP to ADP and P transports H+ outside the cell; this establishes an electrochemical gradient allowing K+ to enter by way of a channel protein
o the blue-light component of sunlight is a signal that can cause stomata to open
§ there is evidence that flavin pigments absorb blue light
§ this pigment sets in motion a cytoplasmic response activating the proton pump that causes K+ ions to accumulate in guard cells
o abscisic acid (ABA) produced by cells in wilting leaves, also causes stomata to close; photosynthesis cannot occur but water is conserved
o in plants kept in dark, stomata open and close on a 24-hour basis as if responding to sunlight in daytime and the absence of sunlight at night; some sort of internal biological clock must keep time
- Organic Nutrient Transport
o Marcello Malpighi (1679) suggested bark transferred sugars from leaves to roots
§ today, we know phloem was removed but xylem remained; therefore, phloem does transport sugars
o radioactive tracer studies using 14C confirmed phloem transports organic nutrients
§ similar studies confirm phloem transports amino acids, hormones, and mineral ions
- Pressure-Flow Model of Phloem Transport
o the pressure-flow model explains the transport of sap through sieve tubes by a positive pressure potential
o the buildup of water creates a positive pressure potential within the sieve tubes that moves water and sucrose to a sink (e.g., at the roots)
o pressure exists from the leaves to the roots; at the roots, sucrose is transported out and water also flows through due to the pressure
o consequently, this pressure gradient causes a flow of water from leaves to roots
o the conducting cells of phloem are sieve tubes lined end to end
o cytoplasm extends through the sieve plates of adjoining cells to form a continuous tube system
o during the growing season, leaves produce sugar
o sucrose is actively transported into phloem by an electrochemical gradient established by a H+ pump
o water flows passively into sieve tubes by osmosis
o a sink can be at the roots or any other part of the plant that requires nutrients
o because phloem sap flows from source to sink, sap can move any direction along phloem.
Chapter 27: Control Of Growth And Responses In Plants
Plant Responses
- Organisms Respond to Stimuli - a defining characteristic of life
o adaptive organisms respond to environmental stimuli because it leads to longevity and survival of the species
o animals have nerves and muscles; plants respond by growth patterns
- Tropisms
o a tropism is plant growth toward or away from a directional stimulus
o positive tropism: growth toward a stimulus
o negative tropism: growth away from a stimulus
o by differential growth, one side elongates faster; the result is a curving toward or away from a stimulus
o reception of the stimulus ® transduction of the stimulus into a form meaningful to the organism ® response by the organism
o phototropism, gravitropism, & thigmotropism
- Phototropism: growth of plants in response to light
o stems show positive phototropism
o occurs because cells on shady side of stems elongate
o a yellow pigment related to riboflavin appears to act as a photoreceptor for light
o following reception, the plant hormone auxin migrates from the bright side to the shady side of a stem
o auxin is also involved in gravitropism, apical dominance, and root and seed development
- Gravitropism: response to earth¹s gravity
o roots demonstrate positive gravitropism; stems demonstrate negative gravitropism
o an upright plant placed on its side displays negative gravitropism; it grows upward opposite gravity
o the hormone auxin underlies both positive and negative gravitropisms
§ auxin inhibits the growth of root cells; cells of the upper surface elongate and the root curves downward
§ auxin stimulates the growth of stem cells; cells of the lower surface elongate and the stem curves upward
- Thigmotropism: unequal growth due to touch (e.g., coiling of tendrils around a pole)
o the coiling of morning glory or pea tendrils around posts, etc., is a common example
o cells in contact with an object grow less while those on the opposite side elongate
o this process is quite rapid; tendrils can encircle an object in ten minutes
o response can be delayed; tendrils touched in the dark respond when illuminated
§ ATP rather than light can cause the response
§ the hormones auxin and ethylene are involved; they induce curvature of tendrils in the absence of touch
o thigmomorphogenesis is a touch response involving the whole plant
§ an entire plant responds to presence of touch, wind or rain
- Nastic Movements: in contrast to tropisms, nastic movements are independent of the direction of stimulus
o seismonastic movements result from touch, shaking, or thermal stimulation
o when a Mimosa pudica leaf is touched, the leaflets fold because the petiole droops
o rapid response (1-2 sec.) and is due to a loss of turgor pressure within cells
o Venus¹s-flytrap has three sensitive hairs at the base of the trap
§ when touched by an insect, an impulse-type stimulus triggers the trap to close
§ turgor pressure in leaf cells then propel the trap
