Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
The end walls of neighbouring vessel elements break
down completely, to form a continuous tube rather
like a drainpipe running through the plant. This long,
non-living tube is a xylem vessel. It may be up to several
metres long. The structural features of xylem vessels are
closely related to their function, as explained in the
next section.
Types of lignified cell wall thickenings
a
From root to stem and leaf in the xylem
The removal of water from xylem vessels in the leaf
reduces the hydrostatic pressure in the xylem vessels.
(Hydrostatic pressure is pressure exerted by a liquid.) The
hydrostatic pressure at the top of the xylem vessel becomes
lower than the pressure at the bottom. This pressure
difference causes water to move up the xylem vessels in
b
lignin ring
lignin spiral
xylem vessel with
annular thickening
pit
lignin ring
before
stretching
after
stretching
before
stretching
after
stretching
xylem vessel with
spiral thickening
annular thickening (ring-shaped)
spiral thickening
c
142
e
d
xylem
vessel
lumen
pit
nucleus
sclerenchyma fibre with
lignified wall and no contents
cellulose cell wall
of parenchyma cell
vacuole
cytoplasm with
all organelles
Figure 7.23 Structure of xylem. a Diagrams to show different types of thickening. b Micrograph of xylem as seen in longitudinal
section. Lignin is stained red (× 100). c and d Micrograph and diagram of xylem as seen in transverse section; lignin is stained
red. Small parenchyma cells are also visible between the xylem vessels (× 120). e Scanning electron micrograph of mature xylem
vessels, showing reticulate (net-like) pattern of lignification (× 130).