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
- vascular plants evolved about 430 million years ago during the Silurian period
- 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
o gymnosperms appear about 400 million years ago, during the Devonian period
- flowers evolved as reproductive structures to attract pollinators; they first appeared about 400 million years ago
- 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
- appearance of the generations among plants varies widely
o in ferns, the gametophyte is a small heart-shaped structure
o the female gametophyte in flowering plants (the embryo) is retained within the body of the plant as a few cells inside an ovule
o in seed plants, pollen grains are mature sperm-bearing male gametophytes
- 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 ("Bryophytes")
- 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
o flagellated sperm swim to the vicinity of the egg in a continuous film of water
o the sporophyte is attached to & nourished by the photosynthetic gametophyte
- 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 mosses store much water; when they dry out, they become dormant; when it rains, they become green
o copper mosses only live near copper & are an indicator of ore deposits
o luminous moss lives in caves and glow with a golden-green light
o some "mosses" are not true mosses:
• Irish moss is an edible red alga of northern seacoasts
• Reindeer moss is a lichen that is a mainstay of caribou
• Club mosses are vascular plants
• Spanish moss, which hangs from trees in the southern U.S., 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
• 3 days of favorable growing conditions produces upright shoots 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
• initially the sporophyte is green and photosynthetic; at maturity it is brown and nonphotosynthetic
- 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
- evolutionary history: Rhyniophytes were dominant from mid-Silurian period of the Paleozoic era to the mid-Devonian; Cooksonia may have been the first vascular plant and colonizer of land
o the photosynthetic stems, not true leaves or roots, have sporangia at tips; they are attached to a rhizome
o similar to bryophytes, they were homosporous, producing one type of spore
- 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
- evolutionary history: seedless vascular plants were dominant from the late Devonian period through the Carboniferous period
o club mosses (35 m), horsetails (18 m), and ferns (8 m) were larger than today's specimens and formed great swamps
- Club Mosses (division Lycopodophyta)
o common in temperate woodlands where they are called "ground pine."
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
o spores germinate into inconspicuous and independent gametophytes
o most club mosses live in tropics or subtropics as epiphytes, plants that live on trees without harming them
o closely related spike mosses (Selaginella) and quillworts (Isoetes) produce heterospores; suggesting that heterospory arose independently at least twice
- 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 horse's tail
o many horsetails have a strobilus at the tip of all stems; others send up special buff-colored stems that bear stroboli
o the spores germinate into inconspicuous and independent gametophytes
o the tough, rigid stems have silica in the cell walls; early Americans used them as "scouring brushes."
- 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 aerial stems with tiny scales fork repeatedly and carry on photosynthesis
o sporangia are located at the ends of short branches
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 adaptation of fern reproduction
• a tiny green gametophyte is independent from the sporophyte for nutrition
• flagellated sperm are released by antheridia and swim to the archegonia in a film of water
o uses of ferns
• ferns are used as ornamental plants by florists & home decorations
• fern wood is very decay- & termite- resistant
• fern medicines are used by natives to stop bleeding after childbirth; also as an expectorant
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 "seed."
- 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 ancient gymnosperms were present in swamp forests of the Carboniferous period.
o Conifers: phylum Coniferophyta (~ 575 species)
• conifers are cone-bearing trees and shrubs such as pines, hemlocks, and spruces
• conifers usually have evergreen needlelike leaves well adapted to withstand extremes in climate
• the oldest and largest trees in existence are conifers:
⋅ the coastal redwood (Sequoia semperivirens) is the tallest living vascular plant and grows to nearly 100 meters high
⋅ bristlecone pines grow in the White Mountains of California Nevada mountains; one is 4,900 years old
• conifer forests cover vast areas of northern temperate regions
• pine needles have a thick cuticle and recessed stomata
• uses of pines:
⋅ pine is a major wood used in construction
⋅ with xylem tissue that lacks some of the rigid cell types, it is a "soft" rather than "hard" wood
⋅ pine resin is an insect and fungal deterrent harvested for 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
• each scale of a seed cone has two ovules surrounded by an integument and with an opening at one end
• 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 the second 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
• the cycad life cycle is similar to that of pine trees except they are pollinated by insects
• the pollen tube bursts in the vicinity of the archegonium and multiflagellated sperm swim to reach an egg
• cycads flourished during the Mesozoic era and probably were food for herbivorous dinosaurs
• 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 is found in U.S. desert regions, and is a many-branched shrub with small, scalelike leaves
• Welwitschia is found in deserts in southwest Africa; most of it exists underground and it has two enormous leaves
• the xylem and stroboli are uniform across all three 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 this group contains six times the number of species of all other plant groups combined
o angiosperms live in all habitats from freshwater to desert and from tropics to subpolar regions
o flowering plant size ranges from microscopic duckweed to Eucalyptus exceeding 100 m tall
o they are important in everyday human life: clothing, food, medicine, and commercial products
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
• flower variety is related to the numerous means by which flowers are pollinated and fruits are dispersed
• 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
• since animals live in certain habitats or have particular migration patterns, they can deliver a fruit-enclosed seed to a suitable location for germination and development.
