Zea mays

These vascular tissues occur nearer to the apex in
slow growing or dormant roots than in actively
elongaring roots. The firsr mature protOphloem
elements appear at the circumference of the
procambial cylindet and differentiate closer to the
root apex than the first mature protOxylem (4.8).
By contrast the prospective met'axylem becomes
demarcated, by its prominent vacuolation growth
(4.5), nearer the apex than the protophloem. Some
distance basal ro the prorophloem, mature protoxylem
elements develop between the prorophloem
files. Maturation of the metaphloem and
metaxylem proceeds centripetally (4.30) on the
radii already demarcated by the protophloem and
protoxylem (4.8), and the vascular system of the
mature primary rOOt is formed (4.31). The outer
layer of the procambium gives rise 10 the
parenchymatous pcricyle (4.8,4.31).
In dicotyledonous roots the tissue between the
xylem and phloem often forms a cambium (4.31),
which later spreads laterally over the protoxylem
poles ro form a COntinuous meristem (1.25). The
vascular cylinder is invested by the parenchymatous
cortex (1.29, 4.30} which frequently
contains conspicuous intercellular air spaces. The
pericycle is bounded by the single layered
endodermis (1.29, 4.31, 4.32) whose radial and
transverse walls are impregnared with lignosuberin
to form Casparian bands (4.33). These
laterally continuous bands are impermeable, so
that all water and solute movement across Ihis
layer is confined to the symplast (4.33). In the
roots of many species, particularly monocotyledons,
the ligno-suberisation of the endodermis
86
later extends to all walls and additional cellulose
thickening may be deposited (4.32). Such cells,
however, still allow symplastic transport from rhe
cortex ro the stele via their plasmodesmata.
The endodermis is an important selective
barrier but it allows the active transport into the
vascular cylinder of certain beneficial ions
(potassium, phosphare) absorbed from the soil.
Calcium, however, apparently moves apoplastially
and cannor corer the vascular cylinder through the
endodermis, but instead passes into the vascular
cylinder in the very immarure root where
Casparian bands have not developed. The active
transport of ions into the vascular cylinder
accounts for the phenomenon of roOt pressute
which sometimes plays an accessory role to
transpiration in the movement of water to Ihe
shoot.
to some roots the ourer cortex diffetentiates as
a one- ro several-layered exodermis (1.29) but the
Casparian bands in the tadial walls are usually
masked by deposition of subetised lamellae
adjacent ro the protoplast (4.34). A short distance
behind the apex a zone of absorptive root hairs
develops from the epidetmis (4.27); but warer
absorption also occurs over the test of the
epidermal surface and in some species roar hairs
are absent. In the aerial roots of some epiphytic
orchids and aroids the multilayered epidermis
develops into a dead velamen (4.34), whose walls
are thickened by bands of cellulose. This is
probably an adaption to absorb watet from the
humid atmosphere of tropical forests.
4.1 lu.s of the plwnule of Pht'lseo/us /lUfgaris (bean) seed.
The hemispherical, densely-staining and small-eelled shoot
apex (1) bears a pair of leaf primordia (2) at its margins
which represent the first foliage leaves. A prominent strand
of procambium (3) demarcates tbe longitudinal rows of
vacuolated pith (4) from the cortex. (LM x 110.)
1 Shoot apex
2 Leaf primordium
3 Procambium
4 Pith