23.4: Specialized Vascular Tissue
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When you looked at vascular tissue during the anatomy portion of this class, you learned that the conducting cells in xylem are tracheids and vessel elements, while in phloem they are sieve cells, sieve tube elements, and companion cells. However, not all plants have the same vascular composition. In SVPs and gymnosperms (with the exception of gnetophytes), the xylem contains only tracheids, while the phloem contains only sieve cells. These were the earliest versions of conducting cells. It was not until the angiosperms that vessel elements, sieve tube elements, and companion cells evolved.
Recall the experiment you did in Lab Roots and the Movement of Water. How did the diameter of the straw influence the height the water moved up the straw?
How does this relate to the ability of angiosperms to compete with gymnosperms?