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 "die back" 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 ring
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
• tracheids are smaller, hollow, thin, long nonliving cells with tapered overlapping ends
⋅ water moves across end and sidewalls because of pits or depressions in secondary cell wall
• vessel elements are hollow non-living cells lacking tapered ends
⋅ they are larger than tracheids & lack transverse end walls
⋅ they form a continuous pipeline for water and mineral transport
• xylem also contains sclerenchyma cells to add support
• vascular rays are flat ribbons of parenchyma cells between rows of tracheids; they 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 ("sieve-tubes"), 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 without entering cells
o endodermis is single layer of rectangular cells that forms the boundary between cortex and inner vascular cylinder
• its cells fit closely together & are bordered on four sides by the Casparian strip
• 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
• the only access to the vascular bundle is through endodermal cells
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 bundle
⋅ 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; the first or primary root grows straight down and remains dominant root of a plant; often fleshy and 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
⋅ a prop root's main function is to anchor a plant (e.g., corn and mangrove plants)
⋅ pneumatophores of mangrove plants project above the water from roots to acquire oxygen
⋅ ivy has holdfast roots to anchor aerial shoots
o haustoria are rootlike projections from stems on parasitic plants (e.g., dodders and broomrapes)
• haustoria grow into the host plant & contact vascular tissue from which they extract water and nutrients
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
• legumes are often planted to bolster nitrogen supply of soil
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
o shoot apical meristem develops into: protoderm (produces epidermis); ground meristem (produces pith cortex); procambium (inner meristem that produces primary xylem and primary phloem)
o differentiation continues; cells become first tracheids or vessel elements within vascular bundle
o mature phloem develops later after all surrounding cells have stopped expanding and vascular cambium has developed
- 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
• secondary phloem is produced each year by vascular cambium but does not build up
• this phloem tissue is soft; therefore it is easy to remove the bark of a tree
o cork cambium is meristem beneath the epidermis that produces new cork cells when needed
• cork cambium begins to divide, producing cork that disrupts epidermis replacing it with cork cells
• 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 year's 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
• it takes energy to support secondary growth and prepare plant for winter in temperate zones
• long-lasting plants need more defense mechanisms against attack by herbivores and parasites
• trees need years to mature before reproducing; they are more vulnerable to accident or disease
- Stem Diversity
o stolons are stems that grow along the ground; new plants grow where the nodes contact the soil
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
• rhizomes survive winter and contribute to asexual reproduction because each node bears a bud
• 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
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 Venus's-flytrap has leaves to trap and digest insects
Chapter 26: Nutrition And Transport In Plants
Plant Nutrition and Soil
- Early Views
o ancient Greeks considered plants "soil-eaters" that converted soil into plant tissues
o the 17th Century Dutchman Jean-Baptiste Van Helmont conducted an experiment
• he planted a five pound young willow tree in a pot with 200 pounds of soil
• after five years of watering, the tree weighed 170 pounds but only a few ounces of soil was missing
• 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 these elements are divided into macronutrients and micronutrients by concentration in plant tissue
o beneficial nutrients: elements required or that improve growth of a particular plant
• horsetails require silicon as a mineral nutrient
• sugar beets show better growth in the presence of sodium
• soybeans use nickel when root nodules are present
- 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 profiles
• the A horizon is the uppermost topsoil layer that contains litter and humus
• the B horizon lacks organic matter but contains inorganic nutrients leached from the A horizon
• the C horizon is weathered and shattered bedrock
• soil profiles vary by parent material, climate and ecosystem
