Zea mays

CHAPTER 3
Plant histology
Disttibution of cells and tissues
The vascular plant is composed of dermal, ground
and vascular tissue systems (3.1 to 3.4). The
phloem and xylem constitute the vascular system;
both arc complex tissues in which the conducting
elements (2.10, 2.68, 2.69) arc associated with
other cell types (1.4, 1.11,3.5). The ground system
surrounds the vascular tissues (3.1 to 3.4) and
comprises parenchyma, collenchyma and selecenchyma
(3.6 to 3.10). The dermal system is initially
represented by the epidermis (3.2, 3.3) containing
various cell types (3.3, 3.lt, 3.12). Its structure is
considered in Chapter 5.
In plants which undergo woody thickening, the
epidermis typically becomes replaced by the cork
and associated tissues (the periderm, 3.13) whose
structure is discussed in Chapter 6. Secretory cells
do not normally develop as distinct tissues (except,
for example, in nectaries) but rather occur
cither as surface structures (3_11, 3.12) or within
other tissues (3.4).
Parenchyma
These cells arc generally thin-walled and have
large vacuoles (2.24, 3.3. 3.6). Thcy form a
continuous tissue in the cortex and pith (1.27, 3.4)
and in the leaf mesophyll (3.3), while vertical
strands and horizontal rays of parenchyma occur
in the vascular system (1.3, 3.14). The leaves and
stem of seedlings and small herbs are largely held
erect by the collective turgor of the parenchyma
cells (3.3) and the shoot wilts if insufficient water
is available from the root system to replace
transpiration losses (1 .12).
Parenchyma cells are often polyhedral or
elongate (3.3, 3.6) but stellate and irregular forms
occur (3.3, 3.15). The mesophyll cell of the leaf is
specialised for photosynthesis and contains numcrous
chloroplasts (3.3) while starch is frequently
stored in the parenchyma of the root and
stem (t.14). In plants growing in marshy conditions
and in aquatics, the parenchyma of the
shoot and root sometimes develops extensive
intetcellular spaces and is termed aerenchyma
(3.15).
Mature parenchyma ceJls of the stem and root
may resume division (3.16) and undergo partial
dedifferentiation to form the lareral meristems of
the phellogen and vascular cambium (3.5) from
which the cork and secondary vascular tissues
originate (3.13, 3.17). In some situations the
parenchyma cells revert to an undifferentiated
state (as found in the apical meristems, 2.2) and
give rise to new plant organs. This phenomenon
results in the formation of lateral and adventitious
roots (3.18) and in the development of primordia
(3.19) which give rise to adventitious shoots on
various partS of the plant (3.20, 3.21).
The development of adventitious organs is
sometimes related ro injury (3.18); the plant's
regenerarive capacity is exploited in horticulture
by the rooting of shoot cuttings (3.J8) and regeneration
from various excised plant organs (3.J9 to
3.21). Parenchyma cells should therefore be
regarded as potentially totipotent and single cells
derived from carrot and tobacco have given rise to
completely new plants when cultured in vitro on a
suitable nutrient medium with exogenous growth
substances.
The parenchyma cell wall is often thin, with irs
growth normally terminating at the end of cell
vacuolation (2.24, 3.3, 3.15)_ However, in seed
srorage tissue the walls may be greatly thichned
(Correa, Phoenix, Phaseo/lts. 2.54) by noncellulosic
reserve carbohydrates, while parenchyma
cells often develop thick lignified secondary
walls (1.3, 2_65) in the xylem and pith. In transfer
cells labyrinthine ingrowths of the wall (2.63)
vastly increase the plasmalemmal surface, while
simpler vertical ingrowths of the wall also occur in
the plicate mcsophyH of several conifers (3.22).
Collenchyma
These living cells arc thick-walled but relatively
pliable (3.7) and are located in the outer ground
tissue (3.23). Collenchyma is of great value in the
mechanical support of the young shoot but is rare
in the root. The proroplasts of such cells are
generally vacuolate (3.7) while their highly
hydrated primary walls are unevenly thickened
(3.7, 3.24) and comain large amounts of peCtins
and hemicelluloscs. Collenchyma cells are usually
elongate (3.7) and in transverse section often
appear angular with thickening localised in the
cell corners (3.7, 3.24) but the thickening may be
confined to the tangential walls (3.7). Intercellular
spaces sometimes occur in collenchyma and in
older cells the walls may become lignified.
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