The Organization of the Plant Body: Cells, Tissues, and

MeristemsChapter 5

Organization of Plant Body

Most vascular plants consist of:

Shoot System

Above ground part

Stems, leaves, buds, flowers, fruit

Root SystemBelow ground part

Main roots and branches

Plant Cells and Tissues

• Cell wall – surrounds each plant cell

• Pectin – glues plant cells together

• Meristems– Groups of specialized dividing cells– Sources of cells and tissues– Not tissues themselves

• Plant organs – leaves, stems,roots, flower parts

Main Tissues of Plants

Ground tissue system

Most extensive in leaves (mesophyll) and young green stems (pith and cortex)

Vascular tissue system

Conducting tissues•Xylem – distributes water and solutes•Phloem – distributes sugars

Dermal tissue system

Covers and protects plant surfaces – epidermis and periderm

Plant Tissues

• Simple tissues– Composed of mostly one cell type– Workhorse cells of plant body– Functions

• Conduct photosynthesis• Load materials into and out of vascular system• Hold plant upright• Store things• Help keep plant healthy and functioning

Simple Plant Tissues

Tissue type Cell types

Parenchyma tissue Parenchyma cells

Collenchyma tissue Collenchyma cells

Sclerenchyma tissue Fibers, sclereids

Parenchyma

• Usually spherical or elongated

• Thin primary cell wall

• Perform basic metabolic functions of cells– Respiration – Photosynthesis– Storage– Secretion

Parenchyma

• Usually live 1-2 years• Crystals of calcium oxalate commonly

found in vacuoles– May help regulate pH of cells

• May aggregate to form parenchyma tissue in– Cortex and pith of stems– Cortex of roots– Mesophyll of leaves

Parenchyma

• Mature cells may be developmentally programmed to form different cell types– Wound healing– Transfer cells

• Have numerous cell wall ingrowths• Improve transport of water and minerals over short

distances• At ends of vascular cells help load and unload

sugars and other substances

Collenchyma

• Specialized to support young stems and leaf petioles

• Often outermost cells of cortex

• Elongated cells

• Often contain chloroplasts

• Living at maturity

Collenchyma

• Walls composed of alternating layers of pectin and cellulose

• Can occur as aggregates forming collenchyma tissue– Form cylinder surrounding stem– Form strands

• Make up ridges of celery stalk

Sclerenchyma

• Rigid cell walls

• Function to support weight of plant organs

• Two types of cells– Fibers– Sclereids

• Both fibers and sclereids have thick, lignified secondary cell walls

• Both fibers and sclereids are dead at maturity

Sclerenchyma

• Fibers– Long, narrow cells with thick, pitted cell walls

and tapered ends– Sometimes elastic (can snap back to original

length)

Sclerenchyma

• Fibers– Arrangements

• Aggregates that form continuous cylinder around stems

• May connect end to end forming multicellular strands

• May appear as individual cells or small groups of cells in vascular tissues

Sclerenchyma

• Sclereids– Many different shapes– Usually occur in small clusters or solitary cells– Cell walls often thicker than walls of fibers– Sometimes occur as sheets

• Hard outer layer of some seed coats

Complex Tissues

Composed of groups of different cell types

Complex tissue Cell types

XylemVessel member, tracheid, fiber, parenchyma cell

PhloemSieve-tube member, sieve cell, companion cell, albuminous cell, fiber, sclereid, parenchyma cell

EpidermisGuard cell, epidermal cell, subsidiary cell, trichome (hair)

Periderm Phellem (cork) cell, phelloderm cell

Secretory structures Trichome, laticifer

The Vascular System

Xylem

• Complex tissue

• Transports water and dissolved minerals

• Locations of primary xylem– In vascular bundles of leaves and young

stems– At or near center of young root (vascular

cylinder)

Xylem Cell Types

Cell Type Description

Trachery element (tracheids and vessel members)

•Water conducting cells

•Not living at maturity

•Before cell dies, cell wall becomes thickened with cellulose and lignin

Fibers

•Strength and support

Parenchyma cells•Help load minerals in and out of vessel members and tracheids

•Only living cells found in xylem

Xylem

• Secondary xylem– Forms later in development of stems and roots

• Water exchanged between cells through tiny openings called pits– Simple pits

• Occur in secondary walls of fibers and lignified parenchyma cells

– Bordered pits• Occur in tracheids, vessel members, and some fibers

Phloem

• Complex tissue

• Transports sugar through plant

• Primary phloem– In vascular bundles near primary xylem in

young stems– In vascular cylinder in roots

Phloem

• Cell types in angiosperm phloem– Sieve-tube members– Companion cells– Parenchyma– Fibers and/or sclereids

