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Plant Center 8
Getting down to the Root of it all
It is time to learn about roots. Complete both worksheets.
1. Read the article Plant Partners. Create a crossword with the following
vocabulary:
 Pollen
 Nectar
 Digestive enzymes
 Carnivorous
 Flytrap
 Lure
 Pollination
 Pollinator
 Dispersal
2. To complete worksheet Root Systems/Inside a Root, skim the article provided.
This article (Roots) is very high level. You are not expected to read and
understand it all. However by looking through it you should be able to find
enough information to complete the worksheets.
ROOTS
Written and Illustrated by Mark Brundrett
CSIRO Forestry and Forest Products
Index
A.
B.
C.
D.
Introduction
Root Systems
Root Growth
Root Structural Diversity
1. Root Tips
2. The Epidermis and Root Hairs
3. The Exodermis or Hypodermis
4. Air Spaces
5. The Endodermis and Vascular Tissues
6. Other Structures You are Likely to See
7. The Periderm and Secondary Growth
E. Roles of Structural Features
F. Terminology
A. Introduction
It is necessary to be familiar with the structure of nonmycorrhizal roots before
examining any changes in root structure due to mycorrhizal associations. Some
root structures have the potential to influence mycorrhizal development.
Additional information about roots can be found in plant anatomy and plant
nutrition texts. The terminology used is explained in the glossary.
WHAT ARE
ROOTS?
Roots are
equal in
importance to
leaves as the
life support
system for
plants and
thus for all
life in
terrestrial
ecosystems
Roots are:




Carbon pumps that
feed soil organisms
and contribute to soil
organic matter
Storage organs
Chemical factories
that may change soil
pH, poison
competitors, filter out
toxins, concentrate
rare elements, etc.
A sensor network
that helps regulate




plant growth
Absorptive network
for limiting soil
resources of water
and nutrients
Mechanical
structures that
support plants,
strengthen soil,
construct channels,
break rocks, etc.
Hydraulic conduits
that redistribute soil
water and nutrients
Habitats for
mycorrhizal fungi,
rhizosphere and
rhizoplane organisms
B. Root Systems
The recognition of different types of roots is important because these can have
different functions. Most plants produce one or more orders of lateral root
branches. Different orders of roots vary in their thickness, branching patterns,
growth rates, capacity for secondary growth, lifespans, structural features, etc.
These variations will influence their capacities to obtain water and nutrients,
support mycorrhizal associations and survive adverse conditions. Higher order
lateral roots are generally thinner, shorter and don't live as long as those of
lower orders.
Types of roots:






Seminal root - from a seed
Adventitious root - from a stem
First order lateral root - from a seminal or
adventitious root
Second order laterals, etc. - from first order
laterals, which in turn produce third order
laterals, and so on . . .
Feeder roots - fine, relatively short-lived roots
that acquire nutrients and water in the topsoil
Primary roots - from primary growth by the
apical meristem


Secondary roots - mature, thicker "woody" roots
with bark and additional vascular tissue
Coarse roots - may live for a long time and have
roles in transport and mechanical support
Root System Diversity
There are differences between plant species in the proportion of relatively fine
or coarse roots they produce. Examples of plants with fine or coarse root
systems are shown below. The nature of plant's root systems is related to their
capacity of to grow without mycorrhizal associations. Plants may also have
distinctive root branching patterns, such as the proteoid roots of some
nonmycorrhizal plants.
It is generally difficult to identify roots with vesicular-arbuscular mycorrhizas
by superficial examinations. Species with ectomycorrhizal associations
generally have short, swollen lateral roots resulting in a distinctive root system.
These characteristic lateral roots can be visible without a microscope and have
been used to identify ectomycorrhizal associations. However, there are also
some plants with VAM associations that appear to have short roots such as the
angiosperm trees Acer and Ulmus, and the Gymnosperm Podocarpus. Thus, it is
advisable to examine roots internally to identify association types.
