1 Review Describe how the structure of the leaf is
adapted to make photosynthesis more efficient
Form a Hypothesis The leave of desert plants often
have two or more layers of palisade mesophyll
rather than one. How might this modification be
advantageous to a desert plant
2 Review How do stomata help plants maintain
homeostasis
Predict Are stomata more likely to be open or
closed on a hot day- explain
CH 23 PLANT STRUCTURE AND FUNCTION
23.4 Leaves
Anatomy of a Leaf

Blade
 Thin,

Petiole
 Thin

flattened part of leaf to collect sunlight
stalk that attaches stem to blade
Have dermal, ground, and vascular tissues.
Dermal Tissue


Epidermis is covered by a waxy cuticle that protects
the leaf and limits water loss through evaporation
Tough irregular cells with thick cell walls.
Vascular Tissue

Xylem and phloem tissues are gathered together
into bundles called leaf veins.
Ground Tissue

Palisade mesophyll
 Closely
packed cells that absorb light that enters the
leaf

Spongy mesophyll
 Has
many air spaces between its cells.
Stomata



Small openings in the epidermis
Allow carbon dioxide, water, and oxygen to diffuse
into and out of the leaf
Connects to spongy mesophyll.
Transpiration




Loss of water through leaves due to evaporation
Replaced by water drawn into the leaf through
xylem
Mesophyll cells are kept moist so that gases can
enter and leave the cells easily
Helps to cool leaves on hot days.
Gas Exchange



Leaves take in carbon dioxide and give off oxygen
during photosynthesis
Cells take in oxygen and give off carbon dioxide
during cellular respiration
Gas exchange uses stomata.

Stomata always open
 Water
loss would be so great, few plants would be
able to survive

Stomata always closed
 Air

exchange couldn’t occur
Keep stomata open just enough to allow
photosynthesis to occur but not enough that they
lose excessive water.
Guard Cells



Highly specialized cells surround stomata
Control their opening and closing
Regulate gases exchange.
Open Stomata


Carbon dioxide can enter
Water is lost by transpiration.
Open Stomata

When water is abundant, it flows into the leaf,
raising water pressure in the guard cells, which
opens the stomata.
Closed Stomata


Carbon dioxide cannot enter
Water is no lost by transpiration.
Closed Stomata

When water is scarce, water pressure within the
guard cells decreases, the inner walls pull together,
and the stoma closes.
Homeostasis


Stomata open during the day for photosynthesis,
and close at night to limit water loss
May close in bright sunlight under hot, dry
conditions to conserve water.
Transpiration and Wilting


Osmotic pressure keeps a plant’s leaves and stems
rigid
High transpiration rates can lead to wilting.
Adaptations of Leaves

Pitcher Plant:
 Leaf
is modified to attract and digest insects and other
small
 Typically live in nutrient-poor soils and rely on animal
prey as their source of nitrogen.

Rock Plant
 Two
leaves are rounded to minimize exposure to air
 Have few stomata.

Spruce
 Narrow
leaves contain waxy epidermis and stomata
that are set below the surface of the leaf
 Reduce water loss.

Cactus
 leaves
are nonphotosynthetic thorns
 Protect against herbivores
 Photosynthesis carried out in stem.
Examining Stomata
1.
2.
3.
4.
5.
6.
Obtain different kinds of leaves from your teacher
Spread a thick coating of clear nail polish on the
underside of each leaf
Wait about 10 minutes for the polish to dry
completely
Attach a strip of clear tap to the polish and gently
peel off the tape and the dried polish
Tape the polish to a clean microscope slide and
examine under a 40x lens
For each leaf, move the microscope stage so you
can count stomata from three distinct fields of view
Examining Stomata
1.
2.
3.
Calculate What is the average number of
stomata per square cm for each leaf
Graph Make a graph that compares these
averages
Form a Hypothesis Write a hypothesis that could
account for differences in stoma density among
plants