15.1 Introduction to the Plant Kingdom

Lesson Objectives

  • Identify traits of plants.
  • Explain the importance of plants.
  • Give an overview of the plant life cycle.
  • Describe how plants are classified.


Chapter 15.1 workbook pages

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  • alternation of generations
    • change back and forth from one generation to the next between haploid gametophyte and diploid sporophyte stages in the life cycle of plants
  • angiosperm
    • type of seed plant that produces seeds in the ovary of a flower
  • cone
    • structure consisting of scales that bear naked seeds in the type of seed plants called gymnosperms
  • flower
    • structure in angiosperms consisting of male and female reproductive structures that attracts animal pollinators
  • fruit
    • structure in many flowering plants that develops from the ovary and contains seeds
  • gametophyte
    • haploid generation in the life cycle of a plant that results from asexual reproduction with spores and that produces gametes for sexual reproduction
  • germination
    • early growth and development of a plant embryo in a seed
  • gymnosperm
    • type of seed plant that produces bare seeds in cones
  • lignin
    • tough, hydrophobic carbohydrate molecule that stiffens and waterproofs vascular tissues of plants
  • ovary
    • one of two female reproductive organs that produces eggs and secretes estrogen
  • plant
    • multicellular eukaryote with chloroplasts, cell walls made of cellulose, and specialized reproductive organs
  • rhizoid
    • hair-like structure in a nonvascular plant that absorbs water and minerals and anchors the plant to a surface
  • seed
    • structure produced by a seed plant that contains an embryo and food supply enclosed within a tough coat
  • sporophyte
    • diploid generation in the life cycle of a plant that results from sexual reproduction with gametes and that produces spores for asexual reproduction
  • vascular tissue
    • type of tissue in plants that transports fluids through the plant; includes xylem and phloem
  • vegetative reproduction
    • asexual reproduction in plants using nonreproductive tissues such as leaves, stems, or roots
  • weed
    • plant that is growing where people do not want it


Like the skunk cabbage, most of the plants you are familiar with produce flowers but many don’t. What traits define a plant?

What Are Plants?

Plants are multicellular eukaryotes with cell walls made of cellulose. Plant cells also have chloroplasts. In addition, plants have specialized reproductive organs. These are structures that produce reproductive cells. Male reproductive organs produce sperm, and female reproductive organs produce eggs. Male and female reproductive organs may be on the same or different plants.

How Do Plants Obtain Food?

Almost all plants make food by photosynthesis. Only about 1 percent of the estimated 300,000 species of plants have lost the ability to photosynthesize. These other species are consumers, many of them predators. How do plants prey on other organisms? The Venus fly trap in Figure below shows one way this occurs.

Venus fly trap plants use their flowers to trap insects. The flowers secrete enzymes that digest the insects, and then they absorb the resulting nutrient molecules.

 BBC – Venus Flytraps

What Do Plants Need?

Plants need temperatures above freezing while they are actively growing and photosynthesizing. They also need sunlight, carbon dioxide, and water for photosynthesis. Like most other organisms, plants need oxygen for cellular respiration and minerals to build proteins and other organic molecules. Most plants support themselves above the ground with stiff stems in order to get light, carbon dioxide, and oxygen. Most plants also grow roots down into the soil to absorb water and minerals.

The Importance of Plants

The importance of plants to humans and just about all other life on Earth is staggering. Life as we know it would not be possible without plants. Why are plants so important?

  • Plants supply food to nearly all terrestrial organisms, including humans. We eat either plants or other organisms that eat plants.
  • Plants maintain the atmosphere. They produce oxygen and absorb carbon dioxide during photosynthesis. Oxygen is essential for cellular respiration for all aerobic organisms. It also maintains the ozone layer that helps protect Earth’s life from damaging UV radiation.
  • Plants recycle matter in biogeochemical cycles. For example, through transpiration, plants move enormous amounts of water from the soil to the atmosphere. Plants such as peas host bacteria that fix nitrogen. This makes nitrogen available to all plants, which pass it on to consumers.
  • Plants provide many products for human use, such as firewood, timber, fibers, medicines, dyes, pesticides, oils, and rubber.
  • Plants create habitats for many organisms. A single tree may provide food and shelter to many species of insects, worms, small mammals, birds, and reptiles (see Figure below).
Red-eyed tree frogs like this one live in banana trees.

We obviously can’t live without plants, but sometimes they cause us problems. Many plants are weeds. Weeds are plants that grow where people don’t want them, such as gardens and lawns. They take up space and use resources, hindering the growth of more desirable plants. People often introduce plants to new habitats where they lack natural predators and parasites. The introduced plants may spread rapidly and drive out native plants. Many plants produce pollen, which can cause allergies. Plants may also produce toxins that harm human health (see Figure below). A terrific book that has some interesting facts about toxic plants is Wicked Plants: The Weed That Killed Lincoln’s Mother and Other Botanical Atrocities. You might be able to check it at out your local library!