Most of the root samples shown below were obtained from natural
ecosystems. Some additional samples were obtained from plants
growing in glasshouse experiments.
The root systems illustrated below were washed from soil,
suspended in water and photographed with a dissecting
microscope or a camera with a macro lens.
Coarse root system of a
plant (Arisaema
atrorubens) for
comparison with the fine
root system below. This
plant with thick,
relatively
unbranched
roots without
long root hairs
is considered to be highly
dependant on
mycorrhizas (Brundrett &
Kendrick
1988).
(finest gridlines =
1 mm)
Deciduous
forest plant
with fine roots
(Geranium
robertianum). This
species with highly
branched roots with long
root hairs is considered to
have a low requirement
for mycorrhizas.
(finest gridlines = 1 mm)
Ectomycorrhiza
l short roots
(arrows) of
birch (Betula
alleghaniensis), an
angiosperm tree. The
mycorrhizal roots are
thicker than other laterals
of the same order due to
mantle hyphae on the
surface and epidermal
cell enlargement in the
Hartig net.
(grid = 1 mm)
Pine species with
ectomycorrhizal
associations have
distinctive short, lateral
roots with equal
(dichotomous) branches.
Root system of
maple (Acer
saccharum).
This tree has
short lateral roots with
constrictions that are
called beads (arrows).
These result from
constrictions when root
growth stops and later
resumes after suberisation
(metacutinisation) of root
cap cells.
(grid = 1 mm)
Cluster roots
produced by a
nonmycorrhizal
plant. This is an
Australian a species of
Hakea in the Proteaceae
family. These localized
proliferation of lateral
roots are thought to be
important sites of nutrient
uptake.
Roots of Hakea baxterii
growing in hydroponic culture.
Root clusters are approx. 2 cm
long. (Plant material courtesy
of Prof Hans Lambers and
Greg Cawthray)
C. Root Growth
Plants must produce new roots to grow larger and to explore new volumes of soil to
acquire nutrients. They must also produce roots to replace old roots that have died, were
lost to predation, or no longer function well. Young roots with living epidermal cells and
root hairs, are often considered to be responsible for most direct nutrient uptake
(Marschner 1986). Mycorrhizal formation by both ECM and VAM fungi requires
actively growing, or recently formed roots. Young roots can be recognised by observing
the distance of xylem and endodermis cell maturation from the root tip. Roots which have
stopped growing have mature xylem vessels at their apex and may also have a suberised
(metacutinized) root cap. These features are readily apparent after roots have been
cleared and stained.
Root tissues are produced by cell division in the root apex and cell expansion in subapical
regions. Cell division at the apical meristem produces new root cap cells in an outward
direction and new root cells in an inward direction. Root tissues progressively mature at
greater distances from the root tip and may develop specialized features of their cell walls
or cytoplasm. In the diagram below, root cell maturation is represented by increasing
color intensity and root tissues by different colors.
Most of what we know about plants comes from scientific studies of crops selected from
weedy ancestors for rapid growth in highly fertile soils. However, this information may
not be relevant to plants in natural ecosystems. Crop plants typically have roots which
elongate 1 cm or more in a day (Russell 1977), while roots of plants from in a natural
ecosystem may grow 1 mm or less a day (Brundrett & Kendrick 1990).



Crop plants tend to be annuals with a relatively "soft" fine roots that only lasts a
few weeks or months.
Plants in natural ecosystems often produce perennial roots by investing in a coarse
well-built root system.
There is a trade-off between root lifespan with its associated features and the
capacity of roots to acquire nutrients directly.
D. Root Structures
These images introduce the most important root structures people who work
with mycorrhizas are likely to encounter. Most of these images were taken
during studies of the roots and mycorrhizas of plants and trees in Canadian
forests (Brundrett & Kendrick 1988, Brundrett et al. 1990, 1991).
A compound microscope was used to take images of portions of roots.
