34.0 Introduction

1. Plants Have a Fundamental Unity of Structure fig 34.1
1. Unity in Construction
1. Growth
2. Manufacture and transport of food
3. Regulation of development

2. Fundamental Differences Between Roots and Shoots

34.1 Vascular plants have root and shoot systems

1. Organization of the Plant Body
1. Basic Plant Body Plan fig 34.2
1. Root system
1. Anchors plant and penetrates soil
2. Absorbs water and ions critical to nutrition

2. Shoot system
1. Stems: Serve as framework to position leaves
2. Leaves: Primary location for photosynthesis
3. Flowers: Serve reproductive functions, become fruit and seeds

2. Tissue Types in Plants
1. Meristems: Give rise to all other cells and tissues
2. Ground tissue: Primarily thin-walled parenchyma cells
1. Initial spherical shape, assume other shapes after division and packing of cells
2. Live for many years
3. Function in storage, photosynthesis, secretion

3. Epidermis: Outer protective covering, one cell thick
1. Fatty cutin layer forms the cuticle
2. Some plants add waxy layer outside cuticle

4. Vascular tissue: Conducts materials throughout plant
1. Xylem: Water and dissolved minerals
2. Phloem: Materials needed for growth, carbohydrates, hormones, amino acids

2. Primary and Secondary Growth
1. Evolution of Vascular Plant Primary and Secondary Tissues
1. Meristems give rise to all other plant cells
2. Primary growth
1. Initiated by apical meristems near tips of plant body
2. Increases plant's length

3. Secondary growth
1. Increases plant's girth
2. Involves activity of lateral meristems

4. Earliest vascular plants were not divided into stems, leaves, roots
5. Evolution of secondary growth resulted in treelike growth forms fig 34.3
1. Resulted in development of forests, domination of land plants
2. Evolved independently in several groups of vascular plants

2. Conducting Systems of Early Land Plants
1. Sieve tube members conduct carbohydrates away from area of manufacture
2. Vessel members and tracheids transport water and minerals up from roots
3. Both kinds of cells are elongate and occur in strands
1. Sieve tube members characteristic of phloem tissue
2. Vessels and tracheids characteristic of xylem

4. In primary tissue two types are associated with each other in same vascular strand

3. Monocots and Dicots
1. Two Classes of Phylum Magnoliophyta (or Anthophyta)
1. Magnoliopsida (Dicotyledonae): Dicots
1. Trees and shrubs
2. Snapdragons, mints, peas, sunflowers

2. Liliopsida (Monocotyledonae): Monocots
1. Lilies, grasses, cattails, palms
2. Agaves, yuccas, pondweeds, orchids, irises

2. Comparison of Monocots and Dicots fig 34.4
1. Most obvious features listed in table tbl 34.1
2. Many dicots are annual plants
1. Complete entire growth cycle within one year
2. Very few monocots are annuals

3. Underground storage organs occur more frequently in monocots
4. Many species of woody dicots, no monocots have true wood
1. Palms and bamboos produce extra bundles of conducting tissues
2. Give stems a woody texture

5. Endosperm is largely absent in mature dicot seeds
6. Differentiated from one another early in angiosperm history
1. Dicots are more primitive
2. Monocots likely evolved from early dicots

34.2 Plants have four basic tissues, each exhibiting several cell types

1. Meristems
1. General Features of Meristems
1. Composed of small, unspecialized cells
2. One cell remains in meristem, other becomes part of plant body
3. Plant body cells divide further and begin to differentiate

2. Apical Meristems Produce Primary Growth
1. Apical meristems give rise to three types of embryonic tissues
1. Called primary meristems, develop into primary plant body fig 34.5
2. Protoderm: Differentiates into epidermis
3. Procambium: Differentiates into primary vascular tissues (xylem, phloem)
4. Ground meristem: Differentiates into ground tissue parenchyma cells

2. Some plants have intercalary meristems
1. Example: Horsetails
2. Arise in stem internodes, add to internode length

3. Lateral Meristems Produce Secondary Growth
1. Causes plants to grow in width
2. Secondary tissues arise from secondary growth
1. Occurs in many nonwoody plants
2. Most dramatic effects in woody plants

