The Shoot System II: The Form and Structure of Leaves

1
The Shoot System II The Form and Structure of
Leaves

• Chapter 6

2
Functions of Leaves

• Photosynthesis????
• Release oxygen, synthesize sugars
• Transpiration????
• Evaporation of water from leaf surface
• Specialized functions
• Water storage
• Protection

3
Comparison of Monocot and Dicot Leaves
Type Shape of blade Venation Description
Monocot Strap-shaped blade Parallel vascular bundles Leaf bases usually wrap(?) around stem
Dicot Thin, flat blade Netted pattern of vascular bundles Petiole holds blade away from stem
blade portion of leaf that absorbs light energy
4
parallel veins ????

stem
blade
Blade??
petiole??
bud
sheath??
node
Midrib??
node
a
b
Fig. 6-1, p. 92
5
Leaf Blade

• Broad, flat surface for capturing light and CO2
• Two types of leaves
• Simple leaves(??)Leaves with a single blade
• Compound leaves (??)

6
midrib
blade??
petiole??
Poplar?? (Populus)
Oak?? (Quercus)
Maple? (Acer)
a Simple leaves
leaflets
honey locust???? (Gleditsia)
red buckeye???? (Aesculus)
black locust?? (Robinia)
b Compound leaves
Fig. 6-2, p. 93
7
Leaf Blade

• Compound leaves
• Blade divided into leaflets(??)
• Two types
• Palmately compound (????)
• Leaflets diverge from a single point
• Example red buckeye
• Pinnately compound (????)
• Leaflets arranged along an axis
• Examples black locust, honey locust

8
Leaf Blade

• Advantages of compound leaves
• Spaces between leaflets allow better air flow
  over surface
• May help cool leaf
• May improve carbon dioxide uptake

9
node
blade
stem
sheath
Crabgrass ?????
Fig. 6-3a, p. 93
10
node
Ligule??
Corn ??
sheath
Fig. 6-3b, p. 93
11
blade
stem
ligule
Auricles??
Barley??
sheath
Fig. 6-3c, p. 93
12
Petiole(??)

• Narrow base of most dicot leaves
• Leaf without petiole sessile (???)
• Vary in shape
• Improves photosynthesis
• Reduces extent to which leaf is shaded by other
  leaves
• Allows blade to move in response to air currents??

13
Sheath(??)

• Formed by monocot leaf base wrapping around stem
• Ligule(??)
• Keeps water and dirt from getting between stem
  and leaf sheath
• Auricles (??)
• In some grass species
• Two flaps of leaf tissue
• Extend around stem at juncture of sheath and blade

14
Sheath

• Why does grass need mowing so often?
• Grass grows from base of sheath
• Intercalary meristem
• Allows for continued growth of mature leaf
• Stops dividing when leaf reaches certain age or
  length

15
Cuticle???
upperepidermis
bundle sheath ????
water moves from roots to stems, and into the
leaf through the xylem
palisade Mesophyll ??????
spongy Mesophyll ??????
sugars and other products of photosynthetic cells
enter the phloem in the vascular bundle
and depart from the leaf
lower epidermis
guard cell ????
xylem
phloem
vascular bundle (vein)
Oxygen and water vapor depart from the leaf
through stomata.
carbon dioxide from the air enters the leaf
through stomata
Fig. 6-5a, p. 94
16
palisade Mesophyll ??????
spongy Mesophyll ??????
Fig. 6-5b, p. 94
17
Stoma??
epidermal Cell ????
guard Cell ????
Fig. 6-6a, p. 95
18
Epidermis

• Covers entire surface of blade, petiole, and leaf
  sheath
• Continuous with stem epidermis
• Usually a single layer of cells
• Cell types
• Epidermal cells
• Guard cells
• Subsidiary cells(????)

19
subsidiary cell (????)
guard cell
Cuticle ???
pore
substomatal chamber
Fig. 6-6b, p. 95
20
Epidermal Cells

• Appear flattened in cross-sectional view
• Outer cell wall somewhat thickened
• Covered by waxy cuticle
• Inhibits evaporation through outer epidermal cell
  wall

21
Stomatal Apparatus

• Cuticle blocks most evaporation
• Opening needed in epidermis for controlled gas
  exchange
• Two guard cells pore stoma
• Subsidiary cells
• Surround guard cells
• May play role in opening and closing pore

22
Stomatal Apparatus

• Guard cells subsidiary cells stomatal
  apparatus
• Functions of stoma
• Allows entry of CO2 for photosynthesis
• Allows loss of water vapor by transpiration
• Cools leaf by evaporation
• Pulls water up from roots

23
Stomatal Apparatus

• Stomata usually more numerous on bottom of leaf
• Stomata also found in
• Epidermis of young stem
• Some flower parts

24
Table 6-1, p. 95
25
Trichomes

• Secretory(??)
• Stalk with multicellular or secretory head
• Secretion often designed to attract pollinators
  to flowers
• Short hairs
• Example saltbush (Atriplex)(???)
• Hairs store water, reflect sunlight, insulate(??)
  leaf against extreme desert heat

26
?????
27
Fig. 6-7b, p. 97
28
Mesophyll

• Two distinct regions in dicot leaf
• Palisade mesophyll??????
• Spongy mesophyll??????
• Substomatal chamber
• Air space just under stomata