Several different techniques including UV-light induced fluorescence,
interference contrast and polarized light microscopy were used to
visualize structures.
Many images are of relatively thin cross sections of roots made by
hand (using a sharp razor blade) processed with different staining and
microscopy procedures. Some whole mounts of roots are also shown.
Staining procedures were used that are specific for certain structures
such as lipids, lignin or suberin. More information on staining
procedures is provided elsewhere.
1. Root Tips
The growing tip of roots is protected by a root cap consisting of
concentric layers of cells surrounding the apical meristem where new
root cells are produced. The surface of the root cap of growing roots is
often covered by a thick layer of mucilage (Rougier & Chaboud 1985).
When roots stop growing the root cap may be protected by suberisation
of its outermost cells, as is shown below. These metacutinised root tips
would generally not be produced by annual species such as crop plants,
but are commonly produced by perennial plants such as trees
(Romberger 1963, Brundrett & Kendrick 1988).
Lowmagnification
microscope of a
perennial
woodland plant
(Smilacina racemosa)
with a metacutinised root
cap (M) and mycorrhizal
fungus hyphae (arrows).
Note how suberin in the
root cap functions as an
extension of the
exodermis to completely
encase the root for
protection during periods
of inactivity.
Cleared root stained with
chlorazol black E.
(Magnification = 85x)
Longitudinal
section of a
sugar maple
(Acer
saccharum) root showing
constrictions (beads)
caused by root cap
metacutinization (arrows)
when roots resume
growth. A Low
magnification view of
these roots is shown
above.
Hand section stained for
suberin and lignin
(berberine/aniline blue) and
viewed with fluorescence
microscopy.
(Magnification = 215x)
2. The Epidermis and Root Hairs
The epidermis is the outmost layer of roots that functions as the
interface between plants and the soil. Cell walls of epidermal
cells may be lignified, or suberised, or be relatively unmodified.
Cells of the epidermis of young roots secrete mucilages (Rougier
& Chaboud 1985). Epidermal cells often have narrow projections
called root hairs that extend between soil particles. Root hairs
may be long or short, dense, spares, or absent altogether (Peterson
& Farquhar 1996). Root hairs are considered to help in direct
mineral nutrient uptake by increasing the surface are of roots.
There is a correlation between the degree of root hair production
by plants and their requirement for mycorrhizas.
Root of a Fern
(Dryopteris intermedia)
which has very long and
abundant root hairs
(arrow). Roots of this
plant are sparsely
colonized by VAM fungi
(10% of root length).
Whole mount.
(Magnification =
85x)
3. The Exodermis or Hypodermis
Cells of the surface layers of roots are often highly specialized. The root cell
layer under the epidermis is called a hypodermis if it is relatively unmodified or
an exodermis if it has suberised cell walls (Peterson 1988). Suberin is a
hydrophobic mixture of lipids and phenolics deposited in the walls of some
plant cells (Kolattukudy 1984). In cases where the exodermis is highly
suberised, it is often similar in structure to the endodermis of roots, with
Casparian bands and suberin lamellae. Many species have a suberised
exodermis, which is thought to function as a permeability barrier and to help
defend the root from parasites and adverse soil conditions (Perumalla et al.
1990, Brundrett & Kendrick 1988). In some cases, the exodermis has two cell
types - long and short cells (called a dimorophic exodermis - Shishkoff 1987). In
these roots, the short cells have less suberin in their walls and are used as
passage cells by mycorrhizal fungus hyphae.
Cross section of
an onion
(Allium cepa)
root showing
modified cell walls of
exodermal (EX) and
epidermal (EP) cells.
Exodermal cells have
Casparian bands (arrows)
in their radial walls and
suberin lamellae. View
larger file (42 KB)
Hand section with a
fluorescent stain for suberin
(Berberine). (Magnification =
340x)
Close-up of the
outer cell layers
of an ash tree
(Fraxinus) root
showing suberin lamellae
in exodermal cells. Note
the passage cell (*)
without suberin lamellae.
