Ch 11- Leaves In eudicots, the typical foliage leaf consists of: blade petiole Many leaves also have stipules at the base of the petiole. The axil is where the leaf joins the stem and it forms an angle between the petiole and the stem. In monocots, such as lilies and grasses, the leaf expands out in a sheath. Some leaves lack petioles and are termed sessile.
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Ch 11- Leaves
In eudicots, the typical foliage leaf consists of:
blade
petiole
Many leaves also have stipules at the base of the petiole.
The axil is where the leaf joins the stem and it forms an angle between the petiole and the stem.
In monocots, such as lilies and grasses, the leaf expands out in a sheath.
Some leaves lack petioles and are termed sessile.
Leaf blade variation
-compound
-simple
There are 2 types of compound leaves:
Leaves also possess a wide variety of patterns
of veins which contain xylem and phloem
Pinnately netted veins Palmately netted veins
Leaves can be arranged along a stem in a number
of different ways
smooth toothed deeply lobed
Pinnately compound leaves:
-leaflets are arranged along
a central axis that is an
extension of the petiole
Palmately compound leaves:
-leaflets arise from a common
point at the end of the petiole
Monocots Eudicots
Parallel Netted
Elm Norway maple
alternate opposite whorled
Leaves are complex structures composed of 4 main types of tissues:
The structure of the leaf is dependent on the plant’s environment
Epidermis
Mesophytes Hydrophytes Xerophytes
-epidermis -tough outer cell layer of the leaf that protects
inner tissues and provides structural support
-mesophyll -main photosynthetic tissue of the leaf
-xylem
-conducting tissues which form the vascular bundles
-phloem or veins of the leaf
Epidermal cells secrete a waxy layer (cuticle) which help retard water loss
hydrophytes plants growing in wet environments
xerophytes plants with leaves adapted to dry environments
mesophytes plants that require moderately moist environments
Epidermis
Trichomes serve a variety of functions:
Mesophyte Hydrophyte
The upper epidermis of grass leaves has another specialized type of cell- bulliform cell.
Epidermal cells may be specialized to form hairs called trichomes.
Epidermal tissue typically contains many thousands of
minute pores called stomata (sing. stoma).
Each stoma consists of a pore surrounded by two guard cells.
Xerophytes
When the stomata open, water vapor and oxygen move out of the leaf
and carbon dioxide moves into the interior of the leaf.
When these cells are filled with water (turgid), the leaf is unfolded to capture sunlight.
When bulliform cells are limp (flaccid), the leaf folds up to reduce water loss.
Xylem and phloem are the conducting tissues of leaf veins
Mesophyll tissue is organized in 2 layers:
The intracellular spaces facilitates gas exchange and is connected to the
external atmosphere by stomata
Mesophyll is located between the upper and lower epidermis
Mesophyll cells are:
Eudicots Monocots
Xylem conducts water and dissolved minerals to the leaf tissues
Veins are surrounded by bundle-sheath which are made
up of parenchyma cells.
In some leaves, the bundle-sheath is connected to the
upper and lower epidermis by bundle-sheath extensions.
-Distinct palisade
and spongy layers
in eudicot
-Mesophyll is not
differentiated into
distinct palisade
and spongy parenchyma
in monocots
-Bundle sheath cells
are also very conspicuous
in monocots
Phloem conducts dissolved sugars from the leaf tissues.
Plants lose large quantities of water through transpiration
Transpiration is the evaporation of water from plant
surfaces via epidermal pores, or stomata.
Stomatal movement control transpiration
Movement of water through plants
Two possibilities for water movements:
Pushing Xylem Sap: Root Pressure
•At night, when transpiration is very low, root cells continue
pumping mineral ions into the xylem of the vascular cylinder
Casparian strip Endodermal cell Pathway along
apoplast
Pathway through symplast
Casparian strip
Plasma membrane
Apoplastic route
Symplastic route
Root hair
Vessels (xylem)
Cortex Endodermis Epidermis Vascular cylinder
The build-up of water
creates turgor pressure that
causes the guard cells to
bend and the pore to open.
The increase of dissolved
substances causes the
movement of water
into the guard cells.
K+ accumulate in the guard
cells during the daylight
-pushed up from the roots
-pulled up by the leaves
Root pressure sometimes results in guttation, the
exudation of water droplets on tips of grass blades or the
leaf margins of some small, herbaceous dicots.
Pulling Xylem Sap: The Transpiration-Cohesion Tension Mechanism
•Water is pulled upward by negative pressure in the xylem.
Cohesion
Adhesion
•Transpirational pull is facilitated by cohesion and adhesion
Water has a high degree of cohesion due to hydrogen bonding that links
together adjacent water molecules.
Xylem sap
Mesophyll cells
Stoma
Water molecule
Atmosphere Transpiration
Xylem cells
Adhesion Cell wall
Cohesion, by hydrogen bonding
Cohesion and adhesion in the xylem
Water molecule
Wa
ter
po
ten
tia
l g
rad
ien
t
Root hair
Soil particle
Water
Water uptake from soil
Xylem cells
Adhesion Cell wall
Cohesion, by hydrogen bonding
Cohesion and adhesion in the xylem
As water evaporates from the surface of mesophyll cells
in the leaf, the water at the surface is replaced by water
from the interior of the mesophyll.
When water is drawn out, solute concentration increases
and more water is drawn from surrounding cells by
osmosis.
The transpirational pull draws water through the xylem, up
the stem all the way from the roots.
the force that allows water
molecules to stick together
the attraction between
different types of molecules
Leaf abscission-complex process that results in dropping of leaves
Leaves perform many functions in addition to photosynthesis
Modifications of leaves
tendrils bud scales
bracts
-Occurs in plants as a whole, e.g., annual plants that live only one season.
-Eliminate leaves before winter
-Declining day length is the signal that triggers aging and death
-Occurs only in parts of plants
1. Abscission zone is formed at the base of the petiole
3.Cork layer forms inside the separation layer to seal wound when leaf falls.
4.
-no photosynthesis
-water loss
-leaves in winter get weighed down by snow
-branches break
-Chlorophyll is broken
down leaving yellow and
orange accessory pigments
to color the leaves.
http://theseedsite.co.uk/lifecycle.html
2. Thin walled cells form in a separation layer.
The plant hormone ethylene promotes leaf abscission.
Senescence and leaf fall are a normal part of plant development
thread-like structures that help plants
climb over objects and gain access to light
waxy modified leaves that prevent
dessication and insulate young
buds in cold weather.
modified leaves, some of which
are colorful and attract pollinators.
Some leaves are specialized for water or food storage
cabbage
lettuce
edible parts of celery and rhubarb onions
Leaves are modified for defense in some plants
Leaves of some plants capture animal prey
-Carnivorous plants which trap live animal prey
Familiar garden plants have leaves modified to store nutrients