Agriculture Reference
In-Depth Information
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The
endoplasmic reticulum
is a complex
mesh of membranes that enables transport
of chemicals within and between cells and links
with the cell membrane at the cell surface.
Specialized units made of RNA and protein
called ribosomes are commonly located on the
endoplasmic reticulum and manufacture proteins,
including enzymes which speed up chemical
processes.
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The
vacuole
is a sac within the cell, bound by a
membrane which contains dilute sugar, nutrients,
pigments and waste materials. It may occupy the
major volume of the cell and its main functions
are storage of waste products and maintaining cell
shape through controlling cell turgor (see p. 120),
which is important for support in herbaceous plants
in particular.
The term 'tissue' may also be used to describe
the different areas within plant organs such as the
cortex of roots and stems or the mesophyll of plant
leaves.
Stem and root anatomy
When roots and stems are first formed, their tissues
are arranged in distinctive ways. In young roots and
stems, herbaceous plants and monocotyledons, and in
the area behind the root and shoot tips, this anatomy
persists while in plants which become woody the
structure eventually changes due to secondary
thickening.
Dicotyledonous stem tissues
The internal structure of various non-woody
dicotyledonous stems, as viewed in cross-section,
is shown in Figures 6.4a, b and d and 6.6a. The
stem is broadly divided into four areas of tissue, the
epidermis, the cortex, the vascular tissues and the
pith.
The protective
epidermis
consists of a single layer
of cells on the outside of the stem which produces
a waterproofing waxy layer of cutin on its surface
called the
cuticle
. Pores called
stomata
punctuate
the epidermis allowing gases to pass through an
otherwise impermeable layer. Opening and closing of
the pores is controlled by a pair of modified epidermal
cells called guard cells (see Figure 10.5).
The
cortex
of the stem lies beneath the epidermis
and is largely made up of
parenchyma tissue
composed of relatively unspecialized parenchyma
cells. These cells are thin walled and maintained in an
approximately spherical shape by osmotic pressure
(see p. 120) with many air spaces between them.
The mass of parenchyma cells combines to maintain
plant shape. Parenchyma cells also carry out other
functions, when required - for example, many of
these cells contain chlorophyll (chlorenchyma), giving
the stems their green colour (see Figure 7.5), and so
are able to photosynthesize. They also release energy,
through respiration, for use in the surrounding tissues.
In some plants such as the potato they are capable
of acting as food stores; the potato tuber is an organ
which stores starch (see p. 93). Parenchyma cells can
sometimes be triggered to undergo cell division, a
useful property when a plant has been damaged. This
property has practical significance when plant parts
such as cuttings are being propagated, since new cells
can be created by the parenchyma to heal wounds and
initiate root development. Plants which are adapted to
waterlogged situations often have parenchyma tissue
The whole of the living matter of a cell,
its membrane, nucleus and cytoplasm, is
collectively called
protoplasm
. Plant cells differ
from animal cells in having a cell wall, a vacuole and
chloroplasts.
Plant tissues
The tissues which make up the structure of a plant
can be grouped into five categories according to
the functions they perform.
Meristematic
tissues
are where new cells are produced by cell division.
Meristems are responsible for lengthwise growth
at the tips of roots and shoots (
apical meristems
)
and, in woody plants, width-ways growth (
lateral
meristems
) as the plant increases in size and
needs to support itself. Meristematic cells are
undifferentiated, that is they are as yet unspecialized
for any particular function. They are cuboid in shape
with small vacuoles, a large conspicuous nucleus
and many mitochondria.
Protective
tissues (e.g.
the epidermis) cover the entire plant surface,
holding the plant together and protecting it from
water loss and damage.
Transport
tissues are the
plumbing system of the plant transporting water
and nutrients in the xylem, sugars in the phloem
and plant hormones in both. The bulk of the plant
is made up of
packing
tissue such as parenchyma,
whose cells are often unspecialized but may
sometimes be adapted for specific functions such
as photosynthesis, aeration in plants growing in
waterlogged conditions or starch storage. Finally,
supporting
tissues reinforce leaves, stems and
roots by having cell walls with extra thickenings of
cellulose (collenchyma tissue) or lignin (sclerenchyma
tissue) (Figure 6.4).