Poison ivy causes allergic skin rashes. It’s easy to recognize the plant by its arrangement of leaves in groups of three. That’s the origin of the old saying, “leaves of three, leave it be.”

Creation Moments TV: Leaves of Three Let Them Be video

Life Cycle of Plants

All plants have a characteristic life cycle that includes alternation of generations. Plants alternate between haploid and diploid generations. Alternation of generations allows for both asexual and sexual reproduction. Asexual reproduction with spores produces haploid individuals called gametophytes. Sexual reproduction with gametes and fertilization produces diploid individuals called sporophytes. A typical plant’s life cycle is diagrammed in Figure below.

Life Cycle of Plants. This diagram shows the general life cycle of a plant.

Most plants reproduce with gametes using pollen and seeds, and they spend most of their life cycle as diploid sporophytes. Many plants can also reproduce asexually using roots, stems, or leaves. This is called vegetative reproduction. One way this can occur is shown in Figure below.

Strawberry plants have horizontal stems called stolons that run over the ground surface. If they take root, they form new plants.

Classification of Plants

The scientific classification of modern land plants is under constant revision. Informally, land plants can be classified into the groups listed in Table below. The most basic division is between nonvascular and vascular plants. Vascular plants are further divided into those that reproduce without seeds and those that reproduce with seeds. Seed plants, in turn, are divided into those that produce seeds in cones and those that produce seeds in the ovaries of flowers. You can read more about each of these groups of plants in the next lesson.

Major divisions and types of modern land plants are organized in Table below. Why do the first five types of plants require a moist habitat?

Major Division Types of Plants No. of Living Species Description
Nonvascular Plants
15.1f Liverworts 7,000
15.1g Hornworts 150
15.1h Mosses 10,000 They lack leaves and roots. They have no stems, so they grow low to the ground. They reproduce with spores. They need a moist habitat.
Vascular Plants
15.1i Clubmosses 1,200 They have roots and tiny leaves. They have no stems, so they grow low to the ground. They reproduce with spores. They need a moist habitat.
15.1j Ferns 11,000 They have large leaves in fronds. They have stiff stems, so they are tall growing; some are trees. They reproduce with spores. They need a moist habitat.
15.1k Ginkgoes 1
15.1l Cycads 160
15.1m Conifers 700
15.1n Gnetae 70 Most are trees with wood trunks. They have adaptations to dryness such as needle-like leaves. They reproduce with seeds and pollen. They produce seeds in cones.
15.1o Flowering Plants 258,650 They have tremendous diversity in size, shape, and other characteristics. They reproduce with seeds and pollen. They produce seeds in the ovaries of flowers. Ovaries may develop into fruits, which enhance seed dispersal.

KQED: Albino Redwoods, Ghosts of the Forest

Albino redwood trees? Really? Yes, these pale ghosts that hide amid their gigantic siblings, only a few dozen albino redwood trees are known to exist. They are genetic mutants that lack the chlorophyll needed for photosynthesis. But how and why they survive is a scientific mystery.

Watch one or both of these OPTIONAL videos:

Lesson Summary

  • Plants are multicellular eukaryotes. They have organelles called chloroplasts and cell walls made of cellulose. Plants also have specialized reproductive organs. Almost all plants make food by photosynthesis. Life as we know it would not be possible without plants.
  • All plants have a characteristic life cycle that includes alternation of generations. Asexual reproduction with spores produces a haploid gametophyte generation. Sexual reproduction with gametes and fertilization produces a diploid sporophyte generation.
  • The most basic division of living plants is between nonvascular and vascular plants. Vascular plants are further divided into seedless and seed plants. Seed plants called gymnosperms produce seeds in cones. Seed plants called angiosperms produce seeds in the ovaries of flowers.

Lesson Review Questions


1. What traits do all plants share?

2. What do plants need?

3. List reasons that plants are important to life on Earth.

4. When is a plant considered a weed?

5. What are vascular tissues? What is their function?

Apply Concepts

6. Draw a diagram of a typical plant life cycle that illustrates the concept of alternation of generations.

Think Critically

7. Compare and contrast gymnosperms and angiosperms, and give an example of each.

Reading Assignment


Read Chapter 7 from Evolution Exposed Biology:

The Origin of Plants


Previous: Protists, Fungi, and Human Disease

Next: Four Types of Plants

4 thoughts on “15.1 Introduction to the Plant Kingdom

  1. In the paragraph called “Life Cycle of Plants,” in the fifth sentence, there isn’t a space between the words “called” and “sporophytes.” Just so you know. Have a great day!

    1. Thank you SOOOOO much for letting us know! We really appreciate you taking the time to leave a comment! <3 It's fixed now! :-)

  2. In the workbook key, the answer to 15.1, vocabulary II, question 3 may be erroneous.
    “________ transports water from the roots, through the stem, and to the leaves.”
    The key says “non-vascular tissue.” I believe the answer is “vascular tissue.”

    1. Thank you so much!! I will get that fixed up asap!

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