Chapter 35

Plant Structure and Growth

Morphology – studies of external structure of plants

1) Root system –

-anchors plant in soil (land adaptation)

-underground

-absorbs minerals, water

-conducts water & nutrients

-stores food

taproot – (as seen in dicots)

– 1 lg., vertical root

fibrous root system – (as seen in monocots)

- mat of threadlike roots

- allows for greater exposure to water

root hairs – increase surface area

*mycorrhizae also help with water/minerals

2) Shoot system - vegetative shoots above ground

stems

axillary buds – embryonic side shoots in the

angle between leaf/stem

terminal buds – at apex

modified stems:

stolons – grow on surface of ground

rhizome – horizontal underground stem

tubers – swollen ends of rhizomes

bulbs – vertical underground shoots

leaves – photosynthetic organs

-consist of blade and petiole

modified leaves:

tendrils – modified leaflets that cling to supports

spines – modified leaves on cacti

modified water-storing leaves in succulents

brightly colored leaves – as in poinsettias

Plant anatomy – concerned with internal structure (tissues)

3 major tissue types:

1) parenchyma – least specialized

- most lack secondary walls

- fleshy tissue of most fruit

- perform most metabolic functions

2) collenchyma – support young parts of plants

without restricting growth

- alive at functional maturity

3) sclerenchyma – more rigid (have lignin)

- may be dead at functional maturity

a) fibers – long, slender, tapered in bundles

b) sclerids – irregular, shorter, found in

nutshells, seed coats

Water conducting cells of xylem

1) tracheids – water moves from cell – cell

through pits; also aid in support

(dead at functional maturity)

2) vessel elements – aligned end – end with

perforations at their tips allowing water to flow

freely through them (dead at functional

maturity)

Food conducting cells of phloem

Sieve tube members (alive at maturity)

-transport sucrose, organic compounds,

mineral ions

-formed by chains of cells

Plant tissue systems

1) epidermal – single layer of cells covering

the entire body of a young plant

- protects

2) vascular – functions in transport & support

3) ground – mostly parenchyma

-storage, support

-between dermal & vascular tissue

Chapter 36

Transport in Plants

Transport in plants

Occurs on 3 levels:

1) uptake of water/solute by individual cells

2) short distance transport to/from neighboring

cells

3) transport of sap over long distances

Absorption by roots- through root hairs and epidermal cells provided by hydrophilic cell walls (touching root cortex)

- water/minerals enter xylem by:1) symplastic pathway – absorbed into

parenchymal cells & to stele by plasmodesmata

2) extracellular apoplastic pathway – along cellwalls until they reach impervious Casparianstrip at endodermal boundary to stele bycrossing endodermis

Ascent of sap by xylem-supplies minerals to shoots

-replaces lost water by transpiration

-water potential determines net movement

-increase in solute concentration lowers water

potential; increase in pressure increases water

potential

-water in stele causes upward push of root

pressure that forces water up the xylem

(causes guttation – exudation of water droplets)

Transpiration-cohesion-adhesion theory

-evaporative water loss during transpiration

lowers water potential of leaf cells compared to

root, cohesion of water due to H bonds,

transpiration pulls water upward & creates

tension (negative pressure)

Control of transpiration-transpiration to photosynthesis ratio is a

convenient way to determine how efficiently

plants use water

-plants with lower ratios do well in arid climates

since water loss/CO2 exchange are trade off

-important for evaporative cooling & delivering

minerals to leaf

Control of transpiration

-turgor pressure & stomatal opening are

regulated by -/+ changes in membrane

potentials by proton pumps in guard cell

membranes

Guard cells-usu. open during the day (at dawn) & closed at

night due to:

1) light stimulates K+ accumulation & makes

cells turgid

2) depletion of CO2

3) inherent circadian rhythms – cycles with

24 hr. intervals

-hormones & water loss in the day can trigger closing

Xerophytes

-plants adapted to arid environments have

modified leaves:

-thick, small (reduces surface area to volume)

-thick cuticle, water storage in fleshy parts

-CAM metabolic pathways (circadian rhythms

are different)

Phloem transport

translocation – process of transporting

photosynthetically produced food throughout

plant’s body

-phloem sap carries:

sucrose, minerals, AA’s, & hormones

-transported from sugar sources to sugar sinks

Chapter 37

Plant Nutrition

Nutrients

-water (for photosynthesis as a solvent used in

elongation/growth)

-CO2 (from atm. used in carbohydrates)

-hydroponic culture is used to determine which

minerals are necessary for plant growth

Nutrients

Macronutrients:

C, O, H, N, S, P, Ca, K, Mg

(necessary for organic compounds)