• grassland soils have deep A horizons from turnover of decaying grasses and lack of leaching
• forest soils have thinner A horizons but enough inorganic nutrients for tree root growth
• tropical rain forest A horizons are shallow due to rapid decomposition; the B horizon is deeper due to extensive leaching
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
• U.S. farmlands lose soil faster than it is formed on one-third of cropland
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
• mineral nutrient concentration in roots may be 10,000 times more than in surrounding soil
• during transport throughout a plant, minerals can exit xylem and enter cells that require them
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
• plants cannot use atmospheric nitrogen because they lack enzymes to break the N=N bond
• Rhizobium makes nitrogen compounds available to plants in exchange for carbohydrates
• bacteria live in root nodules - structures on plant roots that contain nitrogen-fixing bacteria
• Rhizobial bacteria reduce atmospheric nitrogen (N2) to ammonium (NH4+) (nitrogen-fixation)
• other plants have a relationship with free-living, nitrogen-fixing microorganisms in soil
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 fungal hyphae may enter the cortex of roots but do not enter plant cells
• ectomycorrhizae form a mantle exterior to the root, and they grow between cell walls
• 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
• orchid seeds are small with limited nutrients; they germinate only when invaded by mycorrhizae
• nonphotosynthetic plants (e.g., Indian pipe) use mycorrhizae to extract nutrients from nearby trees
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
• tracheids are hollow, nonliving cells with tapered overlapping ends and sidewalls
• vessel elements are hollow, nonliving cells that lack tapered ends and lack transverse end walls; they form a continuous pipeline for water and mineral transport
o phloem conducts organic solutes in plants mainly from leaves to roots; contains sieve-tube cells and companion cells
• sieve-tube cells lack a nucleus, are arranged end to end and have plasmodesmata that extend from one cell to another
• companion cells connect to sieve-tube cells by numerous plasmodesmata, are smaller and more generalized than sieve-tube cells; they have a nucleus
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
o they also rely on the chemical properties of water: polarity of water and hydrogen bonding
- 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 movement of water and minerals in a plant involves entry into roots, xylem, and leaves
o water and minerals enter root cells before they reach xylem by the two routes already described
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 plant's 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 in daytime, the negative water potential created by transpiration extends from leaves to roots; the water column must be continuous
o if a water column within xylem is broken by cutting a stem, the water column will drop back down the xylem vessel away from the site of breakage, making it more difficult for conduction to occur
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 evidence suggests a receptor in the plasma membrane of guard cells brings about inactivation of the proton pump when CO2 concentration rises, as happens when photosynthesis ceases
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
• he observed the results of removing a strip of bark from a tree (girdling)
• bark swells just above the cut and sugar accumulates in the swollen tissue
• 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
• when 14C-labeled carbon is supplied to mature leaves, radioactively labeled sugar moves to roots
• similar studies confirm phloem transports amino acids, hormones, and mineral ions
o aphids used in study
• it is difficult to take samples of sap from just the phloem cells without injuring the phloem
• aphids (small insects) drive their mouth stylets into a sieve-tube cell; then samples are easily taken
• the aphid body is cut off; the stylet becomes a small needle from which phloem is collected
• such research indicates sap can move through phloem from 60-100 cm per hour or more
- 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 earth's 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 Charles Darwin and his son found that if the root cap is removed, roots no longer respond to gravity
• later researchers showed root cap cells contain statoliths, starch grains within amyloplasts; due to gravity, amyloplasts settle to the lowest part of the cell
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 wind or rain
• a plant growing in a windy location has a shorter, thicker trunk
• simple rubbing of a plant inhibits cellular elongation and produces a shorter, sturdier plant.
