15.2 Four Types of Plants

Lesson Objectives

• Describe modern nonvascular plants.
• Give an overview of living vascular plants.
• Outline the classification of seed plants.
• Summarize the characteristics of flowering plants.

WORKBOOK ASSIGNMENT:

Chapter 15.2 workbook pages

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Vocabulary

• bryophyte
  • type of plant that lacks vascular tissues, such as a liverwort, hornwort, or moss
• endosperm
  • stored food inside a plant seed
• nectar
  • sweet, sugary liquid produced by the flowers of many angiosperms to attract animal pollinators
• petal
  • outer parts of flowers that are usually brightly colored to attract animal pollinators
• phloem
  • type of vascular tissue in a plant that transports food from photosynthetic cells to other parts of the plant
• pistil
  • female reproductive structure of a flower that consists of a stigma, style, and ovary
• seed coat
  • tough covering of a seed that protects the embryo and keeps it from drying out until conditions are favorable for germination
• sepal
  • part of a flower that helps protect it while it is still in bud
• spermatophyte
  • type of plant that reproduces by producing seeds
• stamen
  • male reproductive structure of a flower that consists of a stalk-like filament and a pollen-producing anther
• tracheophyte
  • type of plant that has vascular tissues, such as a seed plant or flowering plant
• xylem
  • type of vascular tissue in a plant that transports water and dissolved nutrients from roots to stems and leaves

Introduction

From tiny nonvascular mosses to large flowering and fruiting trees, the types of living plants today reflect God’s amazing handiwork in the plant kingdom.

Nonvascular Plants

Nonvascular plants are called bryophytes. Despite the dominance of vascular plants, more than 17,000 species of bryophytes exist. They include liverworts, hornworts, and mosses.

Characteristics of Nonvascular Plants

Most bryophytes are small. They not only lack vascular tissues; they also lack true leaves, seeds, and flowers. Instead of roots, they have hair-like rhizoids to anchor them to the ground and to absorb water and minerals (see Figure below). Bryophytes occupy niches in moist habitats. Without the features of vascular plants, they are not very efficient at absorbing water.

Bryophytes also depend on moisture to reproduce. Sperm produced by a male gametophyte must swim through a layer of rainwater or dew to reach an egg produced by a female gametophyte. The tiny, diploid sporophyte generation then undergoes meiosis to produce haploid spores. The spores may also need moisture to disperse.

Diversity of Nonvascular Plants

The three types of  nonvascular plants are pictured in Figure below.

• Liverworts are tiny nonvascular plants that have leaf-like, lobed, or ribbon-like photosynthetic tissues rather than leaves. Their rhizoids are very fine, they lack stems, and they are generally less than 10 centimeters (4 inches) tall. They often grow in colonies that carpet the ground.
• Hornworts are minute nonvascular plants, similar in size to liverworts. They also have very fine rhizoids and lack stems. Their sporophytes are long and pointed, like tiny horns. They rise several centimeters above the gametophytes of the plant.
• Mosses are larger nonvascular plants that have coarser, multicellular rhizoids that are more like roots. They also have tiny, photosynthetic structures similar to leaves that encircle a central stem-like structure. Mosses grow in dense clumps, which help them retain moisture.

Vascular Plants

Vascular plants are known as tracheophytes, which literally means “tube plants.”

Vascular Tissues

The vascular tissues for which these plants are named are specialized to transport fluid. They consist of long, narrow cells arranged end-to-end, forming tubes. There are two different types of vascular tissues, called xylem and phloem. Both are shown in Figure below.

• Xylem is vascular tissue that transports water and dissolved minerals from roots to stems and leaves. This type of tissue consists of dead cells that lack end walls between adjacent cells. The side walls are thick and reinforced with lignin, which makes them stiff and water proof.
• Phloem is vascular tissue that transports food (sugar dissolved in water) from photosynthetic cells to other parts of the plant for growth or storage. This type of tissue consists of living cells that are separated by end walls with tiny perforations, or holes.

Diversity of Seedless Vascular Plants

There are 1,200 species of clubmoss and more than 20,000 species of fern. Both types of vascular plants are seedless and reproduce with spores. Two examples are pictured in Figures below and below.

• Clubmosses look like mosses and grow low to the ground. Unlike mosses, they have roots, stems, and leaves, although the leaves are very small.
• Ferns look more like “typical” plants. They have large leaves and may grow very tall. Some even develop into trees.

Seed Plants

Seed plants are called spermatophytes. Vascular seed plants dominate Earth.

Parts of a Seed

As shown in Figure below, a seed consists of at least three basic parts: the embryo, seed coat, and stored food.

• The embryo develops from a fertilized egg. While still inside the seed, the embryo forms its first leaf (cotyledon) and starts to develop a stem (hypocotyl) and root (radicle).
• The tough seed coat protects the embryo and keeps it from drying out until conditions are favorable for germination.
• The stored food in a seed is called endosperm. It nourishes the embryo until it can start making food on its own.

Many seeds have additional structures that help them disperse. Some examples are shown in Figure below. Structures may help them travel in the wind or stick to animals. Dispersal of seeds away from parent plants helps reduce competition with the parents and increases the chance of offspring surviving.

