The Science of Biology
Vascular Plants: Structure and Function
The vascular tissues include xylem, which conducts water and minerals from the roots upward and throughout the plant, and phloem, which transports dissolved foods in all directions within the plant.
The main conducting vessels of xylem are the tracheids and the vessels. Tracheids are long, thin tubes found in most vascular plants, while vessels are large tubes found predominantly in angiosperms. The tracheids and vessels form pipelines that have pores and perforated ends, which allow water and minerals to be conducted from one tube to the next and out to the surrounding tissues. Tracheids and vessels also help support the plant body.
The main conducting cells of phloem are sieve cells and sieve tube members. Both cell types have numerous pores through which substances are exchanged with adjacent cells. Sieve tube members occur in angiosperms, while sieve cells are found in other vascular plants. In angiosperms, small cells called companion cells assist the sieve tube members in their functions.