- Sleep Movements: nastic responses to the daily changes in light level; an example is the prayer plant that folds its leaves each night
o biological clocks (maintain Circadian rhythms) are synchronized by external stimuli to twenty-four-hour rhythms
o stomata and flowers usually open in the morning, close at night; some plants secrete nectar at same time of day
Plant Hormones: chemical messengers for communication & coordination of activities in plants; a response is influenced by several hormones and may require a specific ratio of two or more hormones
- hormones are synthesized in one part of a plant; they travel in the phloem after a plant receives an appropriate stimulus
- auxin is produced in shoot apical meristem and found in young leaves, flowers, and fruits
o when a terminal bud is removed, the nearest buds grow and the plant branches
o auxin production by seeds promotes growth of fruit
o if auxin is concentrated in leaves and fruits rather than stem, they do not fall off
o auxin-controlled cell elongation is involved in gravitropism and phototropism
o when gravity is perceived, auxin moves to lower surface of roots and stems
o auxin mode of action: in a plant exposed to unidirectional light, auxin moves from the bright side to the shady side of a stem
§ auxin binds to receptors and activates the ATP-driven proton (H +) pump
§ as hydrogen ions are pumped out of the cell, the cell wall becomes acidic, breaking hydrogen bonds
§ cellulose fibrils are weakened and activated enzymes further degrade cell wall
§ the electrochemical gradient established causes of uptake of solutes and water follows by osmosis
§ the turgid cell presses against the cell wall, stretching it so elongation occurs
- Gibberellins: group of 70 plant hormones that promote growth (elongate cells)
o GA3 is the most common of the natural gibberellins
o gibberellins are growth promoters that elongate cells
o mode of action: the hormone GA3 binds to a receptor; a second messenger (Ca2+) inside cell combines with calmodulin to activate the enzyme amylase, which acts on starch to release sugars used as a source of energy by the growing embryo
- Cytokinins: class of plant hormones derived from adenine that promote cell division
o a natural cytokinin zeatin is found in corn kernels; kinetin is a synthetic cytokinin
o may activate oligosaccharins, chemical fragments released from cell wall, also direct differentiation
- Senescence: aging processes; large molecules break down and are transported elsewhere in the plant
o cytokinins prevent senescence of leaves and initiate development of leaf growth
o cytokinins initiate growth of lateral buds despite apical dominance
- Abscisic acid (ABA): sometimes called the ³stress hormone²; it maintains seed and bud dormancy and causes closure of stomata
o dormancy occurs when a plant organ readies itself for adverse conditions by stopping growth
§ ABA moves from leaves to vegetative buds in fall; thereafter these buds are converted to winter buds which are covered by thick, hardened scales
o abscisic acid brings about closing of stomata when a plant is under water stress
§ by some unknown mechanism, ABA causes K+ ions to leave guard cells; as a result, guard cells lose water and the stomata close
o although external application of ABA promotes abscission, the hormone ethylene is considered to have this natural function
- Ethylene is involved in abscission, the dropping of leaves, fruits, or flowers
o lower levels of auxin in these areas (compared to stem) probably initiate abscission
o once abscission begins, ethylene stimulates production of enzymes such as cellulase (breaks down cellulose in cell walls) that cause leaf, fruit, or flower drop
o it was an early practice to prepare citrus fruit for market by storage in a room with a kerosene stove; later work revealed incomplete combustion of kerosene produced ethylene which ripens fruit
o ethylene is a gaseous plant hormone; it ripens fruit by increasing the activity of enzymes that soften fruit
Photoperiodism: a physiological response to relative lengths of daylight and darkness
- many physiological changes in plants (e.g., seed germination, the breaking of bud dormancy, and the onset of senescence) are related to a seasonal change in day length
- in some plants, photoperiodism also affects flowering
- plants can be divided into three groups, based on photoperiodism
o short-day plants: flower when day length was shorter than a critical length
o long-day plants: flower when the day length is longer than a critical length
o day-neutral plants: flowering is not dependent on day length
- a long-day and a short-day plant can have the same critical length
- Phytochrome and Plant Flowering
o U.S.D.A. scientists discovered phytochrome, a blue-green leaf pigment that exists in two forms
o Pr (phytochrome red) absorbs red light (wavelength 660 nm); it is converted to Pfr
o Pfr (phytochrome far-red) absorbs far-red light (wavelength 730 nm); it is converted to Pr
o during a 24-hour period, there is a shift in ratio of these two pigments
