Plant Structure and function
Section 1 PLANT CELLS AND TISSUES
Plants have adapted a range of environments over their evolution As they grow their cells become specialized for specific functions Patterns of tissues changes in each plant part root, stem, leaf Also vary depending on plant s stage of growth and taxonomic group
Specialized Plant Cells Remember plant cells have unique structures Cell wall Central vacuole Plastids 3 types of specialized plant cells 1. Parenchyma 2. Collenchyma 3. Sclerenchyma
Paranchyma puh REN kuh muh Usually loosely packed cube shaped or elongated cells Contain large central vacuole Have thin, flexible cell walls Involved in many metabolic functions: photosynthesis, storage of water and nutrients, healing Usually form main part of nonwoody plants Ex. Fleshy part of apple
Collenchyma koh LEN kuh muh Cells walls thicker than parenchyma Cell walls irregular in shape Thicker walls provide more support for plant Usually grouped in strands Specialized for supporting areas of plant that are still lengthening Ex. Celery stalks lots of collenchyma
Sclerenchyma skluh REN kuh muh Thick, even, stiff cell walls Support and strengthen plant in areas where growth is finished Usually dies at maturity Rough texture of pear is from presence of sclerenchyma cells
Tissue Systems Cells that work together to perform specific function make tissue In plants, arranged into systems 1. Dermal system 2. Ground system 3. Vascular system Systems further organized into 3 major plant organs roots, stems, leaves
Dermal Tissue System Forms outside covering of plants In young plants, made of epidermis ep uh DURH muhs the outer layer made of parenchyma cells In some species, epidermis more than 1 cell thick Outer epidermal wall often covered by waxy layer called the cuticle prevents water loss
Some epidermal cells of roots develop hairlike extensions that increase water absorption Openings in leaf and stem epidermis are stomata help regulate the passage of gases and moisture in and out of plant In woody stems and roots, epidermis replaced by dead cork cells
Ground Tissue System Dermal tissue surrounds the ground tissue system Has all 3 types of cells Functions in storage, metabolism, support Paranchyma most common cell Nonwoody roots, stems, leaves made mostly of ground tissue
Cactus stems have large amounts of parenchyma cells for storing water Plants growing in very wet soil have parenchyma with large air spaces to allow air to reach roots Nonwoody plants that need to be flexible to withstand wind have large amount of collenchyma cells Sclerenchyma found where hardness is advantage, i.e. seed coats, cacti spines
Vascular Tissue System Ground tissue surrounds the vascular tissue system Functions in transport and support Xylem and phloem Xylem conducts water and minerals from roots up Phloem conducts organic compounds and some minerals through plant Phloem is alive at maturity, xylem is not
Structure of Xylem In angiosperms, 2 major parts 1. Tracheids 2. Vessel elements Tracheid (TRAY kee id) long, thick walled sclerenchyma cell with tapering (narrowing) ends
Water moves from one tracheid to another through pits thin, porous areas of cell wall
Vessel element sclerenchyma cell that has either large holes in top and bottom walls or no end walls at all
Vessel elements stacked to form long tubes called vessels Water moves more easily in vessels than tracheids
Xylem of most seedless vascular plants and most gymnosperms contains only tracheids Tracheids considered primitive type of xylem cell Vessel elements in angiosperms probably evolved from tracheids Xylem contains parenchyma and sclerenchyma cells
Phloem Conducting parenchyma cell of angiosperm phloem is called a sieve tube member
Sieve tube members stacked to form long sieve tubes Compounds move from cell to cell through end walls called sieve plates Each sieve tube member lies next to specialized parenchyma cell (companion cell) assists in transport
Phloem usually contains sclerenchyma cells called fibers Hemp, flax, and jute fibers are phloem fibers
Vascular tissue systems also modified for environmental reasons Ex. Xylem forms wood of trees Provides plants with strength AND conducts water and minerals Ex. Aquatic plants Xylem not needed for support or water so may be almost absent from plant
Growth in Meristems Plant growth starts in meristems MER istemz regions where cells continuously divide Apical AP i kuhl meristems plant grows in length Located at tips of stems and roots
Some monocots have intercalary in TUHR kah leree meristems located above bases of leaves and stems Allow grass leaves to quickly regrow after being cut
Gymnosperms and most dicots also have lateral meristems allow stems and roots to increase in diameter Located near outside of stems and roots 2 types 1. Vascular cambium 2. Cork cambium
Vascular cambium produces additional vascular tissues Located between xylem and phloem
