Unit 2 - Plant Structures and Functions

2Plant Structures and Functions

What are the external and internal structures of plants?

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Access interactive content relating to this topic on the NGScience website.

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In this chapter you will ...

• identify and list the external structures of plants.

• identify and list the internal structures of plants.

• describe a plant system in terms of its components and their interactions.

• construct an argument that plants have internal and external structures that function to support survival, growth and reproduction.

How do a plant’s structures support its survival, growth and reproduction?

flower

fruit

stem

Plant Structures

Plants are made up of different parts or structures. Many plants, for example, have roots, stems and leaves. There are also parts inside plants, such as the tubes that transport water and nutrients around the plant. All of the structures of a plant interact and work together as a system to support the survival, growth and reproduction of the plant.

What are the functions of the tomato plant structures you can see here?

leaf

roots

Each plant structure has a function that helps the plant to survive, grow and reproduce. The bright petals of flowers function to attract bees and other pollinators.

The thorns on a blackberry bush help to protect its leaves and fruits from grazing by wildlife. If one or more plant structures are removed or do not function as they are supposed to, the survival of the whole plant system may be affected.

bee pollinating flower

Discuss what would happen to a plant if all of its leaves were removed.

AB Activity 2.1

blackberry thorns

External Structures

Structures that are observable on the outside of a plant are external structures. These include roots, stems, leaves, flowers and fruits.

Roots are usually the external structures that grow down into the soil. The primary function of the roots is to take in the water and nutrients the plant needs to survive and grow. Some roots are shallow and branch out to cover a large area. These roots are called fibrous roots. Fibrous roots help the plant to take up lots of water when it rains. Growing out from the roots are microscopic root hairs. The root hairs help in absorbing water and nutrients from the soil.

Root hairs take in water from the soil.

root hairs

Tree roots branch out and hold the tree firmly in the soil.

dandelion with taproot

Other plants have one main root, called a taproot, which grows deep into the soil. This helps to reach water deeper in the soil. Roots also hold plants firmly in place. This helps to prevent them from being washed away by water or blown away by wind.

How are the shapes of roots suited to their function?

Amazing Fact!

The shepherd tree grows in the very dry grasslands of South Africa. It has the deepest roots of all plants – reaching depths of over 60 meters!

AB Activity 2.2

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Observe how roots take in water and minerals in an animated video on the NGScience website. QuickCode: Y3G5

Try This!

Design and construct a device that mimics the external structures of a plant to produce electricity using solar cells. Many plants have a stem. Stems can be different in many ways, but they usually perform two important functions for the plant. Plants need the energy from sunlight in order to make food through the process of photosynthesis – a process that takes place in the leaves. One function of the stems in many plants is to hold up the plant. This allows the leaves to absorb more sunlight. Stems also hold up the plant’s reproductive structures – flowers and fruits.

Some plants have stems that are soft and can bend easily. They are called herbaceous plants. Herbaceous stems grow quickly.

Other plants, such as trees, have stiff, strong stems. They are called woody plants. The strong stems of woody plants grow thicker and taller each year. This allows the plants to reach high into the sky where the leaves can trap the sunlight they need to photosynthesize. For protection against animals, some plants have coverings on their stems. Trees have an outer layer of bark. Some plants like roses and bougainvilleas have stems with thorns for protection against animals.

Try This!

With your classmates, visit a place in your area that has lots of plants. Classify the plants you spot into groups. Draw a diagram or create a chart to show how you classified the plants.

AB Activity 2.3

maple leaves eucalyptus leaves

Amazing Fact!

The bryophyllum plant can reproduce from its leaves. Small plantlets with roots develop along the edges of the leaves. The plantlets can grow into new plants when they fall to the ground. How is the shape of leaves suited to their primary function?

Plants are organisms that absorb the energy in sunlight to make food through the process of photosynthesis. In most plants, photosynthesis takes place in the leaves. To take in lots of sunlight, leaves are external structures that are often flat and branch out to cover a large area. The key to making food is what lies inside the leaf. We’ll take a look at that in more detail later.

Victoria lily pads

Leaves have a large surface area for absorbing sunlight, but this can also lead to water loss. To reduce water loss, many leaves have a waxy outer covering called a cuticle. Some plants in hot and dry environments like deserts store water in their leaves. They have a thick cuticle to prevent water loss.

What is the function of the cuticle on a plant’s leaves?

Think Deeply

The leaves of a cactus are modified into sharp spines and do not contain chloroplasts. How is a cactus able to photosynthesize?

AB Activity 2.4

longhorn beetle covered in pollen hummingbird feeding on nectar

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Watch the process of pollination in an animated video on the NGScience website. QuickCode: E1V7 Flowers are external structures found on flowering plants. Their primary function is to help the plants reproduce. The sepals are often at the base of a flower. Their function is to protect the flower as it develops. For most flowering plants to reproduce, pollination needs to occur. Pollination is the transfer of pollen from the male part of the flower, the stamen, to the female part of the flower, the pistil. The petals are often the brightly-colored part of a flower that surround the reproductive parts. The petals help to attract pollinators, such as insects and birds, which transfer pollen between flowers and other plants.

dandelion fruit

Flowers develop into fruits which help to protect the seeds from which flowering plants reproduce. Fruits can also help plants to reproduce by spreading seeds to new places.

By spreading seeds to new places, the seeds are able to grow in areas away from the parent plant where there is less chance of competition for water, sunlight and nutrients.

Think Deeply

Many plants produce seeds that are dispersed to new places by wind. What characteristic would you expect these seeds to share?