o phytochrome conversion may be a first step in reception-transduction-response pathway resulting in flowering
- Other Functions of Phytochrome: the Pr ® Pfr conversion cycle controls other growth functions in plants
o in addition to being involved in flowering, Pfr promotes seed germination and inhibits stem elongation
Chapter 28: Reproduction In Plants
Reproductive Strategies
- Life Cycles
o in contrast to animals with one type of adult generation, flowering plants exhibit an alternation of generations life cycle that includes a diploid and a haploid generation
o the sporophyte is a diploid generation in an alternation of generations life cycle
§ a sporophyte produces haploid spores by meiotic division
§ spores develop into a haploid gametophyte
o a gametophyte is a haploid generation in an alternation of generations life cycle
§ a gametophyte produces haploid gametes by mitotic division; gametes fuse to form diploid zygote
§ the zygote undergoes mitotic cell division to develop into the sporophyte
o a flower produces two types of spores, microspores and megaspores
§ a microspore is a plant spore that develops into a microgametophyte
· the male gametophyte is a pollen grain; wind or animals carry it to megagametophyte
· when mature, its nonflagellated sperm cells travel down pollen tube to megagametophyte
§ a megaspore is a plant spore that develops into a female gametophyte, the embryo sac which remains within a sporophyte plant
o in flowering plants, the diploid sporophyte is dominant (longer lasting); it is what we commonly recognize
o the sporophyte is the generation that contains vascular tissue and has other adaptations suitable to living on land, including production of flowers
o flowers are unique to angiosperms; aside from producing the spores and protecting gametophytes, flowers attract pollinators and produce fruits to enclose the seeds
- Flowers
o a flower is the reproductive organ of a flowering plant; it develops in response to environmental signals
o the shoot apical meristem stops forming leaves to form flowers; axillary buds can become flowers directly
o monocot flower parts are in threes or multiples; eudicot flower parts are in fours or fives or multiples
o sepals are leaflike, usually green; this outermost whorl protects the bud as a flower develops within
o petals are interior to sepals; coloration accounts for attractiveness of many flowers
§ the size, shape, and color of a flower are attractive to a specific pollinator
§ wind-pollinated flowers often have no petals at all
o grouped about a pistil are stamens, stalked structures that have two parts
§ the anther is a saclike container within which pollen grains develop
§ a filament is a slender stalk that supports the anther
o the carpel is the vaselike structure located at the center of a flower (a carpel is a simple pistil or 1 element of a compound pistil); carpels usually has three parts
§ the stigma is an enlarged sticky knob on end of a style; stigma serves to receive pollen grains
§ the style is a slender stalk that connects stigma with the ovary
§ the ovary is an enlarged base of a carpel that contains a number of ovules
o not all flowers have sepals, petals, stamens, and a pistil
§ complete flowers have sepals, petals, stamens, and a pistil; incomplete flowers do not
§ bisexual flowers have both stamens and a pistil
§ staminate flowers have only stamens
§ carpellate flowers have only carpels
o if staminate and carpellate flowers are on same plant, the plant is monoecious
o if staminate and carpellate flowers are on different plants, the plant is dioecious
- From Spores to Gametes
o in plants, the sporophyte produces haploid spores by meiosis; in animals, meiosis produces gametes
o flowering plants are heterosporous, producing microspores and megaspores that become spermbearing pollen grains and egg-bearing embryo sacs, respectively
o production of the male gametophyte
§ microspores are produced in the anthers of flowers
§ microspores develop into pollen grains containing a tube cell and two sperm
§ the pollen grain is the mature male gametophyte
o pollination release of pollen as walls of pollen sacs within the anther break down
§ self-pollination is transfer of pollen from anther to stigma of the same plant
§ cross pollination is transfer of pollen from anther of one plant to stigma of another plant; plants often have mechanisms that promote cross pollination
§ using a pollinator to carry pollen from flower to flower of only one species increases the efficiency
§ secretion of nectar is one way to attract certain pollinators, and they may be adapted to reach only one type of flower
o production of the female gametophyte
§ the ovary contains one or more ovules
§ the female gametophyte (or embryo sac) consists of 7 cells: 1 egg cell, 2 synergid cells, 1 central cell with two polar nuclei, and 3 antipodal cells
o fertilization
§ when a pollen grain lands on a stigma, it germinates, forming a pollen tube
§ as a pollen tube grows, it passes between the cells of the stigma and the style to reach the micropyle of an ovule
§ double fertilization occurs after the release of both sperm cells into the ovule
