Plant Form and Function

1
Plant Form and Function

• Chapter 18

2
Drought Versus Civilization

• Periodic droughts are normal
• Plants conserve water by closing stomata lack of
  CO2 stops sugar production
• People depend on plants droughts can devastate
  civilizations

3
p.300
4
Impacts, Issues Video
Drought Versus Civilization
5
Shoots and Roots

• Shoots
• Produce food by photosynthesis
• Carry out reproductive functions
• Roots
• Anchor the plant
• Penetrate the soil and absorb water and dissolved
  minerals
• Store food

6
Complex Tissues

• Composed of mixed cell types
• Xylem
• Phloem
• Epidermis

7
Vascular Tissues

• Phloem
• Transports sugars
• Main conducting cells are sieve-tube members
• Companion cells assist in the loading of sugars

• Xylem
• Conducts water and dissolved minerals
• Conducting cells are dead and hollow at maturity

8
Epidermis

• Covers and protects plant surfaces
• Secretes a waxy, waterproof cuticle
• Contains stomata
• In woody plants, periderm replaces epidermis

9
Adapted to Photosynthesis

• Leaves are usually thin
• High surface area-to-volume ratio
• Promotes diffusion of carbon dioxide in, oxygen
  out
• Leaves are arranged to capture sunlight
• Are held perpendicular to rays of sun
• Arranged so they dont shade one another

10
Leaf Structure

• Leaf organization

11
Leaf Epidermis

• Covers every leaf surface
• Specialized cells

12
Roots

• Provide large surface area for water and mineral
  absorption
• Taproot system
• eudicots
• Fibrous root system
• monocots

13
Root Systems
fibrous root system of a grass plant
taproot system of a California poppy
14
Root Structure

• Vascular cylinder
• Xylem
• Phloem
• Root hairs

15
Soil

• Minerals mixed with humus
• Minerals come from weathering of rock
• Humus is decomposing organic material
• Composition of soil varies
• Suitability for plant growth depends largely on
  proportions of soil particles

16
Macronutrients

• Mineral elements that are required in amounts
  above 0.5 of the plants dry weight
• Carbon Nitrogen Magnesium
• Hydrogen Potassium Phosphorus
• Oxygen Calcium Sulfur

17
Micronutrients

• Elements that are required in trace amounts for
  normal plant growth
• Chlorine Zinc
• Iron Copper
• Boron Molybdenum
• Manganese

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Table 18-2, p.310
19
Leaching

• Removal of nutrients from soil by water that
  percolates through it
• Most pronounced in sandy soils
• Clays are best at holding onto nutrients

20
Soil Erosion

• Loss of soil to wind and water
• Often the result of deforestation
• Nutrient loss affects entire food chain

21
Soil

• Soil profile

22
Root Hairs

• Extensions from the root epidermis
• Greatly increase the surface area available for
  absorption

23
Root Nodules

• Swelling on roots of some plants
• Contain nitrogen-fixing bacteria
• Bacteria convert nitrogen gas to forms plants can
  use

24
Root Nodules
a Root nodule
Fig. 18-17a, p.312
25
Root Nodules
Fig. 18-17b, p.312
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Mycorrhizae

• Symbiosis between young plant root and fungus
• Fungal filaments may cover or penetrate root
• Fungus absorbs sugars and nitrogen from plant
• Roots obtain minerals absorbed from soil by
  fungus

27
Mycorrhizae
28
Water Use and Loss

• Plants use a small amount of water for metabolism
• Most absorbed water lost to evaporation through
  stomata in leaves
• Evaporation of water from plant parts is
  transpiration

29
Water Transport

• Water moves through xylem
• Xylem cells are tracheids or vessel members
• Both are dead at maturity

30
Cohesion-Tension Theory of Water Transport

• Transpiration creates negative tension in xylem
• Tension extends downward from leaves to roots
• Hydrogen-bonded water molecules are pulled upward
  through xylem as continuous columns

31
Transpiration Drives Water Transport
Water evaporates from leaves through stomata
Creates a tension in water column in xylem
32
Replacement Water Is Drawn in through Roots
33
mesophyll (photosynthetic cells)
vein
upper epidermis
Transpiration is the evaporation of water
molecules from aboveground plant parts,
especially at stomata. The process puts the
water in xylem in a state of tension that
extends from roots to leaves.
a
stoma
The driving force of evaporation in air
Fig. 18-20a2, p.315
34
vascular cambium
xylem
phloem
The collective strength of hydrogen bonds
among water molecules, which are confined
within the narrow water- conducting tubes in
xylem, imparts cohesion to water. Hence the
narrow columns of water in xylem can resist
rupturing under the continuous tension.
b
Cohesion in root, stem, leaf xylem plus water
uptake in growth regions
Fig. 18-20b, p.315
35
water molecule
vascular cylinder
root hair cell
endodermis
cortex
For as long as water molecules continue
to escape by transpiration, that tension will
drive the uptake of replacements from soil
water.
c
Ongoing water uptake at roots
Fig. 18-20c, p.315
36
Cohesion Tension Theory
37
The Role of Hydrogen Bonds

• Hydrogen bonds hold water molecules together in
  conducting tubes of xylem
• Weak bonds still allow water to evaporate through
  stomata during transpiration

38
Wilting

• Water regulation maintains turgor

39
Cuticle

• Translucent coating secreted by epidermal cells
• Consists of waxes in cutin
• Allows light to pass through but restricts water
  loss

40
Plant Cuticle
leaf surface
cuticle
epidermal cell
photosynthetic cell
Fig. 18-22, p.316
41
Stomata

• Openings across the cuticle and epidermis allow
  gases in and out
• Guard cells on either side of a stoma
• Turgor pressure in guard cells affects opening
  and closing of stomata

42
Stomata
open stoma
guard cells
chloroplast
closed stoma
Fig. 18-23, p.316
43
Stomata and the Environment
44
Phloem

• Carry organic compounds
• Conducting tubes are sieve tubes
• Consist of living sieve-tube members
• Companion cells
• Lie next to sieve tubes
• A type of parenchyma
• Help load organic compounds intosieve tubes

45
TransportableOrganic Compounds

• Carbohydrates are stored as starches
• Starches, proteins, and fats are too large or
  insoluble for transport
• Cells break them down to smaller molecules for
  transport
• Sucrose is main carbohydrate transported

46
Pressure Flow Theory
Water moves in, increasing turgor pressure.
Pressure pushes solutes by bulk flow between
source and sink.
Pressure and solute concentrations decrease
between source and sink.
Solutes unloaded into sink cells, lowering their
water potential water follows.
47
Transport through Phloem

• Driven by pressure gradients
• Companion cells supply energy to start process

48
upper leaf epidermis
photosynthetic cell
sieve tube in leaf vein
companion cell next to sieve tube
lower leaf epidermis
Typical source region Photosynthetic tissue in a
leaf
Fig. 18-26b, p.318
49
Translocation

• Translocation in phloem