Angiosperms have discrete bundles – eusteles. Each little orange bundle has some xylem and
some phloem – typically phloem to outside of stem and xylem to inside
Cell divides longitudinally into empty sieve zombie cell and companion cell
Tracheary elements which are either
Better management of cavitation, but low flow pressure
Water moves faster in a vessel, wider, but high risk of cavitation
Functional xylem all dead at maturity, heavily lignified walls to withstand pressure
At meristem, xylem and phloem are not differentiated 0 procambium. Then, they differentiate into
the vascular bundle.
In some herbs, there is a terminal strip inside vascular bundle that remains meristematic and can
add new vascular tissue, but this gives them a corrugated look. So how get vascular tissue hat
goes around stem?
Procambium bundles differentiate into vascular tissue, with little strip of remaining meristematic
tissue. So, the meristematic cells in bundle (fascicles) send signal to adjacent living cells.
Fascicular cambium recruits adjacent cells to be interfascicular cambium. Creates continuous
cylinder inside stem where all those cells have meristematic identity.
Interfascicular cells were just pith – undifferentiated parenchymal cells
Cambium – fusiform intiials – long narrow cells dividing in plane of the screen, long daughter cells.
If pushed inside stem, differentiate as xylem, if pushed outside differentiate as phloem. This is the
axial system – running up and down.
Little cells give rise to different cell type – ray initials – they comprise the radial system of stem.
We believe rays are parenchyma – undifferentiated living cells, and we think they help in providing
water to phloem from xylem, which are separated by cambium cells.
Xylem is only functional at center of plant for a few years, so it’s the ones on the outside that keep
the flow going, and ray cells play a role in shutting down the heartwood.
Vascular cambium divides periclinally to produce xylem toward interior and phloe