3. Woody plants have lateral meristems
1. Two cylinders of actively dividing cells
2. Cork cambium: Produces cork cells of outer bark
3. Vascular cambium: Gives rise to secondary xylem and phloem
1. Found just beneath bark
2. Adds secondary vascular tissue on opposite sides

4. Secondary xylem is main component of wood
5. Secondary phloem lies near outer surface of woody stem
1. Removing tree's bark damages phloem
2. May ultimately kill tree

2. Ground Tissue
1. Parenchyma fig 34.7a
1. Have large vacuoles, thin walls, average of 14 sides at maturity
2. Most common type of primary tissue, may occur in secondary tissues
3. Possess only primary cell walls
1. Laid down while cell still maturing
2. Least specialized cell type

4. Have nuclei, are capable of further division
1. Commonly store food and water
2. Are alive at maturity

5. May contain chloroplasts when in leaves and outer parts of stems, called chlorenchyma

2. Collenchyma fig 34.7b
1. Living at maturity, contain living protoplasts
2. Elongated cells with unevenly thickened walls
3. Flexible cells, provide support, bend without breaking
1. Form strands or cylinders beneath epidermis, along leaf veins
2. Example: Strings of celery leaf stalk

3. Sclerenchyma
1. Possess thick, tough secondary walls
2. Usually lack living protoplasts at maturity
3. Secondary walls often impregnated with lignin
1. Adds rigidity to cells
2. Cells are thus lignified
3. Common in cells with supportive or mechanical function

4. Two types of sclerenchyma
1. Fibers
1. Long slender cells that form strands
2. Example: Strands of flax woven to produce linen

2. Sclerids fig 34.7c
1. Varied in shape, frequently branched
2. Example: Gritty texture of pears

3. Epidermis
1. Epidermal Cells Cover All Parts of Primary Plant Body
1. Originate from protoderm
2. Outer wall has cuticle that varies in thickness

2. Epidermis Contains Specialized Cells
1. Guard cells: Paired cells flanking stoma
1. Contain chloroplasts
2. Stoma occur in leaf epidermis, occasionally on stems and fruit fig 34.8
3. Stomal openings allow passage of photosynthetic gases, water vapor
4. More numerous on lower surfaces
5. Stoma open and shut in response to external factors
1. Include light, temperature, availability of water
2. Open during active photosynthesis, allow passage of CO₂ and O₂

2. Trichomes: Hairlike epidermal outgrowths fig 34.9
1. Occur in stems, leaves and reproductive organs
2. Surface appears woolly or fuzzy
3. Help regulate heat and water balance
4. Vary in form
1. Some are single cells, others are several cells
2. Glandular trichomes may secrete sticky or toxic substances

3. Root hairs: Tubular extensions of single cells fig 34.5
1. Found behind tips of growing roots
2. An extension not a separate cell, no crosswall separating it from epidermal cell
3. Provide intimate contact between root and soil particles
4. Greatly increases root's surface area, efficiency of absorption
5. New root hairs produced as old ones slough off
6. Responsible for most absorption in herbaceous plants
7. Numbers reduced in plants with ectomycorrhizae, replaced by fungal filaments

4. Vascular Tissue
1. Xylem
1. Principal water conducting tissue
2. Consists of hollow, dead cells fig 34.10
1. Vessel members are cylindrical, arranged end to end
2. Tracheids taper at ends and overlap each other
3. May be only conducting cells in some plants, like gymnosperms

3. Water conducted in an unbroken stream from roots to leaves
1. At leaves much water passes into film on outside of parenchyma cells
2. Diffuses in form of water vapor into intercellular spaces
3. Exits leaves mainly through stomata
4. Process called transpiration

4. Water in xylem contains various dissolved minerals
5. Also provides support for plant body
1. Primary xylem derived from procambium
2. Secondary xylem derived from vascular cambium
3. Wood is accumulated secondary xylem

6. Vessels found almost exclusively an angiosperms
1. In primitive plants are elongated cells that resemble fibers
2. In advanced plants vessel members are shorter and wider

7. Vessels and tracheids have thick, lignified secondary walls
1. Lack living protoplasts at maturity
2. Not living at maturity

