The vascular tissue system

The vascular tissue system is in the form of bundles or cylinders

Vascular tissue appears as individual bundles in young stems and as a solid cylinder in older, woody stems. It consists of water conducting and sugar conducting cells and may also contain strengthening fibre cells (sclerenchyma) and storage cells(parenchyma).

Conducting water: xylem cells

Xylem cells are dead when mature and functional with their empty interior serving as a conduit for the passage of water. They are often subjected to strong negative pressure and their collapse is prevented by the presence of strong secondary cell walls with a high cellulose and lignin content.

(© Pearson Education, Inc (Campbell, N, and Reece, R. J, Biology, 6th ed., p 725) Tracheid cells are narrow and overlapping and have thin circular areas in their walls called pits. Vessel elements (cells) are broad and arranged end to end. They have pits on their side walls but also have large wall-free (perforated) end walls. Both cell types are dead when mature and conduct water and dissolved minerals.

Pinus (pine) sp, stem, stained longitudinal section of wood. Tracheids overlap at their end walls and have large, bordered pits. They are the only water-conducting cell type in the xylem of most nonflowering plants. (photo, Larry Jensen)

Conducting sugars: phloem cells

Phloem cells are living when functional. They are often subjected to strong positive pressure.

©, McGraw-Hill Publishers (Stern, Kingsley, Jansky S., and Bidlack, J. E, The Biology of Plants) Conduction of sugars takes place within the living cytoplasm of sieve tube members and from one member to the next through pores in their adjacent sieve plates.

Longitudinal section of phloem tissue from a flowering plant. The circular sieve plates with pores are located on the end walls of adjacent sieve tube members (cells). (photo, Miss Edgar)