Chapter 15: Structure of plants
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Plant cell walls
•
•
Cell walls define the size and shape of cells,
contribute to metabolic processes and cell-cell
communication
Lying between adjacent cells is the middle lamella,
which comprises pectins that bind the cells
together
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Fig. 15.3a: Cross-section of parenchyma
cells from Coleus stem
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Primary cell wall
•
•
•
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A plant cell wall is made up of a primary wall and a
secondary wall
The primary wall is the first layer formed by a
developing cell
This is composed of cellulose microfibrils, together
with associated pectins, hemicelluloses and
glycoproteins
The primary wall is able to extend as a cell enlarges
and matures
(cont.)
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Primary cell wall (cont.)
•
Primary cell walls vary in thickness, with thinner
areas (primary pit fields), punctuated by numerous
plasmodesmata, which are involved cell-cell
communication
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Fig. 15.4a: Parenchyma cell
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Secondary wall
•
•
•
As cells mature, many lay down secondary wall
materials on the inner surface of the primary wall
The secondary wall is usually formed in three
distinct layers, each with cellulose microfibrils laid
down in different orientations, and strengthened
by the addition of lignin (a polyphenol)
Some cells have many depressions, or primary
pits, in the secondary wall
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Plant tissue systems
The primary plant body is composed of cells produced
by the shoot and root apical meristems.
1. Dermal tissue
– epidermis segregates internal plant tissues from the
external environment
– epidermal cell structure varies, depending on whether it
occurs in the stem, root or leaf, although its main
functions are protection and facilitating exchange of
materials
– dermal tissues include the endodermis and exodermis
(cont.)
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Plant tissue systems (cont.)
2. Ground tissue
– These tissue systems, which include parenchyma,
collenchyma and sclerenchyma, make up the bulk of
tissues in stems, roots and leaves
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Fig. 15.5a: Parenchyma cells of potato tubers
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Fig. 15.5b: Aerenchyma cells of the seagrass
Amphibolis
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Fig. 15.5c: Transfer cells in an embryo of the
seagrass Amphibolis
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Ground tissue
•
Parenchyma: these tissues normally comprise large
cells with thin primary walls and well-defined, pectinrich middle
– chlorenchyma: photosynthetic, found in leaves and some
stems
– storage parenchyma: contain nutrient reserves
– aerenchyma: spongy cells with a large network of air
spaces, adaptation to low O2 environments
– transfer cells: ingrowths that increase cell surface area for
rapid transfer of molecules
(cont.)
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Ground tissue (cont.)
•
•
Collenchyma: supportive tissue, providing strength
to plant parts where bending and flexibility are
required. Cells are elongated, with thickened cells
walls that contain extra cellulose
Sclerenchyma: supportive tissue that imparts rigidity
and strength
– fibres: elongated cells with tapering end walls
– sclerids: branched or even-shaped, and play roles in both
support and protection
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Vascular tissue
•
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Vascular tissues are involved in transport of
materials around the plant body
Comprises xylem and phloem
Xylem transports water and dissolved minerals from
the roots to the leaves and comprises cell types,
commonly vessels and tracheids
Phloem transports the products of photosynthesis
from the leaf to actively growing areas or to storage
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Vessels
•
Vessels, which are common in flowering plants,
have open ends that allow an unimpeded flow of
water from one cell to another
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Fig. 15.8a: Xylem vessels in vascular
tissues
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Tracheids
•
Tracheids, common to all vascular plants, have
intact end walls, with water moving from cell to cell
through bordered pits in the cell wall
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Fig. 15.8b: Tracheids
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Phloem
•
•
•
Phloem comprises sieve tube cells associated with
companion cells (parenchyma in nature)
Sieve tube cells are elongated and possess thin
primary cell walls composed largely of cellulose
Companion cells control the activity of sieve tube
cells by increasing the surface area for cell-to-cell
interchange
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Fig. 15.8c: Phloem
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Stem structure
•
Plant shoots comprise stems and leaves
• The stem provides support, contains vascular
tissues and in some species is a storage organ
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Fig 15.9: The structure of a flowering plant
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Stem primary growth
•
In dicotyledons (e.g. eucalypts and wattles)
vascular bundles consisting of xylem and phloem,
together with parenchyma cells, form a central
cylinder, the stele
(cont.)
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Fig. 15.10a: Ring of vascular bundles in
Acacia
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Stem primary growth (cont.)
