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Vascular Plants
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: July 22, 2012
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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C ONCEPT
1
1
Vascular Plants
• Give an overview of living vascular plants.
How does water move from the roots to the top of these trees?
Redwood trees found in Yosemite National Park in California. Big? Of course. How do these trees grow so tall? It
has a lot to do with a very efficient system to move water, sugars and other nutrients. But the first plants to have such
a "vascular system" were not tall trees, but much smaller plants.
Vascular Plants
Vascular plants are known as tracheophytes, which literally means “tube plants.” The earliest vascular plants quickly
came to dominate terrestrial ecosystems. Why were they so successful? It was mainly because of their tube-like
vascular tissues.
Concept 1. Vascular Plants
2
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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 1.1.
• 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.
FIGURE 1.1
Xylem and phloem are the two types of
vascular tissues in vascular plants.
Evolution of Vascular Plants
The first vascular plants evolved about 420 million years ago. They probably evolved from moss-like bryophyte
ancestors, but they had a life cycle dominated by the diploid sporophyte generation. As they continued to evolve,
early vascular plants became more plant-like in other ways as well.
• Vascular plants evolved true roots made of vascular tissues. Compared with rhizoids, roots can absorb more
water and minerals from the soil. They also anchor plants securely in the ground, so plants can grow larger
without toppling over.
• Vascular plants evolved stems made of vascular tissues and lignin. Because of lignin, stems are stiff, so plants
can grow high above the ground where they can get more light and air. Because of their vascular tissues, stems
keep even tall plants supplied with water so they don’t dry out in the air.
• Vascular plants evolved leaves to collect sunlight. At first, leaves were tiny and needle-like, which helped
reduce water loss. Later, leaves were much larger and broader, so plants could collect more light.
With their vascular tissues and other adaptations, early vascular plants had the edge over nonvascular plants. The
could grow tall and take advantage of sunlight high up in the air. Bryophytes were the photosynthetic pioneers onto
land, but early vascular plants were the photosynthetic pioneers into air.
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Diversity of Seedless Vascular Plants
Surviving descendants of early vascular plants include clubmosses and ferns. 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 1.2 and 1.3.
• 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.
FIGURE 1.2
Clubmosses like these are often confused with mosses.
Vocabulary
• clubmoss: Vascular plant similar to the nonvascular moss, but with roots, stems, and leaves; similar to early
vascular plants.
• ferns: Early vascular plants; more "plant-like" than the clubmosses; have stems, leaves, and roots.
• leaves (singular, leaf): Vascular plant organ specialized to collect sunlight for photosynthesis.
• lignin: Tough, hydrophobic carbohydrate molecule that stiffens and waterproofs vascular tissues of plants.
Concept 1. Vascular Plants
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FIGURE 1.3
There’s no confusing ferns with mosses.
Why do these ferns look more plantlike?
•
•
•
•
•
phloem: Type of plant vascular tissue; transports food from photosynthetic cells to other parts of the plant.
root: Vascular plant organ that can penetrate soil and rock; absorb water and minerals.
stem: Plant organ that holds plants upright, so they can obtain sunlight and oxygen.
tracheophyte: Type of plant that has vascular tissues, such as a seed plant or flowering plant.
xylem: Type of plant vascular tissue; transports water and dissolved nutrients from roots to stems and leaves.
Summary
• 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.
Practice
Use this resource to answer the questions that follow.
• http://www.hippocampus.org/Biology → Biology for AP* → Search: The Rise of Vascular Plants
1.
2.
3.
4.
Distinguish between xylem and phloem.
Distinguish between the shoot and the roots.
Describe the roles of the leaf and the stem.
What is a rhizome?
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5
5. List three types of seedless vascular plants.
6. What happened to forests of seedless vascular plants?
Review
1. Compare and contrast xylem and phloem.
2. How did vascular tissues and lignin allow vascular plants to be “photosynthetic pioneers into air”?
Concept 1. Vascular Plants