Roots, Stems and Leaves

Chapter 23: Biology II

Structure of Seed Plants

• Roots

• Stems

• Leaves

Roots

• Branching networks in the soil

• Absorb water

• Dissolve nutrients from moist

soil

• Anchor plants in the ground

• Hold plants upright

Stems

• Supports the plant body

• Transports nutrients within the

plant

• Tall stems hold leaves up to the

sun, up to 100 m tall

• Vascular tissues within supply the

plant with water and nutrients

Leaves

• Principal organs for

photosynthesis

• Broad, flat surface

increases SA for sunlight

absorption

• Adjustable pores conserve

water and allow for the

exchange of gases

Tissue Systems • Meristematic

– The only plant tissue that produces new cells by mitosis

– All the cells of a plant originate in meristems and look alike

– Divides into the 3 main tissues

• Dermal

– “skin”

• Vascular

– “bloodstream”

• Ground

– “everything else”

Meristematic Tissue

• Found in several places in a plant

• Apical Meristem: found at the tip of each growing stem and root

Dermal Tissue

• The outer covering of a plant

• Epidermal Cells in non-woody plants

• Cuticle: thick, waxy covering that protects against water loss and injury

• Trichomes: tiny cellular projections which help protect the leaf and give it a fuzzy appearance

• Root Hair Cells

• Guard Cells: underside of leaves

Vascular Tissue

• Transports water and nutrients throughout

the plant

• Xylem: water-conducting tissue

• Phloem: food-conducting tissue

Xylem: Tracheids

• Found in seedless vascular plants and gymnosperms

• Long, narrow cells with walls impermeable to water

• Have openings that connect neighboring cells to one another

• Mature and die; their cytoplasm disintegrates

Xylem: Vessel Elements • Found in angiosperms

• Wider than tracheids

• Arranged end to end, on top of each other, like a stack of tin cans

• When the cell dies the cell walls at the end die, making one long tube

Phloem: Sieve Tube Elements

• Cells arranged end to end to form sieve tubes

• The end walls have many holes in them

• As the mature, they lose most nuclei and organelles; the space becomes a pipeline for sugar and other foods to be carried

Phloem: Companion Cells

• Surround sieve tube elements

• Keep their nuclei and other

organelles throughout their lifetime

• Support the phloem cells

• Aid in the movement of substances

in and out of the phloem stream

Ground Tissue

• The cells between dermal and vascular tissues

Ground Tissue: Parenchyma

• Thin cell wall

• Large central vacuole

• Site of the most of a

plant’s photosynthesis

Ground Tissue: Collenchyma

• Strong, flexible cell wall

• Helps support larger

plants

• Example: the “strings” of

a stalk of celery

Ground Tissue: Sclerenchyma

• Extremely thick, rigid

cell wall

• Makes ground tissue

tough and strong

Types of Roots: Taproots

• Found mainly in dicots

• Primary root grows long and thick

• Secondary roots remain small

• Long taproots: Oak trees, hickory trees,

• Short taproots: carrots, dandelions, beets, etc.

Types of Roots: Fibrous Roots

• Found mainly in monocots

• Branch to such an extent that

no single root grows larger

than the rest

• Helps prevent topsoil from

being washed away by heavy

rain

Root Structure and Growth

Root • Root Hairs: Tiny projections from the

outer surface, or epidermis, of a root

• Cortex: Spongy layer of ground tissue just

inside the epidermis of a root

• Endodermis: Layer of cells that

completely encloses vascular cambium

• Vascular Cylinder/Cambium: Central

region of a root that includes the vascular

tissue-xylem and phloem

• Root Cap: Tough structure that protects a

root as it forces its way through the soil

Root: Functions

• Uptake of plant nutrients

• Active transport of minerals: requires ATP!

• Movement into the vascular cylinder: Casparian strip: waterproof strip that surrounds plant endodermis cells

• Root pressure

Stem Structure and Function

• Produce leaves, branches and flowers

• Hold leaves up in the sunlight

• Transport substances between roots and leaves

Stem Structure and Function

• Node: point on a stem where a leaf is attached

• Internode: region between nodes on plant stems

• Bud: plant structure containing undeveloped tissue that can produce new stems and leaves

Monocot and Dicot Stems

• Monocot Stems: vascular bundles are scattered throughout the

stem

• Dicot Stems (and most gymnosperms): vascular bundles are

arranged in

a cylinder

• Pith: parenchyma

cells inside the ring

of vascular tissue in

dicot stems

Growth of Stems

• Primary Growth: Type of plant growth that occurs at the

tips of roots and shoots

– Cell divisions in the apical meristem

– Takes place in all seed plants

• Secondary Growth: Pattern of plant growth in which stems

increase in width

– Enables the plant to support more weight and more fluid through

the vascular tissues

Formation of the Vascular Cambium

• New layers of vascular tissue are created each year after secondary growth begins

• New meristematic tissue forms between the xylem an phloem of each vascular bundle

• Divisions in the vascular cambium give rise to new layers of xylem and phloem

Formation of Wood

• Most of what we call “wood” is actually layers of xylem

• Heartwood: older xylem near the center of a woody stem that no longer conducts water; darkens with age

• Sapwood: area in plants that surrounds heartwood and is active in fluid transport; lighter in color

• Heartwood and sapwood make up tree rings

Formation of Bark

• Tree structure that includes all tissues outside the vascular cambium, including phloem, the cork cambium, and cork

Leaf Structure and Functions

• Optimized for absorbing light and carrying out photosynthesis

• Blade: thin, flattened section of a plant leaf that collects sunlight

• Petiole: thin stalk by which a leaf blade is attached to a stem

Photosynthesis

Transpiration

Gas Exchange

Water Transport in Plants

• Root pressure

• Capillary Action

• Transpiration

• Together provide enough force to move water

through the xylem tissue of even the largest plant!

Capillary Action

• Tendency of water to rise in

a thin tube

• Adhesion: attraction

between unlike molecules

Transpiration

• The evaporation of water from leaves

• Controlled by the guard cells

• The biggest contributor to moving water in the plant

Nutrient Transport

• Water is pulled upward in plants; nutrients are pushed through phloem

• Phloem: In cold climates, sugar is stored in the roots in the winter and in the trunk and branches in the spring…how does it move?

Movement from Source to Sink

• Pressure-Flow Hypothesis: When nutrients are pumped into or removed from the phloem system, the change in concentration causes a movement of fluid in that same direction

• As a result, phloem is able to move nutrients in either direction to meet the nutritional needs of the plant