Zea mays

secretion may temporarily accumulate between the
outer periclinal wall and the overlying cuticle.
Sevcral types of trichomes may be present on the
same leaf (4.6). l\"on-glandular trichomes have
several possible functions in the leaf, such as
slowing the rate of transpiration by impeding
wind movement over its surfacc, deterring insect
anack and protection against excessive sunlight.
The outer wall of the leaf epidermis is impregnated
with cutin, while the cutiele lies externally
(5.21, 5.23, 5.24, 5.26). Cutin is a highly hydrophobic
lipid polyester of high molecular weight.
The cuticle is especially thick in xerophytes (3.25,
3.26,5.24, 5.35). It can reach 6 mm in thickness
and a thin deposit may sometimes be present on
the anticlinal and innermost periclinal epidermal
cell walls. The cuticle is very thin or absent in
submerged shoots of aquatics. In bifacial leaves
the adaxial cuticle is usually thicker than the
abaxial (5.36). Wax is usually present on the
surface of the cuticle where it occurs as a crust of
filaments, granules, or flakes. However, the wax is
often dissolved from the surface during the
fixation and processing of material for
examination under the microscope.
Mesophyll and sclerellchyma
In mOSt bifacial leaves a layer of palisade cells
occurs adaxially (3.3, 5.21, 5.36). These highly
vacuolate, cylindrical cells are anticlinally elongated
and contain numerous chloroplasts in the
peripheral cytoplasm adjacent to their walls (2.24,
3.3). There is often a well·developed system of
intercellular spaces allowing gaseous diffusion
through the apoplast in rclation to photosynthesis
and transpiration. The palisade tissue may be
several cells thick and in isobilaterallcaves usually
occurs both ad- and abaxially (5.13). Xeromorphic
leaves often show a more compact mesophyll
in which the intercellular spaces are reduced (3.22,
3.26,5.31,5.32).
In bifacial leaves a layer of spongy mesophyll
occurs abaxially (3.3, 5.21). In this non-homogenous
tissue, with its large intercellular spaces,
the total volume of apoplast usually exceeds the
symplast (5.26). However, the surface area of wall
in this tissue is often much less than in the
palisade mesophyll. In some xeromorphic plants
and succulents the adaxial palisade is completel)'
or partly replaced by compact non-photosynthetic
parenchyma, whose large cells are highly vaCllOlate
and probably represent a water storage tissue
(5.23,5.37). In thick leaves, the central tissue may
be achlorophyllous (5.34) and sometimes degenerates
(5.15).
Differential patterns of colouring commonly
occur in leaves, particularly in decorative foliage
plants. Variegated, chimaeral leaves (5.6, 5.38,
5.39) usually originate from a nuclear mutation in
the shoot apex which prevents derivatives of the
mutated cell from developing chloroplasts. If the
mutation occurs in [he inner tunica or corpus, it
may resul! in the formation of achlorophyllous tissue
(5.39) in place of normal green mesophyll
(3.31.
In PilllfS and several conifers the mesophyll is
plicatc (3.22); vertical invaginations from the
main wall protrude into the protoplast and thus
increase the surface area of cytoplasm occupied by
the chloroplasts. Many tropical grasses and
various ocher taxa photosynthesise by the C4
pathway and these leaves often show a 'Kr:lOZ'
(wreath) anatomy, with the mesophyll cells
tadiating outwards from the bundle sheaths
enclosing the veins (5.40). The chloroplaSts within
the sheath are usually agtanal and commonly
larger than the granal mesophyll chloroplasts.
The margins of many leaves are strengthened
by tracks of fibres and prominent strands or
girders of sclerenchyma may also be interspersed
in the mesoph)'l1 (3.26, 5.34). Additionally,
sclereids may intrude within the mesophyll (5.35).
Sc1erenchyma and collenchyma are also frequently
present in the ribs developed over the maior veins
and the mid-rib (5.36, 5.40).
Vascular tissue
In the lamina of dicotyledons tbe finest branches
of the minor veins delimit the areoles (5.17), hut
in monocotyledons the minor nenvotk is less well
defined (1.24, 5.16). Minor veins are embedded in
a sheath of photosynthetic mesophyll (3.3) but the
larger veins arc often enclosed within ground
tissue containing few chloroplasts (5.15, 5.36,
5.41,5.42). The veins are typically collateral (5.4l
to 5.43) with the xylem normally located
adaxially (5.14B, 5.36) but their orientaTion may
vary (S.41). Limited secondary thickening may
occur in (he main veins of some dicotyledonous
leaves and conifer leaves (5.21, 5.36, 5.43).
The minor veins (5.17 to 5.19) are concerned
with the loading of sugars formed by photosynthesis
into the phloem (3.43) and the
unloading of water from the xylem into the
mesophyll. Vascular parenchyma and transfer cells
(2.63) are especially well developed in relation to
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