8. In tracheids, water flows through openings, pits, in secondary walls
1. Tracheids have pits in common side walls
2. Separated by porous pit membrane, controls water flow

9. Vessel elements have side wall pits and open or perforated end walls
1. Vessels conduct water more efficiently than tracheids
2. Vessels evolved from tracheids are specialized for conduction
3. Some fibers evolved from tracheids are specialized for support

10. Xylem also includes fibers and parenchyma cells
1. Parenchyma cells produced in horizontal rows called rays
1. Produced by ray initials on vascular cambium
2. Function in lateral conduction and food storage
3. In woody cross sections radiate from center like wheel spokes fig 34.22

2. Fibers are abundant in dense and heavy wood like oak

11. Types and arrangement of xylem cells used to identify many species of plants

2. Phloem
1. Located toward outer part of roots and stems
2. Principle food conducting tissue
1. Girdling a plant removes strip of bark down to vascular cambium
2. Plant dies due to starvation of roots

3. Conducting cells: Sieve cells and sieve-tube members fig 34.11
1. Sieve cells occur in seedless vascular plants and gymnosperms
2. Sieve-tube members found in angiosperms

4. Both types possess clusters of pores called sieve areas
1. More abundant on overlapping ends of cells
2. Connects protoplasts of adjoining sieve cells and sieve-tube members
3. Both types of cell are living, but most lack nucleus at maturity

5. Sieve-tube member pores may be larger, called sieve plates
1. Occur end-to-end, forming longitudinal series called sieve tubes
2. Less specialized than sieve-tube members
3. Pores are all same size
4. Sieve-tube members more advanced, more efficient

6. Sieve-tube members associated with companion cells
1. Specialized parenchyma cells
2. Carry out metabolic functions that maintain sieve-tube members
3. Possess components of normal parenchyma cells, including nuclei
4. Plasmodesmata connect their cytoplasm with conducting cells

7. Fibers and parenchyma cells often abundant

34.3 Roots have four growth zones

1. Root Structure
1. Plants Possess Three Vegetative Organs: Roots, Stems, Leaves
1. Roots have simpler pattern of organization and development than stems
2. Developing roots have four regions fig 34.12
1. Root cap
2. Zone of cell division
3. Zone of elongation
4. Zone of maturation

2. The Root Cap
1. Has no equivalent in stems
2. Thimble-shaped mass of parenchyma cells that covers tip of young root
3. Protects delicate tissues as root extends into soil
1. Golgi bodies secrete slimy substance, passes through walls to outside
2. Cells replaced from inside
3. Form mucilaginous lubricant that eases root through soil
4. Provides medium for growth of nitrogen-fixing bacteria in some plants

4. New root cap produced when old one is artificially or accidentally removed
1. Also functions to perceive gravity until cap is mature
2. Amyloplasts collect on sides of cells facing pull of gravity
3. Amyloplasts change position if potted plant is put on side
4. Calcium ions in amyloplasts may influence distribution of growth hormone

3. The Zone of Cell Division
1. Apical meristem is inverted dome of cells at center of root tip
1. Protected by root cap
2. Activity takes place toward edges of dome
3. Cells divide every 12 to 36 hours

2. Cells are essentially cuboidal with small vacuoles, large central nuclei
3. Subdivides into three primary meristems
1. Protoderm
2. Procambium
3. Ground meristem

4. The Zone of Elongation
1. Cells become several times longer than wide
2. Small vacuoles merge, occupy nearly 90% of cell volume
3. No further increase in ell size above this point
4. Mature parts of root remain stationary, root increases only in girth

5. The Zone of Maturation
1. Elongated cells differentiate into specific cell types
2. Cells at root surface cylinder mature into epidermal cells
1. Have very thin cuticle
2. Many epidermal cells develop a root hair
1. Protuberance not separated by crosswall from epidermal cell
2. Increase root surface area and absorptive capacity
3. Usually alive and functional for only a few days, sloughed off