- 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 a pulvinus is a thickening at base of such leaflets where turgor pressure can rapidly drop (potassium ions move out of cell and water follows by osmosis)
o a single stimulus such as a hot needle can cause all of the leaves to respond; this requires a nerve impulse-like stimulus for communication
o Venus's-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 movement is due to changes in turgor pressure of motor cells in a pulvinus
o some plant movements correspond to environmental changes in light, temperature, etc.
o biological clocks (maintain Circadian rhythms) are synchronized by external stimuli to twenty-four-hour rhythms
o photoperiod is better indicator of seasonal changes than temperature change
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
- plant growth regulators: naturally-occurring plant growth hormones & their imitators
- 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 a weak solution of auxin causes roots to develop from the ends of cuttings
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 Darwin: phototropism would not occur in oat seedlings if the tip of a seedling is cut off or covered by a cap; chemical moved from tip to the rest of the shoot
o Frits W. Went (1926): coleoptile tips curve away from gravity regardless of light
• named chemical causing curved growth auxin (Greek word for "to grow")
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 that elongation occurs
• auxin-mediated elongation occurs in younger cells; older cells may lack auxin receptors
- 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 discovered in 1926 by Ewiti Kurosawa, a Japanese scientist investigating a fungal disease of rice plants called "foolish seedling disease."
o mode of action: the hormone GA3 binds to a receptor; a second messenger (Ca2+) inside cell combines with the protein calmodulin
• the Ca2+-calmodulin complex activates a gene coding for the enzyme amylase
• amylase 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 oligosaccharins, chemical fragments released from cell wall, also direct differentiation
o researchers hypothesize that auxin and cytokinins are part of a reception-transduction-response pathway that activates enzymes that release these fragments from the cell wall
- 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 "stress hormone"; 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
• reduction in ABA and increase in gibberellins break seed and bud dormancy; seeds germinate and buds send forth leaves
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, it is not believed to function in this process; 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
o a barrel of ripening apples can induce ripening of a bunch of bananas some distance away
o ethylene releases from site of a physical wound; therefore one rotten apple spoils the whole bunch
o the presence of ethylene in air inhibits growth of plants in general
o ethylene is present in auto exhaust and in homes heated with natural gas
o inhibition of plant growth occurs in low concentrations (one part ethylene per 10 million parts of air)
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
- research by U.S. Department of Agriculture in the 1920s in controlled greenhouses revealed this mechanism
- 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
• examples include cocklebur, poinsettia, and chrysanthemum
o long-day plants: flower when the day length is longer than a critical length
• examples include wheat, barley, clover, and spinach
o day-neutral plants: flowering is not dependent on day length
• examples include tomato and cucumber
- a long-day and a short-day plant can have the same critical length
o spinach is a long-day plant that flowers in summer when day length increases to 14 hours; ragweed is a short-day plant that flowers in fall when day length shortens to 14 hours or less
- in 1938, K. C. Hammer and J. Bonner experimented with artificial lengths of dark and light periods & found that length of the dark period controls flowering, not length of the light period
- 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
• direct sunlight contains more red than far-red light; Pfr is present in plant leaves during the day
• shade and sunsets have more far-red than red light; Pfr is converted to Pr as night approaches
• there is a slow metabolic replacement of Pfr by Pr during night
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
o following germination, the presence of Pr dominates; the stem elongates and grows toward sunlight while the leaves remain small
o once a plant is exposed to sunlight and Pr is converted to Pfr, the plant begins to grow normally; leaves expand and the stem branches
o the Pfr form of phytochrome triggers activation of one or more regulatory proteins in the cytosol
o these proteins migrate to the nucleus and bind to "light-stimulated" genes coding for proteins found in chloroplasts.
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
• an anther has four pollen sacs; each contains many microsporocytes (microspore mother cells)
• microsporocytes undergo meiotic cell division to produce four haploid microspores
• the haploid nucleus then divides mitotically forming two cells enclosed in a finely sculptured wall; this is a pollen grain containing a tube cell and a generative cell
• the larger tube cell will eventually form the pollen tube
• eventually each generative cell will divide mitotically to form two sperm
• once both events have occurred, the pollen grain is the mature male gametophyte
o pollination
• walls separating the pollen sacs in the anther break down when the pollen grains are to be released
• the shape and pattern of pollen grains is distinctive and allows close identification
• pollen grains have strong walls resistant to chemicals and also become readily fossilized
• 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 such as the carpel only maturing after anthers have released their pollen
• 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
• an ovule is covered by parenchymal cells except for one small opening, the micropyle
• one parenchyma cell enlarges to become a megasporocyte that undergoes meiotic cell division to produce four haploid megaspores
• three megaspores are nonfunctional; one megaspore nucleus divides mitotically into eight nuclei in a female gametophyte
• when cell walls form around the nuclei later, there are seven cells, one of which is binucleate
• 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
• a germinated pollen grain, containing a tube cell and two sperm, is the mature male gametophyte
• 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
• during this 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; by dividing with the cell plate perpendicular to the surface, they produce one outer cell layer
• dermal tissue 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 a pea pod breaks open on both sides and releases seeds; legumes are fruits that split to two sides when mature
o legumes and cereal grains are examples of dry fruits; such fruits are mistaken for seeds because a dry pericarp adheres to the seed within
o for plants to be widely distributed, seeds must be dispersed away from parent plant
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; a cross-section shows it came from a compound ovary with several chambers
- 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; this ensures that seeds do not germinate until a favorable growing season has arrived
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 respiration and metabolism continue throughout dormancy but at a reduced level
o some seeds have a surface coating that attracts water; imbibing plant cells can swell dramatically
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
• as dicot seedling emerges, 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 "seed" 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 "suckers"
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 in 1902, German botanist Gottleib Haberlandt suggested producing entire plants from tissues
o tissue culture is process of growing tissue artificially in a liquid or solid culture medium
o Haberlandt stated plant cells were totipotent; each cell has full genetic potential of the organism
o in 1958, Cornell botanist F. C. Steward grew a complete carrot plant from a tiny piece of phloem
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 that is virus-free; the plants produced are also 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 ("artificial seeds") can be shipped anywhere
• somatic embryos are cultured by the millions in large tanks (bioreactors)
• plants generated from somatic embryos vary because of mutations; these somaclonal variations may produce new traits
o anther culture cultures mature anthers in a medium of vitamins and growth regulators
• the haploid tube cells within a pollen grain divide, producing proembryos made of 20 to 40 cells
• finally the pollen grains rupture, releasing haploid embryos
• the researcher can then generate a haploid plant
• chemical agents are added to encourage chromosomal doubling; the resulting plants are diploid and homozygous for all alleles
• this produces plants that express recessive alleles
o cell suspension culture uses rapidly growing calluses cut into small pieces and shaken in a liquid nutrient medium
• single cells or small clumps form a suspension of cells; all produce the same chemicals as the entire plant
• this technique is a more efficient way of producing chemicals used in drugs, cosmetics, and agricultural applications than farming plants simply to acquire chemicals they produce
- 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 in particular lend themselves to direct 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, adult plants glowed when sprayed with the substrate luciferin
• regeneration of cereal grains from protoplasts has been difficult; corn and wheat protoplasts produce infertile plants
• foreign DNA can be inserted into a plasmid of Agrobacterium; this bacterium infects plant cells and can be used to deliver the recombinant DNA to target cells
• John C. Sanford and Theodore M. Klein of Cornell University developed a gene gun to bombard a callus with DNA coated metal particles; later, adult plants are generated
• crops have been engineered to resist frost, fungal and viral infections, insect predation, and herbicides
• future crops could have higher protein content and require less water and fertilizer
• sequencing the genomes of a dicot Arabidopsis thaliana and rice will give a blueprint to the genes of other monocots and dicots.
o Agricultural Plants with Improved Traits
• corn, potato, soybean, and cotton plants have been engineered to be resistant to insect predation or herbicides
• if the crops are resistant to herbicide and the weeds are not, then less tillage is needed
• salt-tolerant Arabidopsis has been developed by cloning a gene for sequestering sodium ions in a vacuole
• such techniques would allow development of crops that could grow where irrigation causes salinization
• genes from Vernonia and castor bean seeds have been inserted into soybeans to produce vernolic acid and ricinoleic acid used as hardeners in paints and plastics
• other genetic engineering goals could be the increase in productivity by altering water and carbon dioxide intake
• 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
• researchers can introduce a human gene into tobacco plants using tobacco mosaic virus
• tobacco plants produced antigens to treat non-Hodgkin's lymphoma