§ one sperm nucleus unites with the egg nucleus, forming a 2n zygote
§ the other sperm nucleus migrates and unites with the polar nuclei of the central cell, forming a 3n endosperm nucleus
§ the zygote divides mitotically to become the embryo; the endosperm nucleus divides mitotically to become the endosperm
§ the embryo, in most plants, is a young sporophyte
§ the endosperm is tissue that will nourish the embryo and seedling as they undergo development
Seed Development
- Stages
o development of the seed is the next event
o plant growth and development involves cell division, cell elongation, and differentiation of cells into tissues and then organs
- Development of the Eudicot Embryo
o immediately after double fertilization, the endosperm nucleus divides to produce a mass of endosperm surrounding the embryo
o the single-celled zygote also divides, but asymmetrically, forming two parts: embryo and suspensor, which anchors the embryo and transfers nutrients to it from the sporophyte plant
o globular stage: the proembryo is a ball of cells
§ the root-shoot axis is established; cells near the suspensor will become a root, those at the opposite end will become a shoot
§ the outermost cells become dermal tissue that prevents dessication and also has stomata that regulate gas exchange
o the heart-shaped and torpedo-shaped embryos
§ the embryo has a heart-shape when the cotyledons appear; it then grows to a torpedo shape
§ with elongation, the root and shoot apical meristems are distinguishable
§ ground meristem responsible for most of the interior of the embryo is also present now
o the mature embryo
§ after differentiation into embryo and suspensor, one or two cotyledons develop
§ the embryo continues to differentiate into three parts:
· the epicotyl is between the cotyledons and first leaves; it contributes to shoot development
· the hypocotyl is below the cotyledon and contributes to stem development
· the radicle is below the hypocotyl and contributes to root development
§ the cotyledons are quite noticeable in a eudicot embryo, and may fold over
- Monocots Versus Eudicots
o monocot embryo only has one cotyledon
§ in monocots, the cotyledon rarely stores food
§ it absorbs food molecules from the endosperm and passes them to embryo
o eudicot embryo has two cotyledons
§ during development of a eudicot embryo, cotyledons usually store the nutrients the embryo uses
§ the endosperm seemingly disappears as the nutrients are consumed
Fruit Types and Seed Dispersal
- Fruits
o a fruit is a mature ovary enclosing seeds; sometimes they retain other flower parts
o fruits serve to protect and disperse offspring
o the fruit protects the peach seed well but makes germination difficult; the peas escape easily but are lightly protected
- Simple Fruits
o simple fruit develops from a single carpel or several united carpels of a compound ovary
o legumes (pea pods), nuts and cereal grains are examples of dry fruits; such fruits are mistaken for seeds because a dry pericarp adheres to the seed within
o hooks and spines of clover, bur, and cocklebur attach to the fur of animals
o woolly hairs, plumes, and wings disperse by wind
o a fleshy fruit has a fleshy pericarp (e.g., peach, plum, olive, grape, tomato, apple, and pear)
§ birds and mammals eat fruits, including seeds, and defecate them at a distance
§ squirrels and other animals gather seeds and fruits and bury them some distance away
o an apple is an example of an accessory fruit; the bulk of the fruit is not from the ovary but from the receptacle
- Compound Fruits
o a compound fruit develops from several individual ovaries
o an aggregate fruit develops from ovaries from a single flower (e.g., blackberry)
o an aggregate fruit where each ovary becomes a one-seeded fruit is called an achene (e.g., strawberry)
o a multiple fruit develops from ovaries from separate flowers fused together (e.g., pineapple)
- Seed Germination
o seed germination occurs when growth and metabolic activity resume
o the embryo forms with both shoot and root apical meristem enclosed in a seed
§ protoderm gives rise to the epidermis
§ ground meristem produces the cells of the cortex and pith
§ procambium produces vascular tissue
o seeds retain their viability for varying times: maples seeds only last a week while lotus seeds are viable for hundreds of years
o some seeds do not germinate until they have been dormant for a period of time
§ seed dormancy is a time during which no growth occurs even though conditions are favorable
§ in temperate zones, seeds may have to be exposed to cold weather before dormancy is broken
§ in deserts, germination requires rain
o germination has environmental requirements
§ oxygen must be available for increased metabolism
§ adequate temperature allow enzymes to act
§ adequate moisture hydrates cells
§ light may also be required
o seeds that must be planted near the surface probably require light (e.g., lettuce)
o when a seedling grows in the dark, it becomes long and spindly (etiolated); phytochrome induces normal growth in light
o germination in eudicots and monocots
