Unit 2 - Plants

• list the ways in which plants can be classified and the characteristics that define the plants in each group.

• list the types and describe the function of vascular tissue.

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

• compare and contrast flowering and non-flowering plants by the stages in their life cycles.

How are plants classified? What are the characteristics of the plants in each group?

How do a plant’s internal and external structures help it survive?

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

How are plants different in the ways they reproduce?

Plants are a very diverse group of organisms. Approximately 400,000 species have been named, described and classified by scientists and thousands of new species are discovered every year. With such biodiversity comes the need to classify plants into groups based on shared characteristics. Scientists do this in a number of ways. One way plants are classified is as vascular plants or non-vascular plants.

Vascular plants have vascular tissue – internal, tube-like structures to transport water, minerals and food throughout the plant. Non-vascular plants do not have vascular tissue.

Plants can also be classified by how they reproduce. Non-vascular plants and ferns do not produce seeds – they are seedless plants. They reproduce from spores. The spores are typically made of just one cell. Each spore is capable of developing into a new organism. In mosses, spores are contained in a capsule. In ferns, spores are usually located on the underside of the fronds in clusters called sori.

What four plant groups are represented in the photographs on this page?

Gymnosperms, such as conifers and firs, are cone-bearing plants. They reproduce from seeds held in cones. Angiosperms are flowering plants. They reproduce from seeds that develop in flowers and are held in fruits.

Discover more about classification and diversity within the plant kingdom on the NGScience website. QuickCode: B3G2

Many mosses are pioneer species. A pioneer species is one of the first organisms to grow in an area with little or no other plants. As more pioneer species grow, weather rocks, then die and decay, soil begins to form and the environment becomes suitable for a greater diversity of organisms.

If you walk along a forest stream, chances are you will spot mosses. Mosses are non-vascular plants. They do not have vascular tissue – xylem and phloem – to transport substances throughout the plant. They also lack true roots, stems and leaves.

To survive without vascular tissue, non-vascular plants are small and live close to the ground. They absorb the water and nutrients they need from their environment. The water then travels within the plant by moving from one cell to the next. Mosses, liverworts and hornworts are the main groups of non-vascular plants.

Non-vascular plants have root-like structures called rhizoids that anchor them in place. They can often be seen carpeting the moist forest floor, clinging to rocks or growing on the trunks of trees.

liverwort

Mosses and liverworts have tiny leaf-like parts called leaflets which are just one-cell thick. Most photosynthesis occurs in the leaflets.

Non-vascular plants are seedless plants. They reproduce from single-celled microscopic spores held in a part called the capsule. When the capsule opens, the spores are spread by water and wind to new places. Under the right conditions, new plants will grow from the spores.

Why are mosses small in size and found mostly in moist environments?

moss capsules containing spores

capsule

seta

foot

leaflets sporophyte

gametophyte

phloem xylem

Cross-section of a sunflower stem as observed under a microscope.

Imagine visiting your local park or garden. What plants would you see? How are the plants different? In what ways are they similar?

Almost all of the plants you spot in a park or garden are similar in that they are vascular plants. Vascular plants are plants that have tube-like vascular tissue to transport food, water and waste throughout the plant.

There are two main types of vascular tissue – xylem and phloem. Xylem transports water and nutrients, usually from the roots to the upper regions of the plant. Phloem transports food from the leaves to all parts of the plant.

The cells in vascular tissue are rigid and provide plants with support, allowing them to grow taller than non-vascular plants.

xylem phloem

water food

mosquito fern tree fern

Vascular plants can be further classified based on whether they produce seeds and where the seeds form.

Ferns are a diverse group of seedless vascular plants often identifiable by the feather-like leaves called fronds. The smallest of ferns, the mosquito fern, has fronds that are just a few millimeters in length. The largest of ferns is the tree fern which can reach heights of more than 20 meters and have fronds up to three meters in length.

Ferns reproduce from spores which are usually held on the underside of the fronds. The spores are dispersed by wind and water.

Horsetails are another kind of seedless vascular plants that reproduce from spores which are usually held in capsules.

Fern spores are held in enclosures called sporangia on the underside of fronds.

Why can ferns grow much larger than mosses?

In most horsetails, spores are held in capsules.

giant sequoia

Gymnosperms are the oldest seed-producing plants. Fossil evidence suggests they first appeared on Earth about 320 million years ago. Angiosperms, the other main group of seed-producing plants, first appeared about 135 million years ago.

ginkgo tree Plants that reproduce from seeds are called seed plants. The two main types of seed plants are gymnosperms and angiosperms.

Gymnosperms are non-flowering plants. They reproduce from seeds, but do not produce flowers or fruits. The word gymnosperm means ‘naked seed’ as the seeds are not enclosed in an ovary, but instead sit on the surface of leaf-like structures called bracts. The bracts form the reproductive organ called a cone.

The largest group of gymnosperms are conifers which include pines and cypresses. Some of the oldest and tallest plants are conifers, including the most massive tree on Earth, the giant sequoia.

