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Plant Structure and Function
The Fascinating World of
Plant Structure
With added feature:
Plant Function
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Root Structure and Function
• There are two types of root systems:
– Taproot - one root grows much larger than the
other roots with many small roots growing from
it.
• These roots usually grow deep into the soil to get
water and nutrients.
– Fibrous roots - a dense network of many thin
string-like roots.
• These roots spread out to absorb water and nutrients
that are close to the surface.
Root Structure and Function
• Root Growth
– The first root to grow from a seed is called the
primary root.
– As it grows downward other roots grow from it,
called secondary roots.
– Both types of roots have root hairs, which
increase the surface area of the root.
– The root tip is covered with a root cap which
protects it as the meristem tissue grows.
• The root cap also lubricates the root so it can move
through the soil easier.
Root Structure and Function
• Parts of a Root: (From the outside/in)
– Epidermis - Outer layer
• The root hairs grow out of this layer.
– Cortex - This layer stores food and water.
– Vascular tissue - This is made up of xylem and
phloem.
• Xylem carries water and minerals from the soil to
the leaves.
• Phloem carries food from the leaves to other parts of
the plant, including the roots.
Root Structure and Function
• Specialized Roots
– Aerial root - Absorbs water from the air.
– Prop root - This supports the stem and leaves of
the plant.
Aerial Roots
Prop Roots
Stem Structure and Function
• Stems are made up of the same four tissues
that make up the roots.
– Epidermis
– Cortex
– Vascular Tissue - Xylem and Phloem
• A layer that is different from the roots is the
pith -a layer of cells in the center of the
stem that store water.
Stem Structure and Function
• The stems are of two types:
– Herbaceous - soft, green and filed with water.
• Most of these plants live only a few weeks, a year at
the most.
• They do not grow very tall, usually 2 meters (6.5
feet) tall at the most.
• They have the Xylem and Phloem in vascular
bundles.
– In Dicots the bundles are arranged in a ring.
– In Monocots the bundles are scattered in the cortex.
– The Xylem is always on the inside, Phloem to the outside.
Stem Structure and Function
– Woody Stem - These are usually non-green,
thicker and harder than herbaceous stems.
• These plants grow tall.
• The parts of this stem are (from outside/inward)
– Cork (Bark) - Dead layer of cells that protects the plant
from disease, bacteria, and fungi.
– Cortex (Bark) - Stores food and water.
– Phloem (Bark) - Carries food from the leaves to the roots
and other parts of the plant.
– Cambium - This is meristem tissue, this part grows.
» Outer part forms new phloem cells.
» Inner part forms new xylem cells.
– Xylem - Carries water and nutrients to the leaves.
– Pith - Dead xylem cells, forms the wood of the tree.
Stem Structure and Function
• Stem Growth
– The xylem goes through two growth periods a
year:
• Spring - Large xylem cells produced.
• Summer - Small xylem cells produced.
• Together they form the annual rings.
– These show the age of the tree.
– Stems grow in lengths from buds:
• Terminal Bud - at the tip of the branch.
• Lateral Bud - forms new leaves, branches and
flowers.
Leaf Structure and Function
• Most leaves are broad and flat, which is
called the blade of the leaf.
• The blade is attached to the stem by a stalk
called the petiole.
– Some plants do not have a petiole, because
their leaves are directly attached to the stem.
– In most leaves the petiole becomes the main
vein of the leaf.
• The veins of the leaf carry the food, water, and
nutrients.
• The veins are made of vascular tissue.
Leaf Structure and Function
• Plants have either a simple leaf or a
compound leaf.
– Simple - this has one blade and one petiole.
– Compound - this has a blade divided into
smaller parts, leaflets.
Leaf Structure and Function
• Leaf Layers (Top of leaf to bottom of leaf)
– The leaf has an upper epidermis covered by a
thin waxy layer, the cuticle, that protects the
leaf from drying out and from insects and
disease.
– Below the upper epidermis is a layer of cells
called the palisade layer:
• This is made up of parenchyma cells that contain
chlorophyll.
• Most of the photosynthesis takes place here.
• The veins run through this layer.
Leaf Structure and Function
– The next layer is the spongy layer.
• This layer contains many air spaces.
• Gas is exchanged in this area.
– The next layer is the lower epidermis.
• This layer contains small openings called stomatas
(the singular of “stomatas” is stomata).
