Zea mays: Detail of a single atactostelic vascular bundle
This micrograph shows a typical Zea mays
vascular bundle in cross section. The phloem consists of angular,
thin-walled sieve tubes, between which you can see small, almost
square companion cells. You can see the xylem tissue below
the phloem. It consists of two large-diameter metaxylem vessels (MX)
which are separated from one another by smaller tracheids. A
solitary early protoxylem vessel (PX) is all that is left of
the first-formed xylem in this vascular bundle. Beneath this, is a large,
irregularly shaped lacuna (PXL) - which represents the space
previously occupied by the first protoxylem
element to be formed in this vascular bundle. Beneath and surrounding the
protoxylem lacuna, are mostly lignified, fibrous, thick-walled
The bundle is surrounded by a parenchymatous bundle sheath (BS).
Question: How does the protoxylem lacuna come about? Explain what has happened here.