Phloem

• Sieve-tube members– Conducting elements of phloem– Join end-to-end to form long sieve tubes– Mature cell contains mass of dense material

called P-protein• May help move materials through sieve tubes

– Usually live and function from 1 to 3 years

Phloem

• Sieve-tube members– mature sieve-tube members have aggregates

of small pores called sieve areas• One or more sieve areas on end wall of sieve-tube

member called a sieve-plate• Callose (carbohydrate) surrounds margins of pores

– Forms rapidly in response to aging, wounding, other stresses

– May limit loss of cell sap from injured cells

Phloem

• Companion cells– Connected by plasmodesmata to mature

sieve-tube member– Contain nucleus and organelles– Thought to regulate metabolism of adjacent

sieve-tube member– Play role in mechanism of loading and

unloading phloem

Phloem

• Parenchyma– Usually living– Function in loading and unloading phloem

Phloem

• Fibers and/or sclereids– Long tapered cells– Lignified cell walls

Phloem

Gymnosperms and ferns• Sieve cells instead of sieve-tube members• Conducting elements in phloem• Long cells with tapered ends• Sieve areas but no sieve plates• Usually lack nuclei at maturity• Albuminous cells

– Adjacent to sieve cells– Short, living cells– Act as companion cells to sieve cells

The Outer Covering of the Plant

Epidermis

• Outer covering• Usually one cell layer thick

– Epidermis of succulents may be 5-6 cell layers thick

• Functions– Protects inner tissues from drying and from

infection by some pathogens– Regulates movement of water and gases out

of and into plant

Epidermis

• Cell types– Epidermal cells– Guard cells– Trichomes (hairs)

Epidermis

• Epidermal cells– Main cell type making up epidermis– Living, lack chloroplasts– Somewhat elongated shape– Cell walls with irregular contours– Outer wall coated with cutin to form cuticle

• Cuticle found on all plant parts except tip of shoot apex and root cap

• Cuticle often very thin in roots

Epidermis

• Guard cells– Found in epidermis of young stems, leaves,

flower parts, and some roots– Specialized epidermal cells– Small opening or pore between each pair of

guard cells• Allows gases to enter and leave underlying tissue

– 2 guard cells + pore = 1 stoma (plural, stomata)

Epidermis

• Guard cells– Differ from epidermal cells

• Crescent shaped• Contain chloroplasts

Epidermis

• Subsidiary cell– Forms in close association with guard cells– Functions in stomatal opening and closing

Epidermis

• Trichomes– Epidermal outgrowths– Single cell or multicellular

• Example: root hairs• Increase root surface area in contact with soil

water

Periderm

• Protective layer that forms in older stems and roots

• Secondary tissue

• Several cell layers deep

Periderm

• Composed of– Phellem (cork)

• On outside• Cells dead at maturity• Suberin embedded in cell walls

– Phellogen (cork cambium)• Layer of dividing cells

– Phelloderm• Toward inside• Parenchyma-like cells• Cells live longer than phellem cells

Periderm

• Secretory structures– Primarily occur in leaves and stems– May be single-celled or complex multicellular

structure– Examples

• Trichomes– Could secrete materials out of plant to attract insect

pollinators

• Laticifers– Secrete latex which discourages herbivores from eating

plant

Meristems

Meristems

• Special region in plant body where new cells form

• Area where growth and differentiation are initiated– Growth

• Irreversible increase in size that results from cell division and enlargement

– Cell differentiation• Structural and biochemical changes a cell undergoes in

order to perform a specialized function

Meristems

• Categories of meristems– Shoot and apical meristems

• Ultimate source of all cells in a plant

– Primary meristems• Originate in apical meristems• Differentiate into primary tissues

– Secondary meristems• Produce secondary tissues

Root and Apical Meristems

• RAM – root apical meristem

• SAM – shoot apical meristem

• New cells produced by cell division

• Theoretically could divide forever– Does not occur

• Scarcity of nutrients• Branch of plant can only carry so much weight• Genetic regulation of growth

Primary Meristems

• Functions– Form primary tissues– Elongate root and shoot

Primary Meristems

• Types of primary meristems– Protoderm

• Cells differentiate into epidermis

– Procambium• Cells differentiate into primary xylem and primary

phloem

– Ground meristem• Differentiates into cells of pith and cortex of stems

and roots• Differentiates into mesophyll of leaves

Secondary Meristems

• Functions– Cell division– Initiation of cell differentiation – Lateral growth

• Increases thickness and circumference of stems and roots

Secondary Meristems

• Not found in all plants– Lacking in plants that grow only one season– Leaves usually lack secondary growth

• Types of secondary meristems– Vascular cambium

• Differentiates into secondary xylem and secondary phloem

– Cork cambium• Differentiates into periderm

Additional Meristems

• Intercalary meristems– In stems– Regulates stem elongation

• Leaf specific meristems– Regulates leaf shapes

• Repair of wounds

• Formation of buds and roots in unusual places