3. New root hairs produced on new cells toward zone of elongation

3. Parenchyma cells produced by ground meristem immediately interior to epidermis
1. Parenchyma tissue called the cortex
2. May be many cells wide, functions in food storage
3. Inner boundary of cortex differentiates into cylinder of endodermis
4. Primary walls of endodermis impregnated with suberin, fatty waterproof substance
1. Suberin produced in bands called Casparian strips
2. Surround adjacent endodermal cell walls perpendicular to root surface
3. Fused to plasma membranes of endodermal cells
4. Prevent water and dissolved minerals from passing through them
5. Must pass through intercellular spaces or plasmodesmata
6. Has a regulatory effect, can exclude harmful substances from center of root

4. Epidermis, cortex, endodermis may be sloughed off as root expands in girth
5. Waxes, cellulose, lignin added to endodermal cell walls of other roots fig 34.13
6. All tissues interior to the endodermis are collectively called the stele
1. Pericyle is immediately interior and adjacent to endodermis
1. Cells can divide even after maturity
2. May give rise to lateral or branch roots fig 34.14
3. May give rise to vascular cambium in dicots

7. Primary root growth may exhibit a number of patterns
1. Dicot roots have central core of primary xylem, arms radiate toward pericycle
2. Monocot xylem forms ring of vascular bundles surrounding central cylinder of pith
3. Strands of primary phloem alternate between xylem arms

8. In dicots and plants with secondary growth fig 34.15
1. Vascular cambium arises from cells between primary xylem and phloem
2. Division produces cells that become secondary xylem (inward) or secondary phloem (outward)
3. Primary phloem, cortex, epidermis crushed, sloughed off with new secondary tissue
4. In woody plant pericycle, cork cambium produces new layers
1. Cork cells to outside
2. Phelloderm parenchyma to inside
3. Tissue associated with cork cambium is the periderm (outer bark) fig 34.23

2. Modified Roots
1. Plants Produce One of Two Types of Root Systems
1. Taproot system has one large root with several to many branches
2. Fibrous root system has many smaller roots with the same diameter

2. Some Plants Produce Special Root Modifications
1. Aerial roots
1. Epiphytic orchids have roots that extend out into the air
2. Unconnected to the ground, not parasitic
3. Velamen roots
1. Epidermis is several cells thick
2. Velamen reduces water loss
3. May also be green and photosynthetic

4. Prop roots
1. Located on lower part of stem of some monocots like corn
2. Grow down into ground, anchor against wind
3. Climbing ivies produce roots from stems, anchor them to substrate

5. Adventitious roots
1. Any root that arises along stem or location other than base of plant
2. Includes prop and stem anchoring roots

2. Pneumatophores fig 34.16a
1. Found on plants growing in swamps and wet places
2. Are spongy outgrowths from underwater roots
3. May extend above water, facilitate oxygen supply to underwater roots

3. Contractile roots
1. Roots of certain plants with bulbs
2. Contract by spiralling, pull plant deeper into soil each year
3. Roots may contract to a third of their original length

4. Parasitic roots
1. Dodder and other plant stems lack chlorophyll
2. Produce peglike roots called haustoria, penetrate host plants
3. Establish contact with host, parasitize them

5. Food storage roots
1. Xylem of branch roots of plants like sweet potatoes produce extra parenchyma cells
2. Store large quantities of carbohydrates
3. May be combinations of stem and root: Carrot, beet, parsnip, radish, turnip

6. Water storage roots fig 34.16b
1. Produced by certain members of pumpkin family
2. Stored water used when soil water supply is inadequate

7. Buttress roots fig 34.16c
1. Produced by certain varieties of fig and tropical trees
2. Found at base of trunk, provides stability

8. Mycorrhizae

34.4 Stems conduct liquid and support the shoot

1. Stem Structure
1. External Form fig 34.17
1. Woody twig has leaves attached to it in specific arrangements
1. Spiral around stem
2. Pairs opposite each other
3. Whorls of three or more

2. A node is the region of leaf attachment
1. Stem area between nodes is the internode
2. Two basic leaf parts
1. Blade: Flattened portion
2. Petiole: Slender stalk
3. If petiole is missing leaf is sessile

