Fig. 11.5-5. Transverse section of corn stem (Zea mays). This monocot vascular bundle shows several interesting and common features.
1) The bundle sheath is very thin, only a single layer on the sides and just a few fibers at the top and bottom of the bundle (the top would be the toward the outside of the stem, the bottom of the micrograph is toward the center of the stem). This vascular bundle was located near the center of the stem; those nearer to the surface of the stem have thicker sheaths with more fibers.
2) The phloem is an excellent example of the monocot type of phloem in which the sieve tube members and companion cells form a regular pattern.
            3) The two metaxylem vessels (marked by v) are very large and prominent.
            4) The protoxylem vessel has been torn because the stem continued to elongate long after the vessel finished differentiation and died; its primary wall and its secondary wall (which would have had either an annular or helical pattern) were ripped apart creating a tubular space called a lacuna (marked by L).

            Vessels are extremely common in dicots, and we can more or less safely assume that any wide tracheary element in dicots is a vessel. But in many monocots, vessels are present in only certain parts of the plant, for example, just in the root or just the stem or just the inflorescence, so it can be difficult to be certain in transverse sections if a wide element is a vessel or a tracheid.