Biology: Plant Organs, Tissues, and Cells: Part Five
Vascular Tissue: Xylem
Xylem is one of the major types of transport tissue found in vascular plants. Xylem cells conduct water and minerals from the plant’s roots to its leaves. These cells also provide a great deal of structural support for the plant as well. The cell walls contain the complex chemical, lignin. Parenchyma cells can occur within xylem tissue, but the more identifiable cells are tracheids and vessel elements, which tend to retain a red stain. The best analogy for xylem cells is a series of soda straws joined end-to-end to make a very long straw through which water moves.
Xylem. a) cross section view of xylem vessels (the large red cells) and tracheids (the smaller red cells amongst the vessels) in a vascular bundle of a cucumber; b) three-dimensional view of the xylem cells.
Tracheids are the more primitive of the two main cell types that make up xylem tissue. They occur in the earliest vascular plants (some 400 million years ago). Tracheids are long and tapered. Angled end plates connect the cells where they join end-to-end. Vessel elements are shorter, much wider, and lack end plates. Vessel elements stack on top of one another to form long tubes known as vessels. These vessels occur only in the flowering plants, the most recently evolved division of plants. The evolutionary trend in vessels is toward shorter cells. To allow lateral passage of material in and out of the tracheids or vessels, the cells have many pits on their walls.
Plants produce two types of xylem. Primary xylem is produced from cells of the root or apical meristem. This xylem adds to the height or length of the plant. When some plants get older, they need a stronger stem. To meet this need, they produce large quantities of secondary xylem from the vascular cambium meristem. Secondary xylem cells occur in rows or ranks and form what we commonly call wood.
Vascular Tissue: Phloem
Phloem cells conduct food from the leaves to the rest of the plant. Phloem is usually located outside the xylem. The two most common cells in the phloem are companion cells and sieve cells. Both types of cells in the phloem are alive at maturity and tend to retain the stain fast green. Companion cells retain their nucleus and control the adjacent sieve cells. Dissolved food, such as sucrose, flows through the sieve cells by the bulk flow process. When maple trees are “tapped” to gather sap for the making of maple syrup, some of the tree’s phloem sap is diverted and collected in a bucket.
Phloem: a) sieve cell with sieve plate and companion cell, showing the size and position relationships; b) lengthwise section of the phloem showing the sieve plate from the side. Note: students often will not see the sieve plate as it is not in the slice cut for a prepared slide.
Now go on to the next part. 
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