View larger file (53 KB)
Hand section with a
fluorescent stain for lipids
(Fluorol). (Magnification =
340x)
Root of
Smilacina
racemosa (a
Canadian forest
plant) with a dimorphic
exodermis with
alternating long (L) and
short (S) cells. Casparian
bands are seen as wavy
lines between cells (thin
arrows). A short cell
contains hyphae (thick
arrow).
Surface view of whole root
cleared and stained with
chlorazol black E. (Bar = 100
um)
4. Air Spaces
Plants which are adapted for growth in habitats where soils are often
waterlogged typically have large air spaces in their roots (Armstrong 1979).
These structures would greatly reduce the habitat available for VAM fungi in
these roots. Narrow air channels occur in the roots of many species that grow in
moist or dry soil.
Intercellular air
channels
(arrows) in a
leek (Allium
porrum) root. These
channels run continuously
from the apex to the base
of roots and influence
mycorrhizal development
if they are present in
roots.
Whole mount of a living root
in water. (Magnification =
140x)
Cross-section of
a willow (Salix
nigra) root
from wet soil
with four large air spaces
(stars) occupying most of
the cortex. This root is in
an early stage of
secondary growth.
Polarized light microscopy of
an unstained hand section.
(Magnification = 85x)
5. The Endodermis and Vascular Tissue
Conducting elements, which consist of xylem and phloem, occur within the
vascular cylinder (stele) in the centre of roots. The vascular cylinder also
contains other less-specialized cells and is enclosed by the endodermis. Both
xylem and phloem are made of long narrow cells that are connected by their
ends to form continuous networks of "plumbing". These networks interconnect
plant organs.


Xylem cells are dead at maturity and have thick, strong (lignified) cell
walls. They transport water containing minerals and other solutes
primarily towards the shoot.
Phloem cells are alive at maturity and have thin walls. They transport
metabolites, especially sugars resulting from photosynthesis, primarily

from the leaves to the rest of the plant.
The endodermis is cylinder of cells with suberised walls surrounding the
stele and is thought to be a barrier to solute transport in the apoplast (cell
wall space) (Clarkson & Robards 1975). The endodermis likely has an
important role in regulating exchange processes in mycorrhizal
associations as these fungi do not cross into the stele, so are confined to
root spaces where the availability of resources is regulated by the
endodermis.
Cross section of
an ash tree
Fraxinus sp.
root showing
intense yellow
fluorescence of suberin
lamellae in the exodermis
(EX) and endodermis
(EN). Some endodermal
cells are without suberin
lamellae (arrows). These
are called passage cells.
Xylem cells (X), the
exodermis (EX) and
lipids in VAM fungus
hyphae in the cortex can
also be seen.
Hand section stained with
fluorescent for lipids (Fluorol).
(Magnification = 230x)
Phloem sieve
tube elements
in a root cross
section. This
image shows callus on the
sieve plates which occur
between longitudinally
connected phloem cells.
Endodermal (En) and
xylem cells (X) are also
visible.
Hand section stained with
Aniline blue and viewed with
fluorescence (Magnification =
340x)
Numerous large
xylem vessels
(X) in a thick,
low-order
lateral asparagus root.
Other smaller lignified
cells are present in the
stele, as are phloem sieve
tube elements (arrows)
and the endodermis (En).
Hand section with fluorescent
stains for lignin, suberin and
phloem (Berberine / Aniline
blue). (Magnification = 85x)
Endodermal
Casparian
bands (arrows)
and
longitudinally connected
xylem cells (X).
Whole cleared root stained
with chlorazol black E. (Bar =
100 um)
6. Other Structures You are Likely to See
Roots may contain cells with other types of modified walls or cell inclusions
such as crystals or secondary metabolites. Secondary metabolites often provide
color to roots and may result in UV-induced auto fluorescence. Many plants,
especially coniferous species, accumulate large amounts of dark brown
phenolics called tannins. Pigments of other colors are also common. The natural
pigmentation of roots can help us distinguish young roots from older roots,
which are often darker in color.
