Plant Structure and Growth

Topic 9.1

To review

• Study the chart on pg 239 for your different types of plants

• The seeded vascular plants include the angiosperms and gymnosperms

• Do you remember the difference between angiosperms and gymnosperms???

• In this unit we will be referring to angiosperms unless otherwise noted.

3 tissue systems in plants

• Dermal

• Vascular

• Ground

Dermal tissue

• Outer protective covering

• Single cell layer

• Root hairs are extensions of epidermal cells

• Most dermal tissue is covered by waxy cuticle which prevents water loss

Vascular tissue

• Includes xylem and phloem

xylem

– xylem is dead at maturity, only secondary cell wall remains

– Consist of 2 cell types, either vessel elements (wide/short) or tracheids (long, thin, tapered).

– carries water and minerals– Pits between cells allow water to flow through

phloem

• Carries sucrose through cells called sieve-tube members.

• Alive at maturity but lack nuclei, ribosomes and vacuoles.

• Companion cells lie adjacent and provide for sieve tube members via plasmodesmata

Ground tissue (3rd type!)

• Photosynthesis, support, storage

• “filler” tissue

• In dicots, center of stem is GT referred to as pith, outer stem GT is cortex.

Ground tissue made of 3 cell types

• Parenchyma – unspecialized cells. Lack secondary walls, have large central vacuole. Important in psyn and food storage. All plant cells begin as unspecialized parenchyma cells.

• Collenchyma – lack secondary walls but have thick primary walls. Form strands which support plant parts.

• Sclerenchyma – thick secondary walls w/ lignin (strength). Include fibers and sclerids

• All 3 types of tissue originate from meristematic tissue.

• Meristematic tissue retains the ability to divide.• If a meristematic cell divides one cell begins

differentiation (the derivative) and the other cell remains meristematic (the initials)

• Where would you expect to find meristematic cells in a plant?

Plant organs

Roots

• Obtain water and minerals• Anchor plant• May store food• Have a protective epidermis• Cortex conducts water from soil to interior

vascular tissue. May also store material• Endodermis surrounds vascular tissue• Vascular tissue

• Shoot system – stem and leaves. Absorb light and carbon dioxide.

Shoot system

• Stem – alternating nodes (point of leaf attachment) w/ internodes in between.

• Axillary bud – at angle between leaf and stem

• Terminal bud – developing leaves and compacted nodes and internodes at tip of plant

• Apical dominance – when terminal bud inhibits growth of axillary bud.

• Vascular tissue arranged in rings in dicots and scattered in monocots.

Leaves

• Blade – flat, photosynthetic

• Petiole – stalk of leaf

monocots – leaves lack petiole, veins are parallel

dicots – leaves have netted venation, petiole.

Leaf Anatomy

• Covering of wax over epidermis. Stomata, tiny pores surrounded by guard cells permit gas exchange

• Mesophyll – parenchyma and ground tissue containing chloroplasts. 2 layers, spongy (air spaces) and palisade (lots of chloroplasts)

• A branch of the vascular bundle continues into petiole and divides in leaf blade, providing support and transport

Monocot vs Dicot

• See page 242 to review

Modification of plant organs

Roots - 2 main types

• Taproot – one main vertical root with branch roots from main root

• Fibrous root – no main root. Roots are thinner and spread throughout soil. Good for preventing erosion

Root modifications

• Prop root – adventitious root growing from lower part of stem as a brace (corn)

• Storage root – parenchyma cells store carbohydrates and water (beet, carrot)

• Pneumatophore (air root) – extend above soil or water surface. Help with oxygen uptake (mangrove, cypress knee)

• Buttress root – at bottom of tree for stability (fig tree)

Prop

Pneumatophore Buttress

Storage

Stem Modifications

• Bulb – vertical underground stem with enlarged base for food storage (onion)

• Tuber – horizontal underground stem for starch storage (potato)

• Rhizome – horizontal stem just below surface for asexual reproduction (ginger)

• Stolon – horizontal above ground stem for asexual reproduction (strawberry)

Bulb Tuber

Rhizome

Stolon

Leaf Modifications

• Tendril – coil around objects for support (peas)

• Reproductive leaves – tiny plants form on leaf margins. Fall to ground and take root

• Bracts – AKA floral leaves. Surround flowers to attract pollinators.

• Spines – reduce water loss

TendrilBract

Reproductive Leaf

Spine

Meristem – 2 types

• Apical meristem – found at tips of root and shoot. Produces primary tissue (non-woody) and primary growth (growth in length)

• Lateral meristem – produces secondary (woody) growth. 2 types:– Vascular cambium produces secondary xylem

and phloem (see next slide)– Cork cambium produces cork cells of outer

bark

Apical M

eristem - shoot

Apical Meristem - Root

Secondary Growth

• Wood is the accumulation of secondary xylem with lignified walls.

• Growth rings occur due to seasonal cycles of growth (dormant, fast growth, slow growth)

• In secondary growth epidermis splits and is replaced by tissue made by cork cambium which produces cork cells with suberin – impregnated walls. This layer is called periderm.

• Lenticels are splits in periderm through which gas exchange occurs.

• Bark = phloem + periderm

• In old trees, heartwood is old resin filled wood while sapwood is actively conducting

Auxins

• Plants have hormones too! • Auxins (a class of hormones) are found in

embryos (seeds), apical meristems and buds. They increase the flexibility of cell walls so shoots can bend toward light

• The auxin indoleacetic acid (IAA) collects on the side of the stem AWAY from the light causing that side to elongate and the plant to bend TOWARD light

Tropisms – growth or movement toward or away from a stimulus

• Phototropism – light

• Thigmotropism – touch

• Geotropism – gravity

• Chemotropism – chemicals

• Ex) plant shoots exhibit positive phototropism and roots exhibit negative phototropism