3. Two kinds of leaf buds
1. Axil: Angle formed by leaf between the attachment and the stem
2. Axillary bud is produced in each axil
3. Terminal bud is produced at tip of twig
4. Axillary buds may develop into branches
5. Terminal buds extend length of stem
6. Most buds protected over winter by bud scales
7. Bud scales drop off leaving bud scars

4. Stipules are paired leaf-like organs near base of petiole
1. May fall off and leave stipule scars
2. Deciduous leaves fall off and leave leaf scars with tiny bundle scars

5. Herbaceous stem structure
1. Stems do not make cork cambium, lack periderm
2. Stems are usually green and photosynthetic
3. Outer layers of cortex contain chloroplasts
4. Stems have stomata, various types of trichomes

2. Internal Form
1. Stems possess apical meristem at tip
1. Produces primary tissue to increase stem length
2. Stays dormant until growing season
3. Bud scales fall off reveal leaf and bud primordia fig 34.18
1. Leaf primordia: Embryonic leaves
2. Bud primordia: Found in axils

2. Three primary meristems develop from apical meristem protoderm
1. Epidermis
2. Cylinder of procambial strands, further produce primary xylem and phloem
3. Ground meristem produces parenchyma cells
1. Pith: Parenchyma in center of stem
2. Cortex: Parenchyma cells away from center

3. Trace, strand of xylem and phloem, branches off from main xylem and phloem
1. Enters developing leaf or bud
2. Leaves thumbnail-shaped spaces called leaf or bud gaps

4. Vascular cambium develops between primary xylem and phloem in dicots fig 34.19
1. Produce secondary xylem and secondary phloem
2. Cause increase in girth of stem

5. Woody dicots have cork cambium arising in outer cortex
1. Produces boxlike cork cells to outside
2. May produce parenchymalike phelloderm cells to inside
3. Cork tissues are impregnated with suberin, die and comprise outer bark

6. Gas exchange in young stems occurs through stomata
7. Gas exchange in older stems occurs through lenticels on outer bark fig 34.20

2. Stem Tissue Patterns
1. Composition of Young Stems (and Roots)
1. Central core of tissues called the stele
2. Composed of primary xylem, primary phloem, pith

2. Herbaceous Dicot Stems
1. Vascular bundles arranged in a ring fig 34.21
1. Pith on interior of ring
2. Cortex to exterior of ring

2. Vascular cambium may be confined to vascular bundles or extend between bundles
1. Produces secondary xylem and phloem within bundles
2. Produces parenchyma cells between bundles

3. Woody Dicot Stems
1. Similar appearance to herbaceous stems in early stages
2. Differences appear when vascular and cork cambium produce secondary tissues
3. Most obvious difference associated with secondary xylem, wood
4. Annual rings
1. Growth confined to spring and summer, forms concentric annual rings
1. Active division with larger cells in spring or early wood
2. At end of growing season cells are smaller
3. More tracheids produced than vessel members
4. Ultimately only tracheids (or fibers) produced in summer or late wood

2. Switch between previous summer wood and new spring wood makes band
3. Annual rings are visible in conifers even though only tracheids produced
1. Tracheids in spring are larger
2. Tracheids in summer are smaller

4. More xylem than phloem cells
1. Xylem cells stronger
2. Phloem cells crushed, sloughed off with older bark

5. Most of woody dicots stem is wood
6. Age of tree determined by counting annual rings
7. Can estimate climatic conditions from annual rings
1. Rings thicker in years with plentiful water
2. Can determine presence and year of nonlethal fire
3. Can accurately date pieces of wood
4. Recent thin rings in red pine attributed to acid precipitation, other environmental hazard

5. Wood
1. Lighter lines called rays seen in cross section fig 34.22
1. Tiers of parenchyma cells produced by vascular cambium
2. Live for many years, provide lateral transport of water and minerals
3. Xylem rays are found in xylem
4. Phloem rays extend across phloem

2. Heartwood
1. Located near central region of trunk
2. Denser wood, darker in color
3. Conduction of water blocked, wood is nonfunctional

3. Sapwood
1. Located nearer the vascular cambium
2. Actively involved in transport

4. Proportion of heartwood to sapwood varies widely
5. Outer bark called periderm, composed of cork cambium and derivative tissues
6. Inner bark includes phloem