29
spongy parenchyma
phloem
xylem
upper epidermis
palisade parenchyma
lower epidermis
Fig. 6-8, p. 97
Midrib??
30
Mesophyll

• Dicot midrib(??) (midvein)
• Xylem in upper part of bundle
• Phloem in lower part of bundle
• Bundle sheath(????)
• Single layer of cells surrounding vascular bundle
  (???)
• Loads sugars into phloem
• Unloads water and minerals out of xylem

31
vein endingTracheid ???????
bundle-sheath parenchyma
spongy parenchyma
mesophyll cell
intercellular space
Fig. 6-9, p. 98
32
Fig. 6-10a, p. 98
33
bundle sheath cell ??????
Fig. 6-10b, p. 98
34
Formation of New Leaves

• Originate from meristems(????)
• Leaf primordia (???) early stages of development

35
leaf primordium ???
SAM
Fig. 6-11a, p. 99
36
next leaf primordium
leaf primordium ???
Procambium ?????
young leaves
Fig. 6-11b, p. 99
37
Formation of New Leaves

• Steps in leaf formation
• Initiated by chemical signal
• Location in leaf depends on plants
  phyllotaxis(??)
• Cells at location begin dividing
• Becomes leaf primordium(???)
• Shape of new leaf determined by how cells in
  primordium divide and enlarge

38
Cotyledons

• Seed leaves
• Primarily storage organs
• Slightly flattened, often oval shaped(????)
• Usually wither(??) and die during seedling growth

39
first leaf (simple)
mature leaf (compound)
cotyledons
withered cotyledon
Fig. 6-12, p. 99
40
Heterophylly(???)

• Different leaf shapes on a single plant
• Types of heterophylly
• Related to age of plant
• Example ivy (Hedera helix)(???)
• Juvenile ivy leaves three lobes(??) to leaves
• Adult ivy leaves leaves are not lobed

41
Heterophylly

• Environment to which shoot apex is exposed during
  leaf development
• Example marsh(??) plants
• Water leaves
• Leaves developing underwater are thin with deep
  lobes
• Air leaves
• Shoot tip above water in summertime develops
  thicker leaves with reduced(??) lobing

42
Fig. 6-14, p. 100
43
Heterophylly

• Position of leaf on tree
• Shade leaves
• Develop on bottom branches of tree
• Mainly exposed to shade
• Leaves are thin with large surface area
• Sun leaves
• Develop near top of same tree
• Exposed to more direct sunlight
• Leaves are thicker and smaller

44
Sun leaves
45
Shade leaves
46
Adaptations for Environmental Extremes

• Xerophytes(????)
• Grow in dry climates
• Leaves designed to conserve water, store water,
  insulate against heat
• Sunken stomata
• Thick cuticle
• Sometimes multiple layers to epidermis

47
stoma
Oleander ???
Fig. 6-16a, p. 101
48
multiple epidermis
Fig
Fig. 6-16b, p. 101
49
mesophyll cells
Jade?????
Fig. 6-16c, p. 101
50
Adaptations for Environmental Extremes

• Xerophytes(????)
• Abundance(??) of fibers in leaves
• Help support leaves
• Help leaf hold shape when it dries

51
Adaptations for Environmental Extremes

• Hydrophytes(????)
• Grow in moist environments
• Lack characteristics to conserve water
• Leaves
• Thin
• Thin cuticle
• Often deeply lobed
• Mesophytes (????)
• Grow in moderate climates

52
Leaf Modifications(???)

• Spines(??)
• Cells with hard cell wall
• Pointed and dangerous to potential predators
• Tendrils(??)
• Modified leaflets
• Wrap (??) around things and support shoot(?)

53
tendril
Plantlets ??
Fig. 6-17, p. 101
54
Leaf Modifications

• Bulbs(??)
• Thick leaves sometimes referred to as bulb scales
• Store food and water
• Modified branches with short, thick stem and
  short, thick storage leaves

55
Leaf Modifications

• Plantlets
• Leaves have notches(??) along margins
• Meristem develops in bottom of each notch that
  produce a new plantlet
• Plantlet falls off leaf and roots in soil
• Form of vegetative (asexual????) reproduction
• Example
• Air-plant (Kalanchoe pinnata)????,???

56
Leaf Abscission(??)

• Abscission separation
• Result of differentiation and specialization at
  region at base of petiole called abscission zone
• Weak area due to
• Parenchyma cells in abscission zone are smaller
  and may lack lignin in cell walls
• Xylem and phloem cells are shorter in vascular
  bundles at base of petiole
• Fibers often absent in abscission zone

57
xylem
axillary bud ??
phloem
Separation??
abscission zone
Fig. 6-18, p. 104
58
Leaf Abscission

• Abscission zone weakens
• Cells in vascular bundles become plugged(??)
• Leaf falls off
• Leaf scar
• Scar that remains when leaf falls off
• Sealed over with waxy materials which block
  entrance of pathogens(???)

59
Environmental Abscission Controls

• Cold temperatures
• Short days
• Induce hormonal(???) changes that affect
  formation of abscission zone
• Leaves move nutrients back into stem
• Leaves lose color
• Leaves fall off tree
• Leaves decompose and recycle nutrients