KEY CONCEPT Plants have specialized cells and tissue systems

21.1 Plant Cells and Tissues

KEY CONCEPT Plants have specialized cells and tissue systems.


Plant tissues are made of three basic cell types.

• Parenchyma cells are the most common plant cell type. – store starch, oils and

water– help heal wounds to

the plant – have thin flexible walls


– they are strong and flexible.– celery strings are strands of collenchyma.– they have unevenly thick cell walls.

• Collenchyma cells provide support to a growing plant.


– second cell wall hardened by lignin– die when they reach maturity – used by humans to make linen and rope

• Sclerenchyma cells are the strongest plant cell type.


Plant organs are made of three tissue systems.

• Dermal tissue covers the outside of a plant. – protects the plant – secretes cuticle of leaves – forms outer bark of trees


– provides support – stores materials in roots and stems – most commonly made of parenchyma

• Ground tissue is found inside a plant.


stem

leaf

root

– two networks of hollow tubes

– xylem transports water and minerals

– phloem transports photosynthetic products

• Vascular tissue transports water, minerals and organic compounds.

21.1 Plant Cells and Tissues KEY CONCEPT 20-2 The vascular system allows for the transport of water, minerals, and sugars.

21.1 Plant Cells and Tissues Water and dissolved minerals move through xylem.

• Xylem contains specialized cells. – vessel elements are short and wide– tracheid cells are long and narrow – xylem cells die at maturity

vesselelement

tracheid


– Plants passively transport water through the xylem.– Cohesion is the tendency of water molecules to bond

with each other.

• The cohesion-tension theory explains water movement.

– Adhesion is the tendency of water molecules to bond with other substances.


– absorption occurs at roots

• Water travels from roots to the top of trees.

– cohesion and adhesion in xylem– transpiration at leaves


– water vapor exits leaf stomata– helps pull water to the top

branches

• Transpiration is the loss of water vapor through leaves.

21.1 Plant Cells and Tissues Phloem carries sugars from photosynthesis throughout the plant.

• Phloem contains specialized cells. – sieve tube elements have

holes at ends– companion cells help sieve

tube elements – unlike xylem, phloem tissue is

alive


– plants actively transport sugar from the source– sugar flows to the sink due to pressure differences

sugars

phloem xylem

water

Sugars move from their source, such as photosynthesizing leaves, into the phloem.

1

The sugars move into the sink, such as root or fruit, where the are stored.

3

Water moves from the xylem into the phloem by osmosis, due to the higher concentration of the sugars in the phloem. The water flow helps move sugars through the phloem.

2

• The Pressure-flow model explains sugar movement.

21.1 Plant Cells and Tissues KEY CONCEPT 21.3 Roots and stems form the support system of vascular plants.

21.1 Plant Cells and Tissues Roots anchor plants and absorb mineral nutrients from soil.

• Roots provide many functions.– support the plant– absorb, transport,

and store nutrients – root hairs help

absorption


root cap

– root cap covers the tip

• There are several parts of a root.


apical meristem



– apical meristem is an area of growth



– vascular cylinder contains xylem and phloem

vascular cylinder



– apical meristem is an area of growth



– Fibrous root systems have fine branches.

Fibrous root Taproot

• There are two main types of roots.

– Taproot systems have one main root.

21.1 Plant Cells and Tissues Stems support plants, transport materials, and provide storage.

• Stems have many functions. – support leaves and flowers – house most of the vascular system – store water

Baobab trees Cactus


– grow underground for storage

Ginger rhizomes Potato tubers

Stems support plants, transport materials, and provide storage.



– grow underground for storage

Stems support plants, transport materials, and provide storage.


– form new plants

Strawberry stolons

21.1 Plant Cells and Tissues • Some stems are herbaceous and conduct photosynthesis.

21.1 Plant Cells and Tissues • Some stems can be woody, and form protective bark.


• Secondary growth increases a plant’s width.

• Primary growth increases a plant’s length.


• Tree rings help determine the age of a tree.

heartwood

bands bark

one year of growth

sapwood


KEY CONCEPT Leaves absorb light and carry out photosynthesis.


Most leaves share some similar structures.

• The blade is usually broad and flat. – collects sunlight for

photosynthesis– connects to the stem by a

petiole

blade

petiole


• Mesophyll is between the leaf’s dermal tissue layers.

cuticle

upperepidermis

palisademesophyll

spongymesophyll

lowerepidermis


– Stomata open and close when guard cells change shape.

– When stomata are open, water evaporates and gas exchanges.

– Stomata close at night and when plant loses too much water.

guard cells stoma

• Guard cells surround each stoma.


• Leaves may be simple, compound, or double compound.

Simple leaf Compound leaf Double compound leaf


• Leaf veins may be parallel or pinnate.

Pinnate veinsParallel veins


• Leaf margins may be toothed, entire, or lobed.

Toothed margin Entire margin Lobed margin


Most leaves are specialized systems for photosynthesis.

• There are two types of mesophyll cells. – both types contain chloroplasts – palisade mesophyll absorbs sunlight – spongy mesophyll connects to stomata

xylem

phloem

cuticle

upperepidermis

palisademesophyll

spongymesophyll

stomata

lowerepidermis


– for extreme temperatures, ex: pine needles

• Leaves have many adaptations.




– for water loss,ex: cactus spines


– for aquatic environments, ex: water lily





– for aquatic environments, ex: water lily

– for getting food,ex: Venus’ flytrap




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KEY CONCEPT Plants have specialized cells and tissue systems