Zea mays

absent or may develop some distance behind the
shoot apex from axillary parenchyma. In many
species one or more accessory buds develop in
addition to the axillary buds (4.18).
Buds may also develop adventitiously on the
shoot; when the plumule of Linum is damaged the
hypocotyl forms adventitious replacement buds
(4.19) from dedifferentiated epidermal and cortical
tissue (4.20). Adventitious buds are common
on other stems and may also develop on root'S and
leaves (3.19 to 3.21). Shoots frequently develop
from dormant buds located on the trunk or main
branches of trees (4.21); these buds are commonly
adventitious and arise endogenously from vascular
parenchyma or cambial tissue. A number of
tropical trees (e.g. Artocarpus, Theobroma) are
cauliflorous, developing their flowers from
persistent bud complexes on the mature trunk.
In many tropical species, the axillary buds
develop into laural shoots just beneath the
terminal bud (sylleptic growth). However, in other
plants the terminal bud exerts dominance over the
axillaries; these ate commonly invested by bud
scales and undergo a period of dormancy before
sprouting (proleptic growth, 4.22).
TIssue differentiation in the
young stern
In the terminal bud the procambium (incipient
vascular tissue) develops acropetally into the apex
(4.4) from the older procambial tissue at its base.
Within the apex the procambium becomes
differentiated from the inner flank meristem, with
each leaf linked from its inception to the
ptocambium (4.3, 4.4). In the procambial strand
the first vascular tissues begin to differentiate close
to me apex (2.3, 4.7). The protoxylem normally
develops at the inner margin of the strand
(endarchly) while prorophloem forms exarchly on
the margin nearest to the epidermis (1.11).
Protoxylem usually first differentiates within
the procambial st.rand at the base of a leaf primordium
(4.15), forming a short longitudinal file
of rracheary elements which then differentiates
bidirectionally; both upwards inro the leaf and
downwards into the young internode where it
links with older and larger xylem srrands (4.23).
The longitudinal pathway of protophloem differentiation
in the procarnbium is normally
acropetal into the young axis and leaf primordia
and is in continuity with the phloem elementS in
the older bud. Tbe metaphloem differentiates
somewhat later and is located inwards (centripetally)
to the protophloem, while the metaxylem
develops centrifugally to the protoxylem (4.24). In
dicotyledons and gymnosperms a narrow strip of
procambium remains undifferentiared between the
xylem and phloem and constirures the fasicular
vascular cambium (3.5, 4.14), but in monocotyledons
this is absent (1.11).
In most dicoryledons a large parenchymatous
pith occupies the centre of the primary stem and is
surrounded by a ring of discrete vascular bundles,
with a narrow COrtex situated externally (1.28). In
monocotyledons a distinct pith is uncommon and
the vascular bundles normally occur throughout
the ground tissue (1.27, 4.15).
Root apex
In the grear majority of species the root apex 1S
sub-terminal since it is covered by a protective
root cap (4.25), although in some aquatic plants
this is absent. Due to massive dictyosome activity
in the outer cap cells (2.19), a large quantity of
mucigel is secreted into the soil rhizosphere (4.26).
More mucigel is contributed by the root hairs
(4.27) which develop behind the root apex. In
some plants such as Zea, the rOOt cap has its own
distinct initials (calyptrogen, 4.25). The incipient
epidermis (protoderm) and cortex can be traced to
a single tier of cells adjacent to the calyptrogen,
while tbe central procambial cylinder (4.5)
apparently originates from a third tier of initials
immediately within those of the protoderm-