• The small openings regulate the exchange of gases.
• Two bean-shaped cells, guard cells, surround and
control the size of the stomata.
– When the guard cells swell, the stomata opens.
– When the guard cells shrink, the stomata closes.
Leaf Structure and Function
• Plants lose water through the stomata in
their leaves in a process called
transpiration.
• Specialized leaves
– Needles
– Spines of a cactus decrease the surface area of
the leaf to prevent water lose.
– Vines have tendrils to help them attach to
objects.
– Carnivorous plants have special leaves to trap
insects or other organisms.
Cactus Spines
Tendrils
Carnivorous Plant
Venus Fly Trap
Pitcher Plant
Flower Structure and Function
• Parts of a flowers:
– The green leaf-like structures that surround and
protect the petals are called sepals.
– The petals surround the male and female
reproductive organs.
• The petals of some plants are used to attract
pollinators.
– The male reproductive organ is called the
stamen, a thin stalk that contains the anther,
where pollen is produced.
• The sperm cells are contained inside the pollen
grains.
Flower Structure and Function
– The female reproductive organ is called the
pistil.
• Several stamens usually surround the pistil.
• The bottom of the pistil is called the ovary, which
contains ovules, where the eggs are produced.
• Pollination
– The transfer of pollen grains from the stamen to
a pistil.
• Self-pollination - pollen from the same plant is
transferred to the pistil.
• Cross-pollination - when the pollen from another
plant is transferred to the pistil.
Flower Structure and Function
• After pollination the pollen grain develops a
pollen tube.
– The tube grow the entire length of the pistil.
– The sperm cell then travels down the tube to the
ovary.
• When the sperm cell comes in contact with
the egg’s cell fertilization occurs.
– The embryo plant then begins to develop.
– As the embryo grows the ovary gets larger.
Seeds and Fruits
• A fertilized ovule is called a seed.
– The seed contains an embryo plant and stored
food for the embryo.
• The seed that is formed remains inside the
ovary and we call this the fruit.
– All fruits contain seeds.
Seeds and Fruits
• Seed Dispersal:
– Some plants have fruits that are light and
shaped for travel by air (wind).
• Maple Tree, Ash Tree, or Elm tree - wing-like
structures
• Dandelion and Milkweed - cotton-like structures
Seeds and Fruits
– Some plants have fleshy fruits and animals
disperse them.
• Berries - animals eat them and the seeds are
dispersed in the animals waste.
Seeds and Fruits
– Some plants have fruits that float on water.
• Milkweed - air-filled pod
• Coconut
Seeds and Fruits
– Some plants have hook-like fruits that stick to
the fur of animals.
• Burdock
• Smooth beggartick or Smooth bur marigold (boogie
lice)
Seeds and Fruits
• Seed Germination
– The growth of an embryo plant from a seed is
called germination.
– The two main requirements for germination are
warmth and moisture.
Plant Growth and Responses
• The basic growth requirements for all plants
are:
– Light
• To carry out photosynthesis.
• Not all plants require the same amount of light.
• The amount of sunlight a plant receives determines
the time that it will flower.
– Summer flowering plants are long-day plants, they need
at least 12 hours of light a day.
– Fall flowering plants are short-day plants, they need less
than 12 hours of light a day.
– If the plant flowers anytime they are day-neutral plants,
amount of light is not a requirement.
Plant Growth and Responses
– Water
• Not all plants need the same amount of water.
• Too much water kills a cactus.
• Too little water kills a fern.
– Minerals
• The main minerals that plants need to survive are:
– Nitrogen (Nitrates)
– Phosphorous (Phosphates)
– Potassium
• These minerals are found in fertilizers.
Plant Growth and Responses
– Proper Temperature
• The seeds will not germinate until the soil is warm.
• Most plants grow best in temperatures between 10oC
and 38oC (50oF and 100oF).
• Growth Hormones
– Any organic chemical that makes an organism
grow is a growth hormone.
– Growth hormones are located in the root tips,
lateral buds, and terminal buds of the plants.
• Most plants are taller than they are wide because the
terminal bud makes hormones that prevent the
lateral bud from growing too fast.
Plant Growth and Responses
• Tropism
– The response to a stimulus is called tropism.
• Phototropism - Growth towards light.
• Geotropism - Stems grow up and roots grow down.
• Thigmotropism - Response to touch, like tendrils growing
around an object.