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