Accumulation
of tannins in
epidermal cells
(E) visible in a
cross section of a Tsuga
canadensis (hemlock).
Other structures in this
ectomycorrhizal root are
explained elsewhere.
Hand section stained with
Chlorazol black E.
(Magnification = 540x)
Cross section of
a cedar (Thuja
occidentalis)
root with many
lignified phi thickenings
in cortex cell walls
(arrows). Endodermal
(EN), xylem (X) and
phloem (P) cells are also
visible.
Hand section with fluorescent
stain for suberin and lignin
(Berberine/Aniline blue).
(Magnification = 340x)
Primary root of
black walnut
(Juglans nigra)
with clusters of
crystals in many cells
(arrow). These are
probably calcium oxalate.
Polarized light microscopy of a
root cross section.
(Magnification = 215x)
7. The Periderm and Secondary Growth
Some roots are genetically determined to have the capacity to undergo
additional radial growth, but other roots do not. Only dicotyledons have this
capacity and it is more likely to occur in thicker, lower-order lateral roots,
especially if they have a long life span. This radial growth is called secondary
growth to distinguish it from primary growth at the root apex.
Secondary growth occurs when new meristematic tissue forms in a ring around
the vascular cylinder of roots and produces new xylem inwards and new phloem
outwards. An outer bark layer made up of layers of suberised cells is also
formed. Secondary growth eventually results in the loss of the cortex and
epidermis of roots, so these roots cannot form mycorrhizas.
Fluorescent
staining of
suberised
periderm (bark)
cells (B) encasing a
Quercus root after
secondary growth. Blue
auto fluorescence of
lignified xylem (X) and
phloem fibre (F) cells are
visible.
Hand section with fluorescent
stain for suberin (fluorol).
(Magnification = 215x)
Strong
refringence of
thick cell walls
of xylem (X)
and phloem fibre (P) cells
resulting from secondary
growth in a Juglans nigra
root.
Unstained hand section viewed
with polarized light.
(Magnification = 215x)
E. Roles of structural features
As the above illustrations show, the structural diversity of roots is much greater
than we might expect. Indeed, it is often possible to identify roots of particular
plant species growing in soil by their structural characteristics. Some of the
illustrated structures are known to have important roles, but others have not been
well studied. Possible roles of suberised walls in roots are outlined in the table
below.
Roots of plants growing in natural ecosystems are more likely to have outer cell
layers with a greater degree of suberisation and/or lignifications than the crop
plants we look at more often. Cell wall modifications are thought to provide
structural strength to roots, or have defensive roles since they are most highly
developed in long-lived roots (Brundrett & Kendrick 1988). The chemical
substances roots accumulate may also help to protect them from predators and
parasites.
Possible Roles of Suberin in Root Surface
Layers
 Protection to allow longer root
lifespan
 Increased resistance to pathogens
(physical and chemical barriers to
root penetration)
 Increased drought tolerance (by
withstanding greater hydraulic
pressure)
 Limit nutrient and water loss in
exudates (permeability barriers)
 Protect mycorrhizal fungi within
roots
 Regulate growth of mycorrhizal fungi
by confining them to certain cells
 Contribute decomposition resistant
substances to soil (humus) after root
death
 An exodermis may restrict the
capacity of roots to acquire nutrients
directly
 These structures would increase the
production cost of roots
Most mycorrhizas are formed by relatively fine high-order lateral roots. Coarse
roots of monocotyledons do not have secondary growth, but may not form
mycorrhizas if their primary cortex is protected by heavily suberised or lignified
cells. The influence of root structural features on mycorrhizal formation is
summarized in the table below. These interactions are considered further when
mycorrhizal formation is discussed in the following sections.