AB Activity 2.5

A Closer Look Seed Dispersal

Most flowering plants reproduce from seeds. For seeds to germinate and grow, they often need to be scattered in areas away from the parent plant where there is less competition for the water, nutrients and sunlight the new plants need to survive. This process of scattering seeds to new places is called seed dispersal.

Flowering plants have different adaptations to disperse their seeds in different ways. Some plants have fruits that burst open and throw the seeds into the air. Seeds can also be dispersed by wind and water.

milkweed seeds

Go Online!

Learn all about seed dispersal in a video on the NGScience website. QuickCode: E7X1

coconut floating on water

Many flowering plants develop fruits and seeds that are moved to new places by the animals they attract. Seeds can be dispersed when animals that feed on the fruits move or drop the seeds. Seeds may stick to the fur or feathers of animals and are moved to new places when they fall off. Seeds can also be dispersed when they are eaten. They are moved to new places in the animal’s waste.

How does seed dispersal help plants survive?

Engineer It!

Design and build a seed that can be dispersed by wind. Test and evaluate your design with your classmates.

young wombat with seeds stuck to its fur

squirrel holding an acorn fruit from an oak tree

AB Activities 2.6 – 2.7

Internal Structures

The roots, stem, leaves and flowers are external structures that each play a role in helping the plant function as a whole system. There are structures inside a plant too. They are called internal structures. Like external structures, each internal structure has an important role in helping the plant function as a whole system. One important internal structure is called vascular tissue. Vascular tissue is a series of tubes that transports water, nutrients and food throughout the plant. There are two main structures within the vascular system – xylem and phloem.

Amazing Fact!

Mosses are non-vascular plants. They don’t have xylem to transport water. In fact, they don’t have roots at all. They absorb water and nutrients directly into their bodies. They have root-like rhizoids which help to hold them in place. xylem phloem

root section root hair

phloem xylem Xylem are tubes that transport water and nutrients from the roots, through the stem to the leaves. Phloem are tubes that transport food in the form of sugar from the leaves to all parts of the plant. The vascular tissue in the roots, stem and leaves is arranged in different ways to help the plant function and meet its needs.

Go Online!

Observe how water, food and nutrients are transported through the stem in an animated video on the NGScience website. QuickCode: P6Z5

stem section

AB Activity 2.8

What are the primary functions of the xylem and phloem in vascular plants?

A Closer Look Internal Leaf Structures

The leaves of the plant are where the food-making process of photosynthesis takes place. In order to photosynthesize, the external and internal structures in the leaves work together as a system. Let’s take a closer look! Water and nutrients travel in the xylem from the roots through the stem to the cells in the leaves. Tiny holes, mostly on the underside of the leaves, called stomata, open and close to take in the air the plant needs.

phloem

xylem

stoma

vascular tissue in leaf leaf section

cells on underside of leaf

stoma chloroplast

Inside the cells of a leaf are special structures called chloroplasts. Chloroplasts are the part of the cell where photosynthesis takes place. In chloroplasts, the energy from sunlight is used to convert carbon dioxide and water into food for the plant. The food is transported from the leaves to the rest of the plant in phloem tubes.

AB Activities 2.9 – 2.10 Think Deeply

Would you expect to find chloroplasts in animals cells? Explain your answer.

Go Online!

Observe how water and food are transported within leaves in an animated video on the NGScience website. QuickCode: X9Y7

anther We have learned that flowers are the external reproductive structures of flowering plants. Internal structures within the flower also play an important function in reproduction. When pollination occurs, pollen grains, which are male sex cells, move from the stamen to the pistil. The pollen grains move down a pollen tube to the ovary. The ovary holds the eggs – the female sex cells.

stamen pistil

Fertilization occurs when the male sex cell joins with the female sex cell. The fertilized egg cell then develops into a seed. The petals and reproductive parts of the flower fall away and the ovary develops into a fruit. Once developed and under the right conditions, the seeds will be dispersed and grow to become new plants.

Once fertilized, the ovary in a tomato flower develops into a protective fruit.

Think Deeply

What is the difference between pollination and fertilization?

A Closer Look Parts of a Flower

petal

stamen anther

filament

sepal

AB Activities 2.11 – 2.14

stigma

style pollen tube

ovule ovary pistil

Science Words

Use the words to complete the sentences.

fibrous roots taproot herbaceous plants woody plants cuticle sepals pollination stamen pistil petals seed dispersal vascular tissue xylem phloem stomata fertilization

1.

2. is the scattering of seeds to new places.

are roots that are shallow and branch out.

3. The series of tubes that transport water, nutrients and food through a plant is called .

4. transports water and nutrients from the roots to the upper parts of the plant.

5. transports food from the leaves to all parts of the plant.

6.

7.

the are openings in the leaf that take in air.

is the transfer of pollen from the male part of the flower, , to the female part of the flower, the .

8. Many leaves have a waxy outer covering called a .

9. When a male sex cell joins with a female sex cell, occurs.

10. The are often brightly-colored and surround the reproductive parts of a flower.

11. are often at the base of a flower and help to protect the flower as it develops.

12. have stems that are soft and bend easily. 13. Some plants have one main root called a that grows deep

into the soil.

14. have stiff strong stems often covered in bark.

Review

1. Copy and complete the chart below.

External Structure

roots stem leaves

Functions

2. What is the primary function of root hairs? 3. How can you distinguish between herbaceous plants and woody plants? 4. On leaves, what is the primary function of the cuticle? 5. Draw a diagram to show the process of pollination. 6. List two ways plants disperse their seeds.

7 Copy and complete the chart below.

Internal Structure

xylem phloem chloroplasts

8. Describe the process of fertilization.

Functions