Plant Structure and Function

The Fascinating World of

Plant Structure

With added feature:

Plant Function

See it today! Now in Technicolor.

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 PlantVenus 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.