6. Commercial uses of wood fig 34.24
1. Economically important product of plants
1. Wood for furniture and building materials
2. Pulpwood produces paper
3. Utilize wood from stems not from roots

2. Derivation of commercially used wood
1. Hardwood produced by dicots, many come from tropics
2. Softwood produced by conifers, from north temperate forests

3. Species of wood identified by microscopic characteristics

4. Monocot Stems
1. Lack vascular cambium and cork cambium, are mostly herbaceous
2. Have surface layer of epidermis like herbaceous dicots
3. Internal structure substantially different
1. Vascular bundles scattered through parenchyma of ground tissue
2. Ground tissue not separated into pith and cortex
3. Vascular bundles oriented so xylem points to center, phloem to outside
4. Vascular bundles more numerous toward epidermis than in center

4. Structure of a single vascular bundle
1. Xylem consists of tracheids and two large vessels
2. Phloem consists of sieve tubes and companion cells
3. Bundle surround by sheath of sclerenchyma cells for support

5. Additional meristem found in grasses
1. Monocots lack any cambia
2. Intercalary meristems found in vicinity of nodes
3. Produce tissues to increase length of stem, not girth
4. Stem is same diameter throughout even in tall grasses

6. Parenchyma of palm trees continues to grow after produced

3. Modified Stems fig 34.25
1. Most Stems Grow Erect
1. Stem modifications important in vegetative propagation
2. Modified stems cut into segments, grow into new plants

2. Types of Modified Stems
1. Bulbs
1. Include onions, lilies, tulips
2. Swollen underground stems, really large buds with adventitious roots on base
3. Fleshy leaves attached to small stem

2. Corms
1. Include crocus, gladiolus superficially resemble bulbs
2. Do not have fleshy leaves
3. Mostly underground stem with few nonfunctional leaves and adventitious roots

3. Rhizomes
1. Include perennial grasses, ferns, irises
2. Horizontal underground stems
3. Each node has scalelike leaf with axillary bud
4. Photosynthetic leaves may be produced at tip
5. Adventitious roots produced along entire length

4. Runners and stolons
1. Include strawberry plants
2. Are horizontal stems, long internodes, found above ground
3. Terms are sometimes interchangeable
4. Stolon may describe underground stem associated with white potatoes
5. Potato itself is modified stem, a tuber

5. Tubers
1. Carbohydrates accumulate at tips of stolons, produce tubers
2. Stolons die after tubers are mature fig 34.25a
3. Potato eyes are axillary buds arising in the axil of scalelike leaves
4. Ridge adjacent to eye is a leaf scar

6. Tendrils
1. Include grape, Boston ivy fig 34.25b
2. Pea and pumpkin tendrils are modified leaves

7. Cladophylls
1. Produced by cacti
2. Flattened photosynthetic stems
3. Leaves are modified as spines

34.5 Leaves are organized to transport the products of photosynthesis

1. Leaf External Structure
1. General Features of Leaves
1. Initiated as leaf primordia from apical meristems fig 34.5,18
2. Principal sites of terrestrial photosynthesis
3. Grow via cell division and enlargement within blade
1. Mesophyll established early in development
2. Cell division and enlargement ceases when leaf is fully expanded

4. Features differ greatly in physical appearance

2. External Leaf Anatomy
1. Blade: Flattened portion
2. Petiole: Slender stalk
3. Stipules: Paired organs near base of petiole
1. May be leaflike, modified as spines or glands
2. Vary in size
3. Usually absent in grasses and monocots

4. Veins: Xylem and phloem strands run throughout leaf fig 34.26
1. Parallel in monocots
2. Netted in dicots

5. All leaves have axillary bud in axil between leaf and stem
6. Simple leaves are undivided, may be deeply lobed
7. Compound leaves consist of distinctly separate leaflets
1. Leaflets do not have axillary bud at base
2. Axis of leaflet called rachis (equivalent to midrib of simple leaf)
3. Pinnately compound leaves are arranged in pairs on common axis
4. Palmately compound leaves radiate from common point fig 34.27,28