Cork cambium produces cork Located outside phloem Cork cells replace epidermis in woody stems and roots Protects plant Cork dead cells that provide protection and prevent water loss
Primary growth increase in length Made by apical and intercalary meristems Secondary growth increase in diameter Made by lateral meristems By vascular cambium and cork cambium
Section 2 ROOTS
Functions 1. Anchor plant in soil 2. Absorb/transport water and minerals 3. Store water and organic compounds
Types of Roots When seed sprouts it makes a primary root If it becomes the largest root it s called the taproot
Some plants the primary root doesn t get big Instead many small roots develop to make fibrous root system Many monocots, like grasses, have this Often develop straight from stem instead of other roots
Adventitious roots specialized roots that grow from stems and leaves
Root Structures Root tip covered by protective root cap Covers apical meristem Makes slimy substance that acts like lubricant Allows root to move easily through soil as it grows Cells crushed in root cap as root moves through soil replaced by new cells made in apical meristem
Root hairs extensions of epidermal cells; increase surface area of root so increase plant s ability to absorb
Root structures adapted for several functions Root hairs greatly increase surface area for absorption Most roots also form relationships with mycorrhizal fungi Threadlike hyphae increase surface area also
Highly branched root system increases amount of soil for plant to absorb water and minerals from Also helps in anchoring plant in soil Large amount of root parenchyma functions in storage and metabolism Roots need shoots (stems) for energy must store starch during periods of little/no photosynthesis (night, winter)
Primary Growth in Roots Increase in length through cell division, elongation, and maturation in root tip Dermal tissue matures to form epidermis Ground tissue matures into 2 specialized regions 1. Cortex just inside epidermis 2. Endodermis innermost layer of cortex
Endodermal cell walls have narrow band of waterproof substance that stops further movement of water through cell walls Must pass through selectively permeable membrane of root cell Once in cell membrane, dissolved substances can move from cell to cell
Vascular tissue in roots matures to form innermost part of root In dicots/gymnosperms, xylem makes up center core Has pockets of phloem between xylem
Monocot root xylem happens in patches that circle pith Small areas of phloem occur between xylem patches
Outermost layer of central vascular tissue is called pericycle PER isie kuhl Lateral roots formed from division of pericycle cells
Developing lateral root connects vascular tissues and endodermis to parent root Grows out through parent root s endodermis and cortex Emerges from epidermis
Secondary Growth in Roots Dicot and gymnosperm roots often have secondary growth Begins when vascular cambium forms between primary xylem and primary phloem Pericycle cells form vascular cambium at ends of xylem areas where no phloem is located
Vascular cambium makes secondary xylem toward inside of root and secondary phloem toward outside of root
Expansion of vascular tissues in center crushes all tissues external to phloem, including endodermis, cortex, epidermis Cork cambium develops in pericycle to replace crushed cells with cork
Root Functions Besides anchoring plant in soil roots have 2 other main functions 1. Absorb water and minerals dissolved in soil Roots are selective about which minerals they absorb Absorb some, exclude others Absorbed mainly as ions
Plant cells use some minerals in large amounts macronutrients Required in large amounts More than 1,000 mg/kg of dry matter Use other minerals in smaller amounts micronutrients Required in small amounts Less than 100 mg/kg of dry matter
Adequate amounts of all 13 minerals required for normal growth Plants with shortages show symptoms and reduced growth Serious mineral deficiencies can kill a plant Too many may also be toxic
2. Storing Carbohydrates and Water Roots often adapted to do this Phloem carries carbohydrates made in leaves to roots Carbohydrates not immediately used are stored In roots, these usually converted to starch Stored in parenchyma cells
Section 3 STEMS
Types of Stems Different types show different adaptations to environment Ex. Strawberry stems grow along soil surface, make new plants at nodes
Ex. Edible white potato tuber modified for storing energy
Ex. Cactuses green fleshy stems that store water and carry out photosynthesis
Ex. Black locust and honey locust develop sharp thorns to protect from animals
Stem Structures Similar to roots but more complex Stems, like roots, grow in length only at their tips Apical meristems make new primary tissues Stems, like roots, also grow in circumference through lateral meristems
