Zea mays large vascular bundle

The large  Zea stem bundle shown here illustrates the typical construction of a monocotyledonous vascular bundle at maturity. The phloem consists of large-diameter sieve tubes and smaller companion cells. Some phloem parenchyma occurs at the periphery of the phloem strand and is also located between the large metaxylem vessels (MX) which are bridged by several smaller tracheids, above the remaining protoxylem vessel. Immediately beneath the protoxylem vessel, is a protoxylem lacuna (PXL) which was formed during stem elongation, when the increasing tension on the coiled secondary wall material resulted in tearing and rupturing of the protoxylem, leaving behind a cavity, surrounded by xylem parenchyma cells. The bundle is surrounded by a parenchymatous bundle sheath.

 

Question: What is the function of the parenchyma cells which are associated with the xylem?

Click here to see the phloem in more detail