Root Morphology Characteristics which Influence Mycorrhiza Formation
Association
Anatomical feature
Influence on mycorrhizas
Cortex air channels
hyphal distribution and growth rates,
arbuscule distribution
Epidermis and
hypodermis structure
Appressorium position and path of
root penetration
Hypodermis structure
Hartig net type (epidermis or cortex)
Root growth rate
Efficiency of mycorrhiza formation
VAM
ECM
F. Terminology
A glossary of important terms used to describe root structures is provided below.
You should refer to a plant anatomy text such as Esau (1977) for more
information. Words in italics are defined elsewhere in the list.
A. SUBCELLULAR COMPONENTS
Cell
The basic component of plant organs, consisting of cytoplasm, organelles, vacuoles,
etc, bounded by a plasma membrane.
Cell wall
Structure located outside the plasma membrane of most plant cells. It is primarily made
of structural carbohydrates such as cellulose. Cell walls provide mechanical support
and space for apoplastic transport of substances. They often contain secondary
metabolites, suberin or lignin.
Apoplast
The cell wall space inside living plants is collectively known as the apoplast.
Symplast
The space inside living plant cells is collectively known as the symplast. The cytoplasm
of adjacent plant cells is often connected by channels through the cell wall
(plasmodesmata).
Middle lamella
A cell wall zone rich in the carbohydrate pectin connecting adjacent cells.
Suberin
This is a hydrophobic material, containing lipids and phenolics, which impregnates the
cell walls of specialized cells (Kolattukudy 1984). Suberin is thought to prevent the
passage of water and other materials in the apoplast.
Suberin lamellae
These are concentric layers of suberin deposited on the inner surface of cell walls and
considered to function as barriers to microbial and solute penetration. These are most
often found in endodermal or exodermal cells.
Casparian band
A specialized cell wall structure where suberin is deposited in a radial band. Cells with
these structures are arranged in one or two cylinders within roots to form the
endodermis and exodermis. These bands are thought to provide a barrier to apoplastic
transport of solutes (Esau 1977, Clarkson & Robards 1975, Peterson 1988).
Lignin
A cell wall type that is impregnated by phenolic compounds. These walls are often
considerably thickened to strengthen plant organs. Xylem cells and fibres are typically
lignified, but other cells in the stele or cortex can have lignified walls.
Phi thickenings
These are localized deposits of lignified wall material which form a thickened ring in
cortex cell walls (von Guttenberg 1968).
Crystals
Specialized root cells may contain crystals, along with mucilage, or other substances in
their vacuole.
Secondary metabolites
Plant cells and cell walls often contain secondary metabolites (substances not required
for metabolism). Phenolic compounds, including tannins (dark brown pigments) are
especially common, but many other chemicals, including alkaloids, terpenes,
flavanoids, etc., accumulate in roots of particular plant species. These may color the
root and result in uv-induced auto fluorescence.
Mucilage
High molecular weigh, poorly diffusible substances actively secreted by root epidermal
and root cap cells (Rougier & Chaboud 1985). These primarily consist of
carbohydrates, but also may contain sloughed cells, enzymes, phenolic compounds, etc.
Exudates
Root exudates are defined as substance released into the substrate by healthy and intact plant
roots (Rovira 1969). These include water, sugars, amino acids, etc.
B. CELLAR STRUCTURES
Apex
The root tip which is covered by a root cap (covering sheath) and secreted mucilage
(water soluble polysaccharides which adhere to the root).
Apical meristem
The zone of dividing cells at the root apex which give rise to new cells in a growing
root. Actively growing roots have gradients of maturing tissues away from the apical
meristem.
Epidermis
The outermost layer of cells of the root, in direct contact with the soil. As the soil-root
interface, the epidermis is an important site for nutrient uptake and the initiation of
mycorrhizal associations.
Root hair
narrow cylindrical hair-like cell extension of an epidermal cell on the root surface.
These may be long or short and provide a dense or sparse root covering. Root hairs
increase root contact with the soil and are thought to have a role in water and nutrient
uptake.
