No secondary phloem in monocots

Protophloem may be crushed

A transverse section of a closed vascular bundle in corn (Zea mays) stem. As in other monocotyledons, no secondary vascular tissues are formed, and the descending transport of sugars occurs through the primary phloem, namely in the metaphloem.

In corn, protophloem sieve elements are crushed at an early stage. Note the small densely cytoplasmic companion cells, and "empty" sieve tube members.

1 = sclerenchyma bundle sheath; 2 = crushed protophloem; 3 = sieve tube of metaphloem; 4 = metaphloem companion cell; 5 = metaxylem vessel; 6 = protoxylem vessel; 7 = protoxylem lacuna