plant tissues. plant tissues & organs cells of a vascular plant are organized into different...

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Plant Tissues

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Plant Tissues

Plant Tissues & Organs

• Cells of a vascular plant are organized into different tissues and organs

• Three major organs are: roots, stems, and leaves

Dermal tissue

Vascular tissue

Ground tissue

Stem

Root

Leaf

Plant Organs

• Roots – absorb water and nutrients from soil, anchor plant, hold plant upright

• Stems – supports plant, transports water and nutrients

• Leaves – carry out photosynthesis, have adjustable pores to help conserve water, allow O2 to leave and CO2 to enter

Roots

• Two main types – taproots (found in dicots) and fibrous roots (found in monocots– Taproots – long primary root like that of

carrots or trees– Fibrous roots – branching root system with no

single root growing larger than the rest (many plants, such as grasses)

Root Function

• Anchors plant

• Absorbs water and nutrients– Active transport proteins in the root pump

minerals and nutrients into the plant (moves from low to high concentration and requires ATP/energy to do so)

Stems

• Vary greatly in size and shape

• Three important functions:– Produce leaves, branches, and flowers– Hold leaves up in sunlight– Transport substances between roots and

leaves

Growth in Stems

• Primary growth of stems– For the life of the plant, new cells are produced at the tips of the

roots– Plant gets taller

• Secondary growth of stems– Stem increases in thickness/width– Most evident in woody plants– The vascular cambium is a cylinder of actively dividing cells

between the primary xylem and primary phloem.– Secondary growth adds cells on either side of the vascular

cambium.

Growth in Stems

KeyDermal tissue system

Vascular tissue system

Ground tissue system

Year 1Early Spring

Growth

Primaryxylem

Vascularcambium

Cortex

Epidermis

Primaryphloem

GrowthGrowth

Late Summer

Shedepidermis

Secondaryxylem(wood)

Secondaryphloem

Corkcambium

Cork

Bark

Year 2Late Summer

Secondary xylem(2 years’ growth)

Formation of Wood in Trees

• Actually layers of xylem that build up year after year

• Tree growth is seasonal– In spring, light-colored xylem cells with thin

cell walls form– In fall/winter, xylem cells becomes dark and

have thicker cell walls– This light and dark pattern is called tree rings.

Each ring is composed of a band of light wood and dark wood

Formation of Wood in Trees (continued)

– One ring = one year of growth– Counting rings = approximate age of tree– Thick rings = favorable conditions/good growing

season– Thin rings = less favorable conditions/bad growing

season

Layers of a Tree Trunk

Annual rings

Heartwood

Sapwood

Vascular cambium

Secondary phloemCork cambiumCork

Bark

Leaves

• Main organ of photosynthesis

• Leaves vary in shape, but all collect sunlight on flattened sections called blades

• Epidermis of leaves covered by a waxy cuticle to prevent water loss

Leaf Functions

• Photosynthesis – bulk of leaf tissue is called mesophyll, where many chloroplasts are

• Gas exchange – leaves take in CO2 and give off O2 during photosynthesis

Leaf Functions (continued)

– Leaves have stomata – pore-like openings on the underside of the leaf that allow gas exchange to occur

– Stomata are open just enough to allow photosynthesis to occur, but not so much that they lose excess amounts of water

– Guard cells surrounding the stomata control the opening/closing of the stomata and thus regulate the movement of gases into and out of the leaf (also regulates water loss)

– In general, stomata open during the day and close at night

Function of Guard Cells

Stoma Open Stoma Closed

Guard cells

Inner cell wall

Stoma

Guard cellsInner cell wall

Function of Guard Cells

Stoma Open Stoma Closed

Guard cells

Inner cell wall

Stoma

Guard cellsInner cell wall

The Internal Structure of a Leaf

Veins

Xylem

PhloemVein

Cuticle

Epidermis

Palisademesophyll

Epidermis

Stoma

Guardcells

Spongymesophyll

Tissue Types

• Three tissue types:– Dermal– Vascular– Ground

Dermal tissue

Vascular tissue

Ground tissue

Stem

Root

Leaf

Dermal Tissue

• Dermal Tissue – outermost layer of cells covered with a waxy cuticle to protect against water loss

Vascular tissue

• Vascular tissue – transports water and nutrients through plant by xylem and phloem– Xylem – conducts WATER throughout the

plant– Phloem – conducts NUTRIENTS throughout

the plant

Ground Tissue

• Ground Tissue – cells that lie between dermal and vascular tissues; site of photosynthesis

The Working Plant: The Transport of Water

– To thrive, a plant must be able to transport• water and • dissolved ions from its roots to the rest of the plant.• Xylem is used for this purpose.

– Transpiration, the loss of water vapor from the leaves of a plant by evaporation,

• mostly occurs through the stomata of leaves and• pulls liquid through the xylem up the plant against

gravity.

The Working Plant: The Transport of Water

– Transpiration relies on two special properties of water.1. Adhesion is the sticking together of molecules of

different kinds.

2. Cohesion is the sticking together of molecules of the same kind

– Together, adhesion and cohesion create a continuous string of water molecules that stick• to each other and

• to the inside walls of the xylem tubes.

Xylem sap

Soil particle

Root hair

Water

Water uptake from soil

Root

Flo

w o

f w

ate

r

Cohesion,by hydrogenbonding

XylemcellsCohesion and

adhesion in the xylem

Cell wallWatermolecule

AdhesionStem

Transpiration

Leaf

Outside air

Water molecule

Stoma

Air space within leaf

Mesophyll cells

Xylem sap

1

2

3

4

The Working Plant: The Transport of Water

– Transpiration• helps to distribute water within a plant but• can cause plants to lose large amounts of water.

– Plants adjust their transpiration rates to changing environmental conditions.

The Working Plant: The Transport of Sugars

– Phloem moves sugar in various directions,• from a sugar source, where sugar is produced,• to a sugar sink, where sugar is stored or

consumed.

– Phloem moves sugar from a sugar source to a sugar sink by active transport as well as help from osmosis

The Working Plant: Response to Stimuli

– Plants can respond to physical stimuli from the environment, including

• light,• touch, and • gravity.

The Working Plant: Response to Stimuli

– Tropisms are directed growth responses that cause parts of a plant to grow

• toward or• away from a stimulus.

The Working Plant: Response to Stimuli

• Phototropism is the directional growth of a plant shoot in response to light.

• Thigmotropism• is a response to touch and• occurs when a pea plant tendril coils around a string or wire it

touches for support.• Gravitropism

• is the directional growth of a plant organ in response to gravity and occurs when

– shoots grow upward and– roots grow downward.

The Working Plant: Response to Stimuli

TROPISMS

Phototropism Thigmotropism Gravitropism

Seedlings bending towardthe light

Growth in response to touch Seedlings reacting togravity

The Working Plant: Response to Stimuli

– Light• provides energy for photosynthesis,

• directs growth, and

• regulates a plant’s life cycle, including– flowering,– seed germination, and– the onset and ending of dormancy.

– A photoperiod • is the relative lengths of day and night and

• the environmental stimulus that plants most often use to detect the time of year.