Surfaces of stems have several features that roots don t have Divided into segments called internodes End of each internode = node
At point of attachment of each leaf, stem has lateral bud Bud capable of developing into a new shoot Contains apical meristem and is enclosed by specialized leaves called bud scales
Tip of each stem usually has a terminal bud When growth resumes in spring, terminal bud opens Bud scales fall off Bud scales leave scars on stem surface
Root tips have permanent protective layer (root cap) Stem apical meristem only protected by bud scales when stem is not growing Surface bud forms close to stem tip with 1 or more buds at each node Lateral roots originate farther back from root tip Form deep inside root at no particular location
Primary Growth in Stems Same as roots, apical meristems form dermal, ground, and vascular tissues Dermal tissue represented by epidermis (outer layer of stem) Main functions protect plant, reduce loss of water but allow gas exchange
In gymnosperm and dicot stems, ground tissue forms a cortex and pith Cortex lies just inside epidermis (like root) Cortex usually contains flexible collenchyma cells
Pith located in center of stem Ground tissue of monocot stems not usually separated into pith and cortex
Vascular tissue formed near apical meristem happens in bundles Long strands embedded in cortex Each bundle has xylem and phloem tissues Xylem usually toward inside, phloem toward outside
Monocot stem vascular bundles usually scattered throughout ground tissue Most don t have secondary growth In dicots, organized in single ring Primary tissue replaced by secondary
Secondary Growth in Stems Stems increase in thickness b/c of division of cells in vascular cambium In dicot and gymnosperms first comes between xylem and phloem Eventually vascular cambium forms cylinder
Vascular cambium produces secondary xylem to inside, secondary phloem to outside Usually makes more secondary xylem than secondary phloem Wood secondary xylem
Older parts of xylem eventually stop transporting water Often become darker than new xylem b/c resin and other compounds made by live cells in xylem build up Heartwood darker wood in center of tree
Functional, usually lighter colored wood near outside of trunk is sapwood In large diameter tree, heartwood gets wider, sapwood stays same
Phloem made near outside of stem is part of bark protective outside covering of woody plants Made of cork, cork cambium, phloem Cork cambium makes cork near outside
Cork cells dead at maturity Cannot elongate Cork cracks as stem continues to widen Result: bark of some trees appears rough
During spring, vascular cambium forms new xylem tissue with cells that are wide and thin walled springwood Rains a lot, lots of water to fill cells In summer (drier), vascular cambium makes summerwood smaller cells, thicker walls Change between summerwood and springwood makes the annual ring
Can estimate age of tree b/c 1 ring usually made per year Usually don t happen in tropical trees (environment same year round) Can form in dicot and gymnosperm roots (harder to see)
Stem Functions Transport and store nutrients and water Support leaves Carbohydrates, plant hormones, other organic compounds transported by phloem Movement of carbs occurs from where they re made/stored (source) to where they re stored/used (sink)
Translocation movement of carbohydrates through plant May be made in photosynthetic cells or stores as starch Movement explained by pressure flow hypothesis carbohydrates actively transported into sieve tubes
As carbs enter sieve tubes, water transported by osmosis Positive pressure builds up at source end of sieve tube pressure part of pressure flow hypothesis
At sink, process reversed Carbs actively transported out Water leaves by osmosis Pressure reduced Difference in pressure causes flow
Transport of Water Water and minerals transported through xylem During day, water constantly evaporating from plant (through stomata) Water loss transpiration tran spuh RAYshuhn Result of plant needing CO 2 from air How does water move up a stem?
Cohesion Tension Theory Water pulled up xylem by strong attraction of water molecules to each other (cohesion) Movement also depends on rigid xylem walls and adhesion (attraction of water to xylem walls) Thin columns of water extend from leaves through stems and into roots
As water evaporates, water column has great tension Water column doesn t break b/c cohesion and adhesion Only other possible direction: up Pull at top of tree reaches all the way to bottom As water pulled up, more water enters roots to replace lost water
Storing Water and Nutrients Plant stems adapted for storage in most species Many parenchyma cells in cortex Cacti stems specialized for storing water Roots found close to surface (absorb water quickly and transport to stem) Sugar cane stems store large amount of sucrose Etc.
