Zea mays

Xylem
This complex tissue has twO principal roles: the
transport of large quantities of water from the
rOOt 10 the shoot in the tracheary elements (1.12)
and the mechanical suppOrt of the aerial plant
body (3.44). This support is provided both by the
rracheary elements and the associated (often
thicker-walled) non-conducting fibres (1.3, 3.28).
Additionally, in the secondary xylem the axial and
ray parenchyma (1.3, 3.14, 3.45) store food and
water.
Mature uacheary elements (loll, 3.17) afC
dead and have lost their proroplasts (2.9, 2.l0).
Their secondary walls are thickened relative to the
primary walls (2.8, 2.l0} and, due to lignification,
arc impermeable except at the pits where only
primary wall is present (2.10). The trachcary elements
are elongated and water moves along their
lumina from the root to the shoot in the transpiration
stream (1.12). The absence of a plasmalemma
allows the \"iater to pass fairly freely from
one clement w another via the numerous pits
(3.46).
In nearly all angiosperms the tracheary
elements comprise both tracheids and vessels
(3.47) bll[ generally only tracheids occur in
gymnosperms and lower vascular plants (3.17,
3.27,3.48). A tracheid is derived from a single cell
and has no perforations; it is elongated, with
tapering ends (3.47) and in conifers the tracheids
(3.49) may reach a centimerre in length. By
contrast, vessels are composed of a rube-like series
of two to many vessel elements lying end to end.
They are directly linked through their perforation
plates (3.47, 3.50) which represent the remnants
of their original end walls (3.51). Vessel elements
tend to be shorter but wider than tracheids (3.47).
The perforation plate shows either a single
large pore (3.47) or in compound plates a number
of elongated pores which arc commonly scalariform
(3.14, 3.47). Because of these open pores
vessels generally show a lower resistance to water
movement than trachcids, where the closed pits
impede water flow. In ringporous wood (1.3), the
wide-diametered vessels (3.28) apparenrly extend
many metres along the tree trunk. The tips of vessels
are imperforate bur the numerous pits allow
water to move into adjacent tracheary elements.
The extent and type of the pitting in tracheary
elements is variable (3.47 to 3.52). The protoxylem
in the shoot shows secondary wall deposition
of an annular or helical p:lttern (2.8, 2.9,
3.50). The primary wall between thickenings
becomes greatly extended after the protoxylem
element dies whilst its non-cellulosic components
are digested. The stretched wall often appears
'holey' under TEM (2.66) and may rupture
leaving a protoxylem cavity (1.11). In non-elonga-
60
ting regions of the plant, metaxylem elements
show much more extensive secondary wall
deposition of various patterns (3.52).
In a scalariform element at least half of the
primary wall is covered by secondary wall and the
pits are horizontally elongated and usually
bordered (3.52). In reticulate clements the
thickening is more irregular (3.52) whilst in pitted
elements (3.47, 3.48, 3.52) a greater proportion of
the wall is secondary. Their pits occur in horizontal
rows (opposite pitting, 3.52) or diagonally
(alternate pitting 3.49, 3.57).
In these various modes of secondary wall
thickenings the intervening pits are bordered
(2.10). In conifer tracheids the centre of the pit
membrane (torus, 3.48) is thickened and lignified,
but the periphery (margo) has only a loose
cellulose network and is permeable. The pits
between adjacent rracheary elements are abundant
and bordered (2.9, 2.10, 3.46), but there arc few
connections to fibres. The pits which link with
parenchyma cells are either simple or halfbordered
on the tracheary element side (3.46).
Structure of wood
In woody plantS the formation of secondary
vascular tissue is rypicall)' periodic, since the vascular
cambium becomes dormant in unfavourable
environmental conditions. This typically results in
the formation of growth rings in the secondary
xylem of the tree (3.44, 3.45). In non-tropical
species the cambial activity is limited by temperature
and the rings usually represent annual incrementS
(1.3, 3.17, 3.41, 3.44, 3.45). Generally the
last-formed layers of xylem in a growth ring arc
composed of narrower cells with thicker walls
than the earlier wood (3.53), so that growth rings
are often visible TO the naked eye (3.44). In many
tropical trees (3.54) and desert succulents, growth
rings are not obvious.
The newly-formed tracheary elements of the
sapwood condUCt water for a relatively short time.
In stressful environments (for example nutrient
deficient conditions) they often cease to function
by the end of the first year, but in tropical trees
they may remain active longer. The cessation of
water transport in a tracheary elemenr results
from cavitation (3.46). The rension on the water
columns within tracheal)' lumina is grearest in the
widest clements.
Howe....er, when water is plentifully available in
the soil, most condUCtion occurs within them since
they offer less resistance to water flow than
narrow elements. When the water supply is
restricted, the tension on water columns in wide
elements may become very severe. If a column
breaks, an embolism quickly expands within the