Other gymnosperms include cycads and ginkgos.

Angiosperms are flowering plants. They produce seeds that are held in the ovaries of flowers. As the seed develops, the ovary forms a protective fruit around the seed. Angiosperms are the most diverse group of plants. More than 300,000 different species have been named and classified which accounts for approximately 80 percent of all known plant species.

Angiosperms have a range of adaptations that allow them to live and grow in all areas of the Earth. Many angiosperms are an important source of food for humans and other animals. All grasses, grains, fruits and vegetable plants are angiosperms. As are tulips, sunflowers, oaks and maples.

Botanical gardens and parks are a great way to observe plant biodiversity. Plan a visit with your family or friends. Classify the plants into groups.

vascular tissue

All of the roots of a plant make up its root system. In most vascular plants, the root system is underground. The root system has three main functions:

• take in water and minerals from the soil. • hold the plant in the soil. • store food.

root hair

root hair

root cap The outer layer of a root is called the epidermis. Attached to the epidermis are tiny roots hairs which are just one-cell thick. The root hairs absorb water and minerals which are transported from the root to the upper parts of the plant.

At the tip of a root is a root cap. It helps to protect the root as it grows down into the soil.

Running through the center of roots are vascular bundles which contain xylem and phloem. Xylem transports water and minerals through the roots to the upper parts of the plant, called the shoot system. Food is transported from the shoot system to the roots in phloem vessels.

Plants can have different roots that help them in different ways. Some roots are shallow and branch out to cover a large area. These roots, called fibrous roots, help the plant to take up lots of water when it rains. Branching out over a large area also helps to anchor the plant in place.

phloem

xylem

root hair

root section epidermis

Other plants have one main root, called a taproot, which grows deep into the soil. It allows the plant to reach water deeper in the soil.

Some plants have roots that grow above ground, called aerial roots. An epiphyte is a plant that grows on another plant, often on the trunks of trees. Many ferns and orchids are epiphytes. Their aerial roots take in rainwater and water vapor from the air. Some epiphytes are also able to take in moisture through their leaves.

The stem is usually the part of the plant that connects the root system to the upper portions of the plant. The stem provides support for the plant. It holds up the leaves allowing them to spread out and take in more sunlight. It also supports the reproductive structures.

Stems contain vascular tissue that transports substances throughout the plant. Xylem vessels transport water and minerals from the root system to the leaves. Phloem vessels transport food from the leaves to all parts of the plant.

phloem xylem epidermis

tomato plant stem section

Some plants use their stems for storage. Bamboo and sugarcane store food in their stems. Many cacti store water in their stems.

What are the main functions of the stem?

Some plants, such as the tomato plant on the previous page, have green stems that are thin and can bend more easily than others. They are called herbaceous stems. A plant with a herbaceous stem is called a herbaceous plant. Herbaceous stems get their green color from chloroplasts in the cells. Vegetable plants such as tomatoes, beans, peppers and herbs have herbaceous stems. A soft and flexible herbaceous stem cannot support the weight of the extensive shoot system of a tree.

Trees are plants with thick, strong stems covered in bark. They do not bend easily. These stems are called woody stems. A plant with a woody stem is called a woody plant Many shrubs also have woody stems. Woody stems are usually brown in color. The cells in woody stems do not contain chloroplasts.

The bark surrounding woody stems provides protection against diseases and extreme weather. In cold regions, plants with woody stems survive through the colder months, but may lose their leaves to conserve energy. Plants with herbaceous stems may die back to soil level before re-emerging in the warmer months.

How does the bark surrounding a woody stem help the plant?

Leaves are structures that are highly specialized to carry out photosynthesis. They are often flat, broad and branch out in all directions to maximize their surface area. This enables them to absorb as much sunlight as possible.

Running through a leaf are veins containing vascular tissue. Xylem vessels deliver water that has moved from the roots, through the stem to the leaves. Food produced during photosynthesis is transported from the leaves to all parts of the plant in phloem vessels.

The top of a leaf is a layer of cells called the epidermis. A waxy covering on the epidermis, called the cuticle, helps to prevent water loss due to evaporation.

On the underside of a leaf are many tiny openings called stomata (singular stoma). Air, including the carbon dioxide needed for photosynthesis, enters the leaf through the stomata.

cuticle

epidermis

palisade layer vein

stoma phloem leaf anatomy xylem

Below the epidermis is a layer of cells called the palisade layer. Cells in this part of the leaf contain many chloroplasts which are the organelles that carry out photosynthesis.

Inside the chloroplasts, light energy absorbed by the green pigment chlorophyll combines carbon dioxide and water to produce sugar in the form of glucose. Oxygen is produced as a by-product. The glucose provides the plant with the energy it needs to carry out life processes.