§ prior to germination, a eudicot embryo consists of the following:
· two cotyledons that supply nutrients to the embryo and seedling, but soon shrivel and disappear
· a plumule—a rudimentary plant consists of an epicotyl bearing young leaves
· the hypocotyl, which becomes the stem
· the radicle, which develops into roots
§ in dicot seedling, the shoot is hook-shaped to protect the delicate plumule
§ in monocots, the endosperm is the food-storage tissue and the cotyledon does not have a storage role
§ a monocot ³seed² such as a corn kernel is actually the fruit and the outer covering is the pericarp
§ the plumule and radicle are enclosed in protective sheaths, the coleoptile and the coleorhiza, respectively
§ the plumule and radicle burst through these coverings when germination occurs
Asexual Reproduction in Plants
- Means of Asexual Propagation
o plants contain nondifferentiated meristem tissue and often reproduce asexually by vegetative propagation
o in asexual reproduction, offspring arise from a single parent and inherit genes of that parent only
o vegetative propagation utilizes the meristematic tissue of a parent plant
§ violet plants grow from nodes of rhizomes
§ the nodes of stolons will produce strawberry plants
§ each eye of a potato plant tuber is a bud that produces a new plant
§ sweet potatoes can be propagated from their modified roots
§ many trees can be started from small ³suckers²
o stem cuttings have long been used to propagate a wide array of plants (e.g., sugarcane, pineapple)
o the discovery that auxin will cause roots to develop has expanded our ability to use stem cuttings.
- Tissue Culture of Plants
o tissue culture is the process of growing tissue artificially in a liquid or solid culture medium
o in 1902, German botanist Gottleib Haberlandt stated plant cells were totipotent; each cell has full genetic potential of the organism
o when cultured cells were provided with sugars, minerals, vitamins, and cytokinin from coconut milk, the undifferentiated cells divide and initially formed a callus, an aggregation of undifferentiated cells
o the callus then differentiated into shoot and roots and developed into a complete plant
o micropropagation is a commercial method of producing thousands to millions of identical seedlings, by tissue culture in limited space
o meristem culture micropropagates many new shoots from a single shoot apex culture in a medium with correct proportions of auxin and cytokinin
§ since the shoots are genetically identical, the adult plants that develop are clonal plants
§ clonal plants have the same genome and display the same traits
§ meristem culture generates meristem & plants that are virus-free
o entire plants can be grown from single plant cells
§ enzymes can digest cell walls & produce naked plant cells called protoplasts
§ protoplasts regenerate a cell wall and begin cell division
§ clumps of cells can be manipulated to form somatic embryos
§ somatic embryos encapsulated in a hydrated gel (³artificial seeds²) can be shipped anywhere
o anther culture cultures anthers in a medium of vitamins and growth regulators
o cell suspension culture uses rapidly growing calluses cut into small pieces and shaken in a liquid nutrient medium
- Genetic Engineering of Plants
o traditionally hybridization (crossing different varieties or species) was used to produce new plants
o transgenic plants carry foreign genes directly introduced into their cells
o Tissue Culture and Genetic Engineering
§ genetic engineering alters genes of organisms so they have new and different traits; protoplasts lend themselves to genetic engineering in tissue culture
§ high voltage electric pulses create pores in plasma membrane so new DNA can be introduced
§ when genes for production of firefly enzyme luciferinase were inserted into tobacco protoplasts, plants glowed when sprayed with the substrate luciferin
§ foreign DNA can be inserted into a plasmid of Agrobacterium; this bacterium infects plant cells and can deliver the recombinant DNA to target cells
§ a gene gun is now used to bombard a callus with DNA coated metal particles
§ crops have been engineered to resist frost, fungal and viral infections, insect predation, and herbicides
§ future crops could have more protein and require less water and fertilizer
o Agricultural Plants with Improved Traits
§ corn, potato, soybean, and cotton plants have been engineered to be resistant to insect predation or herbicides
§ salt-tolerant Arabidopsis has been developed by cloning a gene for sequestering sodium ions in a vacuole
§ genes from Vernonia and castor beans were inserted into soybeans to produce vernolic acid and ricinoleic acid used as hardeners in paints and plastics
§ genetic engineering is attempting to improve efficiency of RuBP carboxylase and introduce C4 photosynthesis to rice
o Commercial Products
§ corn has made antibodies to deliver radioisotopes to tumor cells
§ soybeans make an antibody to treat genital herpes
§ human gene can be inserted into tobacco plants using tobacco mosaic virus
§ tobacco plants produced antigens to treat non-Hodgkin¹s lymphoma