Zea mays

(1.15.5.21). In some gymnosperms (e.g. Cycas,
Taxl/s) files of thick-walled and elongate
transfusion tracheids branch off from the main
vein into the mesophyl1 (S.21, 5.22). Most ferns
show dichotomous venation (5.4).
Anatomy of the lamina
Leafepidennis
In angiosperms the epidermis is directly derived
from the outermost tunica layer of the shoot apex
(4.3). In a few genera (e.g. Ficus, Neriflm, Peperomia)
the immature adaxial epidermis undergoes
pcriclinal divisions to form a multiple epidermis,
but it is not possible to distinguish this from a
normal hypodermis in the mature leaf (5.12,
5.23). The epidermis is persistent even in longlived,
evergreen leaves and it is normally coated
exu:rnally by the relatively impermeable layers of
cuticle and wax (3.25, 3_26, 5.21, 5.23, 5.24).
The epidermis always contains ordinary
patenchyma-type cells together with the guard
cells which surround the stomatal pores (5.15,
5.25,5.26). However, stOmata are often confined
to the abaxial surface in bifacial leaves (3.3, 5.2t).
AddiTional cell types frequendy occur, especially
subsidiar}' cells associaTed with the guard cells
(5.27, 5.28, 5.30) and various rrichomes (3.lt,
5.29). Chloroplasts are usually only present in me
guard cells (5.2S); whilst leucoplastS are normally
preseO[ elsewhere in the epidermis (2,47). However,
chloroplasts may differentiate in the ordinary
epidermal cells of shade plants and also in
aquatics.
\Vhen viewed from the leaf surface the anticlinal
(vertical) walls of ordinary epidermal cells
are sometimes sinuous (5.28). In most monocotyledonous
leaves the long axes of the epidermal
cells arc parallel to that of the leaf (5.28). In
dicotyledonous leaves the cells are more randomly
arranged (5.29, 5.30), but over the midrib and
main veins the epidermal cells tend to lie parallel
to each other. In many xeromorphic grasses (that
is species ad:lpted to dry habitats) large, thinwalled
bulliform cells occur in rhe adaxial
epidermis {5.3l to 533}. These cells preferentially
lose water and so contribute to the rolling of the
leaf (5.31). In xeromorphic plants the stOmata
often occur in grooves or pits (3.22) and epidermal
trichames are frequently present (5.12,
5.32). Stomata are usually absent from the
epidermis where it overlies the hypodermal
sclerenchyma (3.22, 5.32) which often occurs in
extensive tracts at the margins of leaves or
98
associated with large veins (5.34).
In bifacial leaves Stomata are usually mOSt
frequent abaxially (3.3, 5.21). On the other hand
in aquatics with floating leaves {5.l} they are
usually confined to the adaxial surface, while in
submerged leaves Stomata are generally absent.
Stomata are often randomly orientated (5.30) but
in elongate leaves the guard cells arc usually
parallel (5.2S). In grasses and sedges a pair of
dumbbell-shaped guard cells surround the pore
(5.27, 5.28), while crescent-shaped guard cells
occur in most other specits (5.27, 5.30).
In the latter the anticlinal walls rtmote from
the pore are relatively thin but the walls adjacent
to the pore are often thickened (5_25, 5.27). The
differential thickening of these walls, and the
radial distribution of cellulose microfibrils in their
pericJinal walls, is apparently associated with the
pore opening when the guard cells are turgid. In
the dumbbell-shaped guard cells of gtasses the
ends are thin-walled while the side walls are
thicker (S.27). An increase of turgor in the guard
cells causes their ends to swell and the anticlinal
side walls are pulled apart at the pore.
A cuticle is always present on the outer
periclinal wall of a guard cell (5.25, 5,26) and
wedges of cuticle-covered wall sometimes
protrude towards the pore. In PinllS and some
other gymnosperms the guard cell and associated
subsidiary cell walls are apparendy lignified
(3.22). In most species where subsidiary cells are
present they da not differ from ordinary epidermal
cells in their cytological appearance, but arc
normally distinguished by their shape and
orientation relative to the guard cells (5.27, 5_28,
5.30). Subsidiary cells may be derived from the
same precursot cell as the guard cells (mesogenous
development) or from neighbouring epidermal
cells (perigenous origin): the two types are not
homologous and their occurence can be of
systematic significance. It is not known whether
subsidiary cells have a distinctive role in stomatal
movement and they are apparently absent from
leaves of a number of species.
Trichomes are COmmon on leaves and stems;
they range fram unicellular to multicellular
structures (5.12, 5.29, 5.31) and may be
branched. Many are secretory and these oftcn
consist of a stalk and glandular head (3.n, 4.6,
4.7). Their secretions are diverse and vary from
essential oils as in Lavandllla to hydrolytic
enzymes in the leaves of carnivorous plants such
as Drosera (3.11, 3.12). In some glands the
,