Xylem-1

	311 Syllabus
Xylem-1
Xylem and Phloem are the Vascular Tissues in MOST plants. They are usually found together. Most people are familiar with vascular tissues in stems. Thus, we will start with stem cross-sections. The vascular tissues in stems are found within bundles which are vertically continuous over a relatively large distance. Cells in the xylem have thick lignified walls which stain red for lignin in most preparations. Cross-Section of a Dicot Stem Cells in the Xylem have stained red for lignin. The Conducting cells in the Xylem are called Tracheary Elements
Both Xylem and Phloem have cytological traits which distinguish them from the surrounding tissues. The Lignified walls of the Tracheary Elements in the Xylem are the most distinctive of these. Xylem & Phloem are said to be "complex tissues" because they contain Fibers and Parenchyma as well as Tracheary Elements (Xylem) & Sieve Elements (Phloem).
The most specialized cells of the Xylem are called Tracheary Elements. These serve two functions, physical support & water conduction. The two kinds of Tracheary Elements are Tracheids & Vessel Members. Vessel Members (VM) have large openings in their end walls. These are the "Perforation Plates". Perforation Plates facilitate water movement in the Xylem Tracheids are called Imperforate because they lack Perforation Plates. Tracheids have Pits on their end walls. Pits are not nearly as efficient for water translocation as Perforation Plates.
Note the ladder-like segments of cell wall which form the Perforation Plate (PP) on the end-wall of this Vessel Member. These restrict the flow of water somewhat but add some strength at the point where the Vessel Members connect. I used an embossing tool to generate the image on the right.
The most advanced VM have no wall obstructions between succeeding members of a Vessel. These have "Simple Perforation Plates". Both of the cells below have simple Perforation Plates. Elongated cells with a narrow diameter are better for support. Shorter wider cells are better for conduction.
Elongate Vessel Member with Simple Perforation Plates (PP).		Wide Vessel Member with a Simple Perforation Plate(PP). This kind of cell is better for conduction than physical support. Note the numerous PITS on the lateral walls.
A Vessel is a longitudinal series of Vessel Members end to end.	Segment from a Vessel: VM = Vessel Member, PP = Perforation Plate	Junction of 2 Vessel Members (VM). PP = Perforation Plate	A Vessel is a long series of Vessel Members end to end.
Tracheary Elements also contain many Pits on their side walls. This provides for lateral transport.
Tracheids are the other type of Tracheary Element. Tracheids are usually more elongate and narrower than Vessel Members. This is very significant because the ability of a tube to conduct water is directly relate to the fourth power of its diameter. Another critical difference is that tracheids are Imperforate. This means that they lack Perforation Plates. Tracheid end walls contain numerous Pits, which facilitate the longitudinal transport of water. These are far less efficient than Perforation Plates. Vessel Members are characteristic for Flowering Plants but are rare in non-flowering terrestrial plants. The most advanced angiosperms have both types of tracheary elements in their xylem.
Comparison of Tracheids and Vessel Members combined in a vertical column. A Vessel is composed of several Vessel Members. There is no collective term for a longitudinal series of Tracheids. The disparity in radial and longitudinal sizes demonstrate that a group of Tracheids can supply more structural support than Vessels & that Vessels can conduct more water.