The oxygen produced during photosynthesis is released into the air through the stomata.

sunlight

Watch photosynthesis in action in an animated video on the NGScience website. QuickCode: S7S2

water

sugar

carbon dioxide oxygen

During photosynthesis, plants use the energy in sunlight to convert water and carbon dioxide into sugar and oxygen.

moss sporophytes

Non-vascular plants are seedless plants with a life cycle that involves both asexual and sexual reproduction. Let’s look at the life cycle of non-vascular plants by looking at the stages in the life cycle of mosses.

In the asexual phase, spores are produced and held in a capsule. When the capsule breaks open, the spores are released into the air. They are spread to new places by wind or moving water.

spores released

sporophyte

Under the right conditions, a single spore will germinate and grow into a leafy gametophyte. The gametophyte has male and female parts.

Fertilization occurs when a sperm cell from the male part fuses with an egg cell in the female part. The fertilized egg will develop into a new moss sporophyte. The sporophyte will produce spores held in capsules and the life cycle continues.

germinating spore leafy gametophytes

gametophyte

fertilization

fern spores held in sporangia

Ferns are seedless plants that reproduce from spores. Like the life cycle of mosses, the life cycle of ferns involves two distinct phases – an asexual phase and a sexual phase.

In the asexual phase, spores form in clusters on the underside of mature fern fronds. The spores are held in cases called sporangia. Once the spores are developed, the sporangia breaks open and the spores are dispersed by wind and moving water.

sporangia

sporophyte

mature sporophyte

Under the right conditions, a single spore will germinate and grow into a heart-shaped plant called a gametophyte. Root-like rhizoids anchor the gametophyte to the ground. The gametophyte has both male and female parts.

Sperm cells make their way from the male part to the female part and fuse with female egg cells. The fertilized egg cell develops into an embryo. It eventually grows larger than the gametophyte and becomes a sporophyte. The sporophyte produces spores and the two-phase life cycle repeats.

spores released

gametophyte with rhizoids developing

fertilization gametophyte

The reproductive structures on gymnosperms are cones. Male cones contain pollen with sperm cells inside and female cones contain eggs cells. Usually, the female cones are larger and are found in the upper regions of a gymnosperm tree. Male cones are smaller and are found mostly in the lower regions.

Pollination occurs when pollen grains, usually blown by wind, are transferred from the male cone to the female cone.

female cone with eggs

male cone with pollen

mature gymnosperm

sapling emerges

The pollen grain develops a pollen tube allowing a sperm cell to reach and fertilize an egg cell. The fertilized egg cell then develops into a seed. Mature seeds are usually dispersed by wind to new places.

Under the right conditions, each seed will grow into a sapling before growing into a mature tree with reproductive cones and the life cycle repeats.

pollen grain develops pollen tube

sperm cell fertilizes egg cell

Male cone releasing pollen into the air.

seeds disburse seed develops

Angiosperms are flowering plants. They reproduce by sexual reproduction whereby male sex cells, called sperms, unite with female sex cells, called eggs.

Pollination occurs when pollen grains from the anther are transferred to the stigma of a flower. This can occur within the same flower, and also between two flowers of the same species.

When a pollen grain lands on the stigma, it produces a pollen tube that grows into the stigma, through the style to the ovary. Inside the ovary are ovules. Each ovule contains an egg cell.

A sperm cell moves from the pollen grain on the stigma, through the pollen tube to the ovary. Fertilization occurs when the sperm cell reaches and fuses with an egg cell.

pollen is released

Compare and contrast the stages in the life cycles of mosses, ferns, gymnosperms and angiosperms.

sperm moves down pollen tube

sperm fertilizes egg cell

Following fertilization, the flower petals fall away and the ovule begins to develop into a seed. The ovary of the flower swells to form a protective fruit. Once developed, the seeds need to be dispersed to new places. This can occur when animals feed on fruits or when seeds and fruits stick to their bodies. Fruits and seeds can also fall away and be carried to new places by wind or water.

In its new location and under the right conditions, the seed will germinate and grow into an adult plant.

seed is dispersed and germinates

fertilized ovule develops into a seed

Use the words to complete the sentences.

1.

2.

3. are plants that do not produce seeds.

transports water and nutrients, usually from the roots to the upper regions of the plant.

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

4. A has a soft, flexible green stem.

5. A has a thick, strong stem covered in bark.

6. One main root which grows deep into the soil is called a .

7. Plants that contain vascular tissue are called .

8. Plants that do not have vascular tissue are called .

9.

10. an ovary.

in ovaries. are seed-producing plants with seeds not enclosed in

are seed-producing plants with the seeds enclosed

11. Roots that grow above ground are called .

12. are shallow and branch out to cover a large area.

1. Create a Venn diagram to compare mosses and ferns.

2. Create a Venn diagram to compare gymnosperms and angiosperms.

3. Draw a simple labeled diagram to show the function of vascular tissue in the stem of a plant.

4. What is the function of stomata? Where are they mostly found?

5. What do plants need to photosynthesize?

What is produced as a result?

6. What is the name given to the spore-producing asexual phase in the life cycle of mosses and ferns?

7. What role does a fruit play in the reproduction of angiosperms?