Vascular Plants

How is vascular tissue important?

For millions of years, early plants grew no taller than a meter high because they lacked vascular tissue. Then, about 420 million years ago, something remarkable happened. The small, mosslike plants on land were suddenly joined by taller plants, some of which were as large as small trees. What happened? Fossil evidence shows that these new plants were the first to have a transport system with true vascular tissue. Vascular tissue carries water and nutrients much more efficiently than does any tissue found in bryophytes. With the evolution of vascular tissue, plants were able to grow high above the ground.

Evolution of a Transport System Vascular plants are also known as tracheophytes (TRAY kee uh fyts), after a specialized type of water-conducting cell they contain. These cells, called tracheids (TRAY kee idz), are hollow tubelike cells with thick cell walls strengthened by lignin, as shown in Figure 22–12. Tracheids were one of the great evolutionary innovations of the plant kingdom.

Tracheids are found in xylem (ZY lum), a tissue that carries water upward from the roots to every part of a plant. Tracheids are connected end to end like a series of tin cans. Openings between tracheids known as pits allow water to move through a plant more efficiently than by diffusion alone.

Vascular plants also have a second transport tissue called phloem. Phloem (FLOH um) transports solutions of nutrients and carbohydrates produced by photosynthesis. Like xylem, the main cells of phloem are long and specialized to move fluids throughout the plant body. Vascular tissues––xylem and phloem––make it possible for vascular plants to move fluids through their bodies against the force of gravity.

ZOOMING IN

VASCULAR TISSUE

FIGURE 22–12 Horsetails are among the most primitive plant species to have specialized vascular tissue. The micrograph at right shows a much-magnified view of the tracheids. You can see the rings of reinforcing lignin surrounding the tracheids.

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