Section 4 LEAVES
This is a tendril Specialized leaf found in many vines Wraps around object to support climbing vine In some species, it is specialized stem (like in grapes)
Unusual leaf modification happens in carnivorous plants like the pitcher plant/venus fly trap Leaves function as food traps Plants grow in soil poor in many nutrients, especially nitrogen Plant gets nutrients when it traps and digests insects and other small animals
b/c spines are so small and nonphotosynthetic, they greatly reduce loss of water from transpiration
Leaf Structures Wide variety of shapes and sizes Important for identifying plants Blade broad, flat portion of leaf Site of photosynthesis
Blade usually attached to stem by petiole
Simple leaf single blade Compound leaf leaf divided into leaflets
In some species, leaflets are also divided Doubly compound leaf
Leaves have 3 tissue systems Dermal tissue system epidermis Epidermis is single layer coated with cuticle
Water, oxygen, carbon dioxide enter/exit through stomata Can have epidermal hairs Protect leaf from insects and intense light
Number of stomata in area depends on species Aquatic plants little or no stomata Corn up to 10,000 per square cm on upper and lower surface Scarlet oak over 100,000 only on lower surface
In most plants, photosynthesis happens in the leaf mesophyll MEZ oh fil Ground tissue made of chloroplast rich parenchyma cells
In most plants, mesophyll organized into 2 layers Palisade mesophyll directly beneath upper epidermis Site of most photosynthesis Palisade mesophyll are in columns Look packed tightly together in 1 or two layers There are air spaces between walls
Beneath palisade layer is spongy mesophyll Consists of irregularly shaped cells surrounded by large air spaces Allow O 2, CO 2 and H 2 O to diffuse in and out of leaf
Vascular tissue system of leaves made of vascular bundles called veins Continuous with vascular system of stem and petiole Embedded in mesophyll Venation arrangement of veins in leaf Most monocots parallel venation Most dicots net venation (branched
Leaf Functions Primary site of photosynthesis Mesophyll cells use light energy, carbon dioxide, water to make carbohydrates Light energy also used in mesophyll cells to make amino acids, fats, and other organic molecules Carbs made in leaf used by leaf as energy or building blocks May also be transported to other parts of plants
Major limitation to photosynthesis is not enough water b/c of transpiration Ex. About 98% water absorbed by corn s roots lost by transpiration Also cools plant and speeds transport of minerals through xylem
Modifications in Capturing Light Leaves often adapt to maximize catching light Leaves develop in full sun are thicker, have smaller area per leaf, and more chloroplasts Shade leaf chloroplasts are arranged so shading of one chloroplast by another is minimized
In dry environment, plants can receive more light than they can use Often have structures that reduce amount of light absorbed Ex. Dense coating of hairs Ex. Growing underground
Gas Exchange Plants have to balance their need to open stomata to get CO 2 and release O 2 with need to close stomata to prevent water loss Stoma bordered by 2 guard cells modified cells that regulate gas and water exchange
Stomata of most plants open during day and closed at night Opening and closing regulated by amount of water in guard cells Epidermal cells of leaves pump K+ into guard cells Water moves into guard cells by osmosis Flood of water makes guard cells swell Bend apart to make pore Stomata now open
During darkness, K+ pumped out of guard cells Water leaves by osmosis Guard cells shrink and stomata close
Stomata also close is plant has shortage of water Closing greatly reduces water loss Helps plant survive until next rain Nearly shuts down photosynthesis (no CO 2 )
Early land plants didn t have leaves and roots, only stems Leaves evolved from branches of stems Parts of a Flower
Botanists consider flowers to be specialized branches and parts of flowers specialized leaves
All specialized leaves form on tip of floral branch called the receptacle The receptacle is the enlarged part of the pedicle/peduncle (stem that ends in the flower)
Flower parts usually found in 4 continuous whorls (rings)
Outermost whorl is the calyx made of sepals SEE puhlz Surround and protect other parts of developing flower before it opens
Petals make up next whorl Most animal pollinated flowers have brightly colored petals Petals and sepals of windpollinated plants are small or absent
Two innermost whorls contain reproductive structures Male reproductive structures are stamens STAY muhnz Each has anther and filament
Anther contains microsporangia produce microspores that develop into pollen grains Filament supports anther
Innermost whorl contains female reproductive structures called carpels KAHR puhlz One or more carpels fused together make up pistil
Large base of pistil is the ovary Style stalklike and rises from ovary Stigma tip of style Usually sticky or has hairs to trap pollen
Most species of flowering plants have flowers with both stamens and pistils
Some have only stamens - Male flowers Some have only pistils - Female flowers