Monocot vs. Dicot
Recall from Diversity unit, that
angiosperms (have seeds & flowers)
can be grouped into 2 classes:
• Monocots – have 1 seed leaf
(cotyledon)
• Dicots – have 2 seed leaf
(cotyledons)
Monocots & dicots will also show
structural differences in their
vascular bundles, leafs and flowers.
Vascular Bundles
Differences in the vascular bundles arrangement:
vs.
Dicot
Monocot
Leaves
Structure of leaves vary – an adaptation to the
biotic factors (such as competitors, predators)
and abiotic factors (such as temperature, soil
conditions, etc)
Leaves must balance the need of maximizing
photosynthesis with the problem of drying out.
Structure of leaves
Blade – flat main body of leaves
Petiole – leaf stalk
Node – points where leaves are positioned on stem
Internode – distance between nodes
Monocot
vs.
Dicot
Structure of leaves
Notice the difference in vascular tissue branching (vein
patterns) of the leaves:
Monocot
Parallel venation
vs.
Dicot
Net venation
Structure of leaves
Can also differentiate between:
Simple leaf
vs.
single, undivided blade
Compound leaf
two or more divided blades
Components of leaves
Chloroplast
• Double-membrane organelle
• Inside chloroplast are flat, disc-like structures
called thylakoids.
• Chlorophyll is green-pigment found in membranes
of thylakoids.
• Stacks of thylakoids is
called grana and act
as solar collector.
Components of leaves
Tiny pores on the leaves (called stomata) allows for:
• diffusion of CO2 into the leaves
• diffusion of O2 out of the leaves
during photosynthesis process.
Q: Most plants have
stomata on the lower
surface of the leaf. Why?
1. Reduce water loss
2. Allows packing of more
cells that can do
photosynthesis on the
upper layer
3. Reduce chances of entry
of foreign organisms
Components of leaves
Stomata also allow water vapour to escape from leaf transpiration.
Opening & closing is controlled by a pair of chloroplastcontaining guard cells which will:
bend outward (to open) OR collapse inward (to close).
Q: How does the plant know when to open /close
their stomata?
A: Stomata need to open during the day (sunlight)
to do photosynthesis. However, plant can then
dry out quickly.
Guard cells can detect water level in the plant to
control opening/closing of stomata.
Lots of water guard cells expand stomata open
Little water guard cells collapse stomata close
Guard cells also respond to CO2 level, light level,
temperature, abscisic acid concentration to control
opening/closing of stomata.
Simple diffusion of gases (from area of HIGH to LOW
concentration):
•
During photosynthesis, CO2 is being used up, so
its level in plant is low compared to atmosphere.
Thus CO2 diffuse in through the stomata.
•
During photosynthesis, O2 is being produced, so
its level in plant is HIGH compared to
atmosphere. Thus O2 diffuse out of the stomata.
Palisade layer: a layer of tall, closely packed parenchyma
cells containing chloroplasts, just below the upper surface of
leaf.
Spongy mesophyll: a region of loosely packed parenchyma
cells containing chloroplasts, in the middle of a leaf. Loose
packing allows rapid diffusion of CO2 into and O2 out of cells.
Both are part of ground tissue system.
Adaptation to abiotic factors
Leaf structure is an adaptation to their environment:
Broad leaves – good for
those in shaded area,
appear early spring
Thin, long needles in conifers:
Small surface area with waxy
cuticle prevents water loss
during winter time
Adaptation to abiotic factors
Cacti spines – modified leaves.
Very few stomata, small surface
area. Photosynthesis on the stems
instead.
Able to survive in area of low
precipitation.
Xerophyte – plants that survive in areas with little
moisture
Hydrophyte – plants living on/in water
Mesophyte – plants living in areas with moderate
moisture
Adaptation to biotic factors
To prevent being eaten by herbivores,
some plants develop:
•Tough, hairy, prickly or bitter leaves
•Toxins (ex: milkweed, nicotine in
tobacco leaves)
However herbivores can adapt as well
by having tough mouth tissues, low
tasting ability, immunity to toxins (ex:
monarch butterfly can eat milkweed).
Other leaf adaptations
Carniverous plants – thrive
better with animal
proteins
Indian pipe – white because no
chlorophyll. Not able to
photosynthesize.
Saprophytes – gets nutrients
from decaying materials in soil.