•
•
•
Vascular tissue often forms a single ring, separating
the stem into outer cortex (ground tissue) from the
inner pith
Primary phloem forms outside the xylem on the same
radius
The earliest-forming xylem, (protoxylem) develops
as thin strands towards the stem centre. Outside of
this develops larger, thicker-walled xylem
(metaxylem)
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Fig. 15.10b: Vascular bundle in Acacia
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Stem secondary growth
•
•
Woody dicotyledons such as trees and shrubs can
undertake secondary, or radial, growth
Cambium, which produces sheets of cells laterally,
may be vascular (wood) or cork (bark)
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Fig. 15.12a, b and c: Secondary growth in the
stem of silky oak (Grevillea robusta)
(a)
(c)
(b)
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Vascular cambium
•
This multilayered cylinder of meristem lies between
the cortex and pith and is responsible for producing
secondary vascular tissues such as secondary
xylem and secondary phloem
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Fig. 15.12d: Cross-section of secondary
xylem and phloem
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Cork cambium
•
These meristematic cells generate the outer
covering, or periderm, that replaces the epidermis
in older stems
•
The cambium produces cork (or phellem) on the
outside of the stem and thin walled parenchyma,
(the phelloderm) on the inside
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Fig. 15.13b: Cross-section of bark
Copyright © Associate Professor Andrew Drinnan, University of Melbourne
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Special functions of stems
•
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Not all plant stems are aerial; underground stems
may be horizontal rhizomes, from which aerial
branches or leaves arise for vegetative propagation
or to serve as food storage, as in the potato tuber
Desert plants
– in arid and semi-arid regions, leaves tend to be absent or
reduced, with photosynthesis carried out by the stem
– e.g. leaves of she-oaks (Allocasuarina sp.) are reduced
to small, scale-like structures in a whorl at nodes of
stems or cladodes
(cont.)
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Special functions of stems
(cont.)
•
Stems of marsh and aquatic plants
– such environments are usually waterlogged, and
stems often contain extensive aerenchyma
– in saline habitats, leaves are reduced and fleshy
stems photosynthesise and store water
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Fig. 15.16: Stems of the saltmarsh plant
Sarcocornia
Copyright © Associate Professor Andrew Drinnan, University of Melbourne
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Root structure
•
Roots are the underground organs of vascular
plants, and grow from apical meristems at their tips
• Roots have several functions, including nutrient
and water uptake from the soil, anchorage and
support, synthesis of plant hormones and storage
of nutritional reserves
• Root systems may be branched, a common trait of
dicots, or consist entirely of adventitious roots, a
trait among monocot species
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Fig. 15.19a, b and c: Types of root systems
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Root primary growth
•
A root consists of an outer epidermis, enclosing a
well-developed cortex and inner vascular cylinder
•
The root cap, located at the tip of the root,
protects the apical meristem
(cont.)
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Fig. 15.20: Organisation of roots
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Root primary growth (cont.)
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The root cap comprises large parenchyma cells
that secrete mucigel, a polysaccharide that
contains a mucilaginous matrix, and sloughed-off
living cells
The root surface area is supplemented by
numerous fine root hairs, which are extensions of
epidermal cells located just behind the growing tip
The soil region around the root hair zone is the
rhizosphere, within which occur interactions
between the plant and its soil environment, and
with free-living and symbiotic microorganisms
(cont.)
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Root primary growth (cont.)
•
•
•
•
The root vascular tissues of dicotyledons comprise
exarch xylem, which forms from the outside to the
inside of the root
The vascular tissue is surrounded by the pericycle,
a thin layer of cells from which lateral roots arise
Outside the pericycle lies a single layer of cells, the
endodermis, the radial walls of which endodermis
are impregnated with suberin, a compound that is
impermeable to water
This Casparian strip forces water to pass through
the symplastic pathway, enabling the plant to
selectively control the movement of water and ions.
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Special functions of roots
•
Roots of aquatic plants
– mangrove species often possess upright aerial roots,
pneumatophores that function in gas exchange for
aerobic respiration
•
Roots and salinity
– river red gums, which are mildly salt tolerant, have an
exodermis, a suberised cell layer that acts as a barrier to
toxic chloride ions
•
Storage roots
– roots adapted for storage often have additional layers of
vascular cambia together with associated storage
parenchyma cells (e.g. sweet potato and beetroot)
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Leaf structure
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Leaves are photosynthetic organs with limited life
spans
Evergreen leaves are shed continuously while
deciduous trees shed their leaves before the onset
of a cold or dry period
Simple leaves have a single lamina while
compound leaves possess many leaflets
Juvenile leaves can differ considerably from adult
leaves in colour and/or form
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Leaf structure and organisation
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A leaf usually comprises upper and lower epidermis,
enclosing mesophyll tissue containing chloroplasts
The epidermis is covered by a thick cuticle and
studded with pores (stomata) which regulate water
loss and gas exchange between plant and
atmosphere
(cont.)
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Fig. 15.33a and b: Leaf anatomy of
Eucalyptus globulus
(a)
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Leaf structure and organisation
(cont.)
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•
•
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Leaf hairs, secretory gland cells and crystalcontaining cells also characterise the epidermis
Mesophyll, which is the ground tissue of leaves,
comprises chloroplast-containing parenchyma cells
that are the sites of photosynthesis
Mesophyll is often differentiated into palisade
mesophyll toward the upper surface of the leaf, with
spongy mesophyll toward the lower
Veins (vascular bundles) of flowering plants form an
interconnected branching system, with xylem on the
upper side and phloem on the lower
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Modifications of leaf structure
•
Aspects of leaf morphology are directly influenced
by the environment
– e.g. shade leaves are generally larger, but have less
palisade mesophyll than sun leaves
•
Plants growing in particular habitats may exhibit
distinctive leaf adaptations
– e.g. arid and semi-arid taxa often have small, scale-like
leaves
•
Some rainforest plants possess pore-like structures,
hydathodes, that enable extrusion—guttation—of
water under conditions of high atmospheric humidity
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