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Leaves
Germinating sunflowers, turgor and nutation
parallel veins
Monocot leaf
Eudicot (or dicot) leaf
blade
petiole
stem
blade
bud
node
midrib
sheath
node
From: http://sunflower.bio.indiana.edu/~rhangart/plantmotion
Germination
light
Nutation isSunflower
due to unequal
rates of growth in
thatcontinuous
is dependent on
cell turgor.
The leaf blade is the thin flat part of the leaf.
Leaves of most dicots have a petiole. This attaches the leaf blade to the stem.
The petiole determines the distance of the leaf from the stem. This
decreases shading of the blade by other leaves. It also allows the position of the
leaf to be changed.
node
The base of the leaf blade of monocot leaves wraps around the stem to form the
sheath.
stem
blade
The sheath may contain a ligule or auricles.
sheath
Crabgrass, no ligules,
no auricles
blade
stem
node
ligule
auricles
ligule
Corn
sheath
Barley
sheath
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midrib
The main function of leaves is photosynthesis.
Leaves are generally thin and flat with a high surface area. This allows
optimal light absorption for photosynthesis. It also promotes water loss that needs
to be regulated.
The cuticle prevents excessive water loss. Stomatal pores allow regulated water
lose and CO2 uptake.
Photosynthesis requires CO2 and light. Open stomatal pores are required
for CO2 uptake but also allow water evaporation (called transpiration).
blade
poplar
(Populus)
oak
(Quercus)
petiole
maple
(Acer)
a Simple leaves
Regulation of stomatal pores provides a balance between CO2 uptake
and water loss.
Transpiration is essential for many reasons:
Evaporative cooling of the leaves.
Generating the negative pressure in the xylem.
Getting nutrients from a large volume of soil solution.
leaflets
This is called
bicompound or doubly
compound
red buckeye
(Aesculus)
black locust
(Robinia)
honey locust
(Gleditsia)
b Compound leaves
Figure 6.07ab: Compound leaves: Two types of compound leaves. The
petiolule is attached to the rachis in (a) and to the end of the petiole in (b).
Figure 6.09: Myriophyllum heterophyllum (two-leaf water milfoil) dissected
leaves of submerged plant.
Figure 6.08: Mimosa, double compound leaf.
When two-leaf water milfoil grows above the surface of the water, it
produces thicker and tougher leaves shown here. Hence the name
“heterophyllum” = different or other leaf.
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Leaf Margins
Figure 6.12: Several common types of leaf margin.
Serrate: having sharp, forward-pointing teeth on the margin
Serrulate: serrate with very small teeth
Dentate: with sharp, outward-pointing teeth on the margin
Undulate: wavy
From: http://www.calflora.net/botanicalnames/botanicalterms.html
Monocot leaves have
parallel venation, the
veins are parallel to each
other.
Eudicot leaves have net
venation.
Ginkgo leaves have
dicotomous venation.
Ginkgo is a
gymnosperm.
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The functions of leaf veins:
cuticle
upper
epidermis
- Leaf veins are vascular bundles composed of xylem and phloem.
- The xylem carries water and nutrients from the soil to mesophyll cells.
- The phloem carries fixed carbon and other metabolites to sink tissues.
Sink tissues are net importing tissues such as roots, flowers,
developing leaves etc.
In the leaf vein
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
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
xylem
bundle and
phloem
depart from
vascular the leaf
bundle
carbon dioxide from the air (vein)
lower
epidermis
guard cell
Oxygen and water vapor
depart from the leaf
through stomata.
Cross section of a lilac leaf (dicot)
palisade
parenchyma
enters the leaf through
stomata
Cross section of a maize leaf (monocot)
xylem
phloem
spongy
upper
epidermis parenchyma
lower
epidermis
midrib
Two types of parenchyma cells in the mesophyll
pallisade parenchyma or pallisade mesophyll - in the upper part of the leaf
spongy mesophyll - usually in the lower part of the leaf, less regular arrangement
of cells.
The mesophyll is the primary site of photosynthesis.
Close-up view of a leaf vein in Zea mays showing the bundle sheath
Pallisade parenchyma cells are
separated as shown in (b) so that there
is airspace between the cells to allow
CO2 uptake
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Leaf structure and regulation of water loss:
stoma
What structures are important for the regulation of water loss from leaves?
epidermal
cell
guard
cell
There is generally a greater density of stomata on the underside of the leaf
(lower epidermis).
The cuticle prevents direct water loss from the epidermal cells. Guard
cells regulate the aperture of stomatal pores and thereby control water loss also called transpiration.
Plants require transpiration to bring nutrients from the soil. But too much
transpiration and water loss causes wilting. So water loss must be tightly
regulated.
Dionaea - venus flytrap
Drosera - sundew
Modified leaves of
carnivorous plants
Table 6.T02: Frequency of Stomata in Leaf Upper and Lower Epidermis
Sarrecenia - pitcher plant
Different types of trichomes on the same plant. Simple, glandular and
stalked-glandular trichomes are visible on this Croton leaf.
Glandular trichomes
Glandular secreting trichomes
from tomato, there are five types.
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Leaf initiation, in the shoot apical meristem.
leaf primordium
next leaf primordium
SAM
leaf primordium
procambium
young
leaves
Heterophylly: different leaf shapes on the same plant. The bean plant is an example
in lab this week.
first leaf
(simple)
mature leaf
(compound)
cotyledons
withered
cotyledon
Figure 6.11a: Two types of leaves on beans.
Heterophylly depends on plant age and enviroment. In this example, the leaves of
white water buttercup (Ranunculus aquatilis) are more more deeply lobed when
submerged. Similar to the example of two-leaf milfoil.
Two types of leaves on Azara lanceolata. This is an evergreen from Chile
that has small round leaves and large lanceolate leaves.
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Leaf acclimation to light intensity, leaves of same species under different light intensity
Leaf adaptations to dry climates: sunken stomata. This adaptation maximizes CO2
uptake per water that evaporates.
Shade leaf
Leaves of plants grown
in the shade will have thinner leaves
and more chlorophyll per reaction
center
Sun leaves are thicker.
Figure 6.38b: Yucca leaves, fiber bundles, high mag.
Leaf adaptations to dry climates:
multiple
epidermis
Figure 6.38c: Yucca leaves, groove with stomata, high mag.
Fig
Cactus spines
Leaf adaptations to dry climates: less air space in leaf
tendril
mesophyll
cells
plantlets
More leaf modifications
Jade
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Figure 6.39a: Many conifers have needle-shaped leaves.
Figure 6.41a: Pine needle, low mag of whole xs.
Figure 6.41b: Pine needle, mag of vascular bundle with secondary phloem.
Figure 6.41c: Pine needle, mag of resin canal.
Leaf senescence (death) is a developmental process that usually culminates in
abscission. Abscission is the process of plant organ separation.
axillary bud
xylem
phloem
abscission zone
separation
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