8. Veins of leaf blades have similar arrangement
1. Pinnately veined
2. Palmately veined

9. Leaves have unique arrangements along length of stem fig 34.29
1. Alternate leaves usually spiral around stem
2. Opposite leaves occur in pairs on opposite sides of stem
3. Whorls are a circle of leaves at same level of a node

10. Microphyll versus megaphyll leaves
1. Microphyll has one vein that does not leave a leaf gap, mostly small in size
2. Megaphylls have several veins, leave gap where it branches from stem

2. Leaf Internal Structure
1. Leaf Exterior
1. Covered by transparent epidermis, generally lack chloroplasts
2. Has waxy cuticle, may have various glands and trichomes
3. Lower epidermis contains numerous stomata fig 34.30
1. Flanked by guard cells
2. Function in gas exchange, water regulation

2. Structure and Organization of the Leaf Interior
1. Mesophyll: Masses of parenchyma through which veins run fig 34.31
2. Dicot leaves have two distinct types of mesophyll
1. Palisade parenchyma: Columnar parenchyma near upper epidermis
1. One to several rows of barrel-shaped or cylindrical cells
2. Chlorenchyma cells, contain chloroplasts
3. Hanging leaves have palisade parenchyma on both sides of leaf

2. Spongy parenchyma: Parenchyma cells within leaf interior
1. Loosely arranged cells with many air spaces
2. Intercellular spaces are connected to stomata
3. Function in exchange of gas and water vapor

3. Monocot mesophyll not differentiated, little distinction of upper and lower surfaces

3. Leaf Abscission
1. Plants Lose Leaves at Different Times
1. In temperate regions produce leaves in spring, loose them in fall
2. In tropics leaf production associated with wet and dry seasons
3. Evergreen plants lose leaves continuously, turnover in one to seven years
4. Loss of leaves called abscission fig 34.32

2. Process of Abscission
1. Changes take place at abscission zone at base of leaf
1. Hormones produced by young leaves prevent their loss
2. Inhibit production of specialized cells in abscission layer

2. Older leaves exhibit hormonal changes produces two layers of cells
1. Protective layer develops on side of petiole base
2. Cells are impregnated with suberin, impervious to moisture
3. Separation layer develops on leaf side
4. Environment triggers enzymes that break down middle lamellae in separation cells
5. Leaf separated from stem by wind, rain
6. Seal leaf scar remains

3. Leaf chlorophyll pigments break down as abscission zone forms
1. Reveals other pigments, like carotenoids, previously hidden
2. Water soluble pigments accumulate in vacuoles of leaf cells
3. Anthocyanins and betacyanins are red and blue pigments
4. Contribute to array of colors in fall leaves

4. Modified Leaves
1. Plants Live in a Wide Variety of Environments
1. Necessitate leaf modifications to adapt to specific habitats
2. Have evolved remarkable modifications

2. Types of Modified Leaves
1. Floral leaves (bracts)
1. Poinsettias and dogwood have inconspicuous yellow-green flowers
2. Large, modified leaves or bracts are mistaken for flowers
3. Perform same function as showy petals fig 34.34a
4. Bracts can also be small and inconspicuous

2. Spines
1. Modified leaves on cactus, barberries fig 34.34b
2. Reduction of leaf surface reduces water loss, deter predators
3. Thorns of honey locust are modified stems
4. Prickles of raspberries and roses are epidermal outgrowths

3. Reproductive leaves
1. Kalanch"e produce tiny, complete plantlets at margins of leaves
2. Plantlet will grow into complete plant
3. Walking fern produces new plantlets at tips of fronds

4. Window leaves
1. Plants growing in arid regions produce succulent conical leaves with transparent tips
2. Leaves often are buried in sand, tips admit light into hollow interiors
3. Photosynthesis occurs beneath surface of ground

5. Shade leaves
1. leaves in the shade are larger in surface area
2. Are also thinner, with less mesophyll than normal light leaves

6. Insectivorous leaves
1. Trap insects and digest soft parts fig 34.34c
2. Plants often grow in bogs deficient in elements, provided by insects
3. Pitcher plants have cone-shaped leaves, accumulates rainwater
4. Sundews have sticky glands that entrap insects
5. Venus flytrap has trigger hairs that close leaf on insect