Hypodermis
The layer of cells below the epidermis is called a hypodermis if it is not suberised
(Peterson 1988).
Exodermis
The hypodermis is called an exodermis if its cell walls contain a Casparian band and
these cells often also have suberin lamellae (Peterson 1988). The exodermis is thought
to reduce root permeability (to apoplastic flow) and increase resistance to pathogenic
organisms, water loss, etc.
Passage cells, short cells
Small exodermal cells that remain unsuberised that are surrounded by longer suberised
cells (long cells). In many plants, long and short cells alternate in a uniform pattern
(called a dimorphic exodermis).
Cortex
The cell layers occurring between the epidermis and stele. Cortex cells typically have a
large central vacuole used to store solutes and are the site of arbuscule formation in
VAM associations.
Aerenchyma
Air spaces within plant organs. These can form between cells, or in the case of large
spaces result from cell death. They often form continuous channels along the length or
organs such as the root. The main role of aerenchyma is to provide gas exchange to
cells in waterlogged soils (Armstrong 1979).
Endodermis
A cortex cell layer found in all roots, next to the vascular cylinder. The cell walls
contain a Casparian band and may develop suberin lamellae (Esau 1965, Clarkson &
Robards 1975).
Intercellular space
The spaces outside the root cells, often in the cortex at the junction of cells. These form
longitudinal air channels in many roots, which can be seen by observing whole-living
roots mounted in water. Air channels provide conduits for gas transport in waterlogged
soils (Armstrong 1979) and influence VAM formation.
Stele, vascular cylinder
The zone internal to the endodermis which contains specialized tissue responsible for
the transport of water and minerals to the shoot (xylem) or organic nutrients, such as
photosynthetically fixed carbon, (phloem). Additional layers of xylem and phloem form
radially during secondary growth and lateral root initiation also occurs in this zone.
Xylem cells develop lignified walls and are dead when mature.
Periderm
The bark layer formed on the surface of roots or branches by secondary growth. Walls
of periderm cells are strengthened by suberin and lignin deposits, which reduce their
permeability and susceptibility to microbial activity and adverse soil conditions.
Metacutinization
This is the modification of dormant root tips by suberization of one or more root cap cell layers
(Romberger 1963). Inactive roots of many perennial plants develop a metacutinized apex, which
functions as an extension of the exodermis, presumably for protection from adverse soil factors
(Brundrett et al. 1990).
C. ORGANS AND ZONES
Seminal root
A root initiated by a germinating seed.
Lateral roots
Any root which grows from another root.
Adventitious roots
A root which arises from a stem.
First order lateral roots
Roots that arise from the seminal root or adventitious roots.
Second and third order laterals, etc.
Roots which arise from first order laterals which in turn may produce third order
laterals, and so on. Higher order laterals may be categorized as feeder roots or fine
roots (see below).
Primary growth
The initial growth of a plant organ caused directly by cell division in its apical
meristem and cell enlargement in subapical regions.
Secondary growth
New growth activity which begins from mature cells in a plant organ. This normally
results from radial enlargement of an organ by a new lateral meristem.
Secondary roots, woody roots
Roots, which develop a periderm and additional vascular tissue due to secondary
growth. These would normally have a much longer lifespan than feeder roots and will
not contain mycorrhizas if secondary growth has resulted in cortex loss.
Coarse roots
The "distributive" root system comprised of lower order roots, which is responsible for
mechanical support and the transport of substances between fine roots and the shoot.
Feeder roots, fine roots
The fine, higher order lateral roots that are thought to be responsible for most nutrient
and water uptake, as well as mycorrhiza formation.
Brown roots, suberised roots, etc.
These additional terms are sometimes use to designate old roots, woody roots, or roots
with a suberised exodermis. These general terms are misleading and should not be
used.
Rhizosphere
The zone surrounding roots where soil properties and microbial populations are
influenced by root exudates.
Rhizoplane
The surface of the root this zone forms a habitat for organisms which live in contact
with the root.