Classification of Seed Plants

The two major divisions of seed plants are the gymnosperms (seeds in cones) and angiosperms (seeds in ovaries of flowers). Figure below shows how the seeds of gymnosperms and angiosperms differ. Do you see the main difference between the two seeds? The angiosperm seed is surrounded by an ovary.

There are only about 1,000 living species of gymnosperms, whereas there are hundreds of thousands of living species of angiosperms. Because angiosperms are so numerous and diverse, they are described separately below. Living gymnosperms are typically classified in the divisions described in Table below. Most modern gymnosperms are trees with woody trunks. The majority are conifers such as pine trees.

	Division	Description
	Ginkgoes	There is only one living species (Ginkgo biloba); some living trees are more than 2000 years old; they originated in Asia but now are cultivated all over the world; they have been used for medicines for thousands of years.
	Conifers	There are more than 700 living species; most are trees such as pines with needle-like leaves; they are often the dominant plants in their habitats; they are valuable to humans for paper and timber.
	Cycads	There are about 300 living species; they are typically trees with stout trunks and fern-like leaves; they live only in tropical and subtropical climates; they have large, brightly-colored seed cones to attract animal pollinators.
	Gnetae	There are fewer than 100 living species; most are woody vines with evergreen leaves; they live mainly in tropical climates; they are the least well known gymnosperms but the most similar to angiosperms.

Flowering Plants

Angiosperms, or flowering seed plants, form seeds in ovaries. As the seeds develop, the ovaries may develop into fruits. Flowers attract pollinators, and fruits encourage animals to disperse the seeds.

Parts of a Flower

A flower consists of male and female reproductive structures. The main parts of a flower are shown in Figure below. They include the stamen, pistil, petals, and sepals.

• The stamen is the male reproductive structure of a flower. It consists of a stalk-like filament that ends in an anther. The anther contains pollen sacs, in which meiosis occurs and pollen grains form. The filament raises the anther up high so its pollen will be more likely to blow in the wind or be picked up by an animal pollinator.
• The pistil is the female reproductive structure of a flower. It consists of a stigma, style, and ovary. The stigma is raised and sticky to help it catch pollen. The style supports the stigma and connects it to the ovary, which contains the egg. Petals attract pollinators to the flower. Petals are often brightly colored so pollinators will notice them.
• Sepals protect the developing flower while it is still a bud. Sepals are usually green, which camouflages the bud from possible consumers.

Flowers and Pollinators

Many flowers have bright colors, strong scents, and sweet nectar to attract animal pollinators. They may attract insects, birds, mammals, and even reptiles. While visiting a flower, a pollinator picks up pollen from the anthers. When the pollinator visits the next flower, some of the pollen brushes off on the stigma. This allows cross-pollination, which increases genetic diversity.

Note: The following video has references to evolution. Whenever it says “evolved”, just think in your mind that God created it that way. 😉

See The beauty of pollination at http://www.youtube.com/v/xHkq1edcbk4?version for an amazing look at this process.

Although flowers and their components are the major innovations of angiosperms, they are not the only ones. Angiosperms also have more efficient vascular tissues. In addition, in many flowering plants, the ovaries ripen into fruits. Fruits are often brightly colored, so animals are likely to see and eat them and disperse their seeds (see Figure below).

Classification of Flowering Plants

There are more than a quarter million species of flowering plants, and they show tremendous diversity. Nonetheless, almost all flowering plants fall into one of three major groups: monocots, eudicots, or magnolids. The three groups differ in several ways. For example, monocot embryos form just one cotyledon, whereas eudicot and magnolid embryos form two cotyledons. The arrangement of their vascular tissues is also different. Examples of the three groups of flowering plants are given in Table below.

Lesson Summary

• Nonvascular plants are called bryophytes. They include liverworts, hornworts, and mosses. They lack roots, stems, and leaves. They are low-growing, reproduce with spores, and need a moist habitat.
• Vascular plants are known as tracheophytes. Vascular tissues include xylem and phloem. They allow plants to grow tall in the air without drying out. Vascular plants also have roots, stems, and leaves.
• Most vascular plants are seed plants, or spermatophytes. They reproduce with seeds and pollen. Some seed plants are gymnosperms that produce seeds in cones.
• Most seed plants are angiosperms that produce seeds in the ovaries of flowers. Ovaries may develop into fruits. Flowers attract pollinators, and fruits are eaten by animals, which help disperse the seeds.

Lesson Review Questions

Recall

1. Describe nonvascular plants.

2. Identify the parts of a seed and the role of each part.

3. Name and describe the division of gymnosperms.

4. Describe the male and female reproductive structures of flowers and their functions.

5. State how fruits help flowering plants reproduce.

Think Critically

6. Compare and contrast xylem and phloem.

Points to Consider

In this chapter, you read about the classification of plants. In the next chapter, you can read more about the special cells, tissues, and organs of plants that make them such important and successful organisms.

• How do you think plant cells differ from the cells of other eukaryotes, such as animals? What unique structures do plant cells contain?
• Besides vascular tissues, what other types of tissues do you think plants might have?

Previous: Introduction to the Plant Kingdom

Next: Chapter 16: Plant Biology

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