Micronutrients:

Cl, Fe, B, Mn, Zn, Cu, Mb

(cofactors in enzymatic reactions)

Soil

-a very important factor

-texture (particle size affects drainage)

*loam is usu. most fertile

-amt. of humus (decomposing organic material)

-clay particles (charged negatively; attract H2O)

*soil management is essential in agriculture:

promotes erosion, depletes minerals, taps

water reserves

Soil

-irrigation drains water reserves, causes NaCl

accumulation but increases crop yield

-bacteria fix N2 in soil (into NH3, NH2)

-legumes, alders, some tropical grasses have

evolved mutualistically

-protein-rich plant breeds genetically engineered

to stop hunger

Nutritional adaptations

A. Parasitic plants – supplement photosynthesis

ex: mistletoe, dodder

or tap vascular tissues of host plants

(epiphytes – free living)

B. Carnivorous plants – obtain N2 by killing &

digesting insects

ex: sundew, pitcher plant, Venus’ fly trap

Indian pipe

Venus’ fly trap & pitcher plant

Sundew

Chapter 38

Plant Reproduction

Angiosperms

Flower (modified leaves) for reproduction

sterile: petals, sepals

fertile: stamens (M), carpels (F)

classified:

monoecious – M & F structures

dioecious – M or F structures

Monoecious

Flower

complete – sepals, petals, stamens, carpels

incomplete – missing 1 or more of these

perfect – stamens & carpels

imperfect – staminate or carpellate (pistillate)

Floral variations have evolved during the 130 million years of angiosperm history.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 38.3

Flower

• Diploid sporophyte dominates

pollination – pollen placed on stigma

by wind or animal

fertilization – pollen grain germinates, growing

pollen tube extending down style, seed develops

Fruit

classification:

simple – ex: cherry

aggregate – ex: strawberry

multiple – ex: pineapple

Seed

germination – seeds imbibe water, frees seed from seed coat & begins metabolic changes triggering growth

radicle – embryonic root

-1st structure to emerge from seed; shoot breaks through soil surface

Asexual reproduction (vegetative)

• fragmentation of parent plant

• cloning

• apomixis

Chapter 39

Control Systems in Plants

Response to the environment occurs on 2 time scales:

1) Individual plants & other orgs. respond adaptively to what goes on around them

2) These control systems themselves are adaptations that evolved over generations of interactions of plant/environment

* plants respond to env. cues by adjusting their pattern of growth & dev. through internal signals & interactions with the env.

Hormones

• traveling chemical signals that coordinate the functions (growth, development, response) between different parts of an organism

• 1st plant hormone identified: auxin

• found when studying experiments on phototropism

5 classes of hormones control plant growth & development

1) auxin – stimulates cell elongation in target tissues

-affects secondary growth & differentiation

-promotes fruit growth

-initiates adventitious root formation

-ex: IAA (indolacetic acid)

2) cytokinins – stimulate cell division (in roots, embryos, fruits) * prolong fruit shelf life-ex: zeatin

3) gibberelins – stimulate leaf/stem growth; -increase elongation in stems (w/ auxin)

-fruit development -support germination (stimulates enzymes for mobilization of seed storage material)

-ex: GA3

4) abscisic acid – slows plant growth & favors

dormant state by inducing bud scale

development

-inhibits cell division in vascular cambium

-suspends growth in buds & seeds

-stress hormone – helps cope w/ adversity

-ex: ABA

5) ethylene – (gaseous) – diffuses through

air spaces

-inhibits root growth & development of

axillary buds in an increase of auxin conc.

-stimulates fruit ripening & induces several

aspects of senescence in plant cells & organs

-ethylene increase & auxin decrease results

in leaf abscission

Tropisms• orient growth of plant organs to/from stimuli

• result in curvature of organs

1) phototropism – enhances phsyn. bending

shoots toward light

2) gravitropism – mediated by statoliths

(specialized plastids w/ dense starch grains)

-negative: shoots grow upward

-positive: roots grow downward

Tropisms

3) thigmotropism – adaptive coiling of tendrils on

touching a support

ex: thickening of stems in response to chronic

strong winds

Turgor movements

• rapid, reversible responses

• stimuli cause changes in turgor pressure

ex: sleep movements in legumes

Circadian rhythms

• physiological cycles w/ 24 hr. frequencies

• absence of env. cues leads to free-running

periods where they may deviate, but still have

their own cycles

• internal biological clocks control this

Photoperiodism

Other responses

• to heat stress

• to salt stress

• to drought

• to flooding

• to cold stress

Defense against pathogens & herbivores

• thorns

• chemical defenses

• airborne attractants