From Cell to Organ-7  

There are two Vascular Tissues, Xylem & Phloem. The conducting cells in the Xylem are called Tracheary Elements. These have thick walls that stain red due to the presence of Lignin. Lignin makes their cell walls watertight, inflexible and strong. Consequently, Tracheary Elements have two main functions, water transport and structural support. They usually have a hollow appearance because they are dead at maturity. These traits make Xylem relatively easy to spot in a cross section.

Phloem is associated with Xylem. Phloem cells do not generally have diagnostic traits like xylem but they can usually be discerned by their position. They have relatively thin cell walls and have a narrower diameter than the largest Tracheary Elements. In some cases,  Phloem exhibits a definite pattern of small cells and large cells. The large cells are Sieve Elements (S). These are the cells that are specialized for Sucrose Transport. The smaller cells are Companion Cells (CC). These regulate the physiological activities of the Sieve Elements. Phloem contains Callose. Callose regulates the size of the openings between adjacent Sieve Elements. Callose is a carbohydrate that stains with Aniline Blue and is fluorescent under Violet light. This is the best way to verify the presence of Phloem because Callose is not present in other cell types.