Biology 102

Study Notes Exam 2

 

Chapter 24: Evolution & Diversity of Plants

 

Evolutionary History of Plants

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

 

Alternation of Generations

-       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

 

